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Benzonitrile as a Proxy for Benzene in the Cold ISM: Low-temperature Rate Coefficients for CN + C 6 H 6
Abstract
The low-temperature reaction between CN and benzene (C 6 H 6 ) is of significant interest in the astrochemical community due to the recent detection of benzonitrile, the first aromatic molecule identified in the interstellar medium (ISM) using radio astronomy. Benzonitrile is suggested to be a low-temperature proxy for benzene, one of the simplest aromatic molecules, which may be a precursor to polycyclic aromatic hydrocarbons. In order to assess the robustness of benzonitrile as a proxy for benzene, low-temperature kinetics measurements are required to confirm whether the reaction remains rapid at the low gas temperatures found in cold dense clouds. Here, we study the C 6 H 6 + CN reaction in the temperature range 15–295 K, using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with pulsed-laser photolysis-laser-induced fluorescence. We obtain rate coefficients, k ( T ), in the range (3.6–5.4) × 10 ⁻¹⁰ cm ³ s ⁻¹ with no obvious temperature dependence between 15 and 295 K, confirming that the CN + C 6 H 6 reaction remains rapid at temperatures relevant to the cold ISM.
... In fragmentation events, for example in reactions of molecules with H 3 + , these destruction routes are rather uninformative for the potential detection of the parent molecule. However, when fragmentation is not the main reaction outcome, for example in the case of group substitutions (e.g., H by CN), the spectroscopic information of the nascent molecule can help in detection of the parent species, the so-called proxy effect (Cooke et al., 2020;Miksch et al., 2021;Barnum et al., 2022). Such is the case of benzene (C 6 H 6 ), an apolar molecule that is functionalized via the reaction C 6 H 6 + CN → C 6 H 5 CN + H forming benzonitrile (Balucani et al., 1999;Woon, 2006;Trevitt et al., 2010), that, as we mentioned before was the onset of the new wave of detections of cyclic species. ...
... with k i,Add the individual CN addition reactions and k Back the back-dissociation rate constant. The total loss rate is 1.33 × 10 −9 cm 3 s −1 at 30 K, a factor ∼ 4.3 higher than the loss rate of 3.15 × 10 −10 cm 3 s −1 for benzene at that same temperature for theoretical calculations (Woon, 2006) and a factor between 2.5 and 3.4 (5.30 × 10 −10 -3.49 × 10 −10 cm 3 s −1 ) with experiments (Trevitt et al., 2010;Cooke et al., 2020) at equivalent temperatures. We believe that the reason behind the increased reactivity of the pyridine in contrast with benzene is due to the different attractive forces implied in the CN capture event. ...
... A careful inspection of the reactivity of radicals with heterocycles, and possibly in a per-radical and per-heterocycle basis, is needed to suggest good proxies of heterocyclic species. For the specific case of pyridine, the subject of this study, we suggest 1-cyano-pyridine (and in a lesser extent 2-cyanopyridine) as good candidates to posses a significant "proxy effect" (Cooke et al., 2020) due to the increased dipole moment and the significant contribution to the branching ratio of reaction (see Table 6; Figure 7). We must emphasize however, that the dipole moment of pyridine should allow for a detection of a pure species in contrast with benzene. ...
The recent detection of cyclic species in cold interstellar environments is an exciting discovery with yet many unknowns to be solved. Among them, the presence of aromatic heterocycles in space would act as an indirect evidence of the presence of precursors of nucleotides. The seeming absence of these species in the observations poses a fascinating conundrum that can be tackled with computational insights. Whilst many arguments can be given to explain the absence of heterocycles in space, one of the possible scenarios involves fast chemical conversion and formation of new species to be detected. We have tested this hypothesis for the reaction of pyridine with the CN radical to find possible scenarios in which the detectability of pyridine, as an archetypical heterocycle, could be enhanced or diminished via chemical conversions. Using a combination of ab-initio characterization of the reactive potential energy surface and kinetic and chemical simulations, we have established that pyridine does react very fast with CN radicals, estimating that the studied reactions is between 2.5–4.5 times faster in pyridine than in benzene, with a total loss rate constant of 1.33 × 10 –9 cm ³ s ⁻¹ at 30 K, with an almost null temperature dependence in the (30–150) K range. Addition reactions forming 1,2,3-cyanopyridine are favored over abstraction reactions or the formation of isocyanides. Besides, for 1 and 2-cyanopyridine there is an increase in the total dipole moment with respect to pyridine, which can help in their detection. However, the reaction is not site specific, and equal amounts of 1,2,3-cyanopyridine are formed during the reaction, diluting the abundance of all the individual pyridine derivatives.
... The rotational emission from these unambiguously detected PAHs has been observed toward TMC-1 and originates from CN-functionalized PAHs (nitriles), with the exception of the asymmetric, pure PAH indene. It has been proposed that, owing to their large dipole moments, nitrile-substituted PAHs can be used as observational proxies for pure PAHs [23,24]. Extracting quantitative abundances of unsubstituted PAHs from these proxies, however, relies on knowledge of the kinetics of their dominant formation and destruction pathways [25]. ...
... The column density of benzonitrile is 1.73 +0. 85 −0.10 × 10 12 cm −2 [17], therefore, assuming a CN + benzene rate coefficient of ∼4×10 −10 cm −3 s −1 [24], we predict a benzene abundance of ∼1.4 × 10 13 cm −2 . Since this is approximately half the abundance that we derive above for pyrene, it is difficult to envision a bottom-up route to pyrene from benzene, unless benzene is destroyed much more efficiently than pyrene. ...
Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in TMC-1.
... then we would expect attack at any of the double bonds to be efficient and barrierless (Balucani et al. 1999;Cooke et al. 2020). It has been shown both experimentally and theoretically that addition-elimination reactions between CN and unsaturated hydrocarbons do not have barriers in their entrance channel and proceed via addition complexes (Carty et al. 2001;Sims et al. 1993;Woon & Herbst 1997). ...
... Of the ratios shown in Figure 4, the outlier, the ratio of C 6 H 6 /C H CN 6 5 , still differs only by a factor of ∼4 from the others. This discrepancy is due to the fact that our astrochemical models use the experimentally determined value for the rate coefficient of Equation (2) of 5.4 × 10 −10 cm 3 s −1 determined at 15 K by Cooke et al. (2020), whereas the other hydrocarbon + CN reactions use the values of (1.0−1.5) × 10 −10 cm 3 s −1 . ...
We present laboratory rotational spectroscopy of five isomers of cyanoindene (2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier transform microwave spectrometer operating between 6 and 40 GHz. Based on these measurements, we report the detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2- C 9 H 7 CN ) in GOTHAM line survey observations of the dark molecular cloud TMC-1 using the Green Bank Telescope at centimeter wavelengths. Using a combination of Markov Chain Monte Carlo, spectral stacking, and matched filtering techniques, we find evidence for the presence of this molecule at the 6.3 σ level. This provides the first direct observation of the ratio of a cyano-substituted polycyclic aromatic hydrocarbon to its pure hydrocarbon counterpart, in this case indene, in the same source. We discuss the possible formation chemistry of this species, including why we have only detected one of the isomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratio across this and other simpler species, as well as compare to those ratios predicted by astrochemical models. We conclude that while astrochemical models are not yet sufficiently accurate to reproduce absolute abundances of these species, they do a good job at predicting the ratios of hydrocarbon:CN-substituted species, further solidifying -CN tagged species as excellent proxies for their fully symmetric counterparts.
... The CN radical is the second species that has ever been detected in space (McKellar, 1940)even before its spectroscopic laboratory characterisation (Jefferts et al., 1970;McGuire, 2018) and is considered quite ubiquitous in the ISM (Savage et al., 2002;McGuire, 2018). Therefore, the reaction c-C 2 H 4 O + CN appears to be a promising reactive pathway, considering that the CN radical has been proved to be a good reactant for barrierless reactions being involved infor examplethe formation of formyl cyanide (Tonolo et al., 2020), cyanomethanimine (Vazart et al., 2015), benzonitrile (Woon, 2006;Cooke et al., 2020), and cyanocyclopentadiene (McCarthy et al., 2021). ...
... Moreover, differently from what has been reported in literature for many cyano-containing molecules (Vazart et al., 2015;Cooke et al., 2020;Tonolo et al., 2020), the title reaction does not form the cyano-derivative of the reaction partner, i.e., cyanooxirane, since the nucleophilic attack of the radical always leads to the opening of the ring. This result demonstrates that cyanooxirane cannot be formed efficiently from its parent oxirane and would explain why the astronomical searches of this species have resulted unsuccessful so far (Dickens et al., 1996;Wirström et al., 2007). ...
The escalating identification of new complex molecules in the interstellar medium claims for potential formation routes of such species. In this regard, the present work considers the reaction between oxirane and the CN radical as a feasible formation mechanism of species having the C 3 H 3 NO molecular formula. Indeed, the compounds of this family are elusive in the interstellar medium and suggestions on which species could be formed at low temperature and low pressure conditions might aid their discovery. The c -C 2 H 4 O + CN reaction has been investigated from the thermodynamic and kinetic points of view. The thermodynamic has been studied by means of a double-hybrid density functional and revealed the presence of several mechanisms submerged with respect to the reactants energy, with the potential formation of oxazole and cyanoacetaldehyde. However, the kinetic results suggest that the main reaction pathway is the H-extraction, leading to 2-oxiranyl radical and HCN. The formation of cyanoacetaldehyde + H and of H 2 CCN + H 2 CO is also possible with smaller rate constants, while the production of oxazole is negligible due to the presence of a high energy barrier.
... The rotational emission from these unambiguously detected PAHs has been observed towards TMC-1 and originates from CN-functionalized PAHs (nitriles), with the exception of the asymmetric, pure PAH indene. It has been proposed that, owing to their large dipole moments, nitrile-substituted PAHs can be used as observational proxies for pure PAHs 23,24 . Extracting quantitative abundances of unsubstituted PAHs from these proxies, however, relies on knowledge of the kinetics of their dominant formation and destruction pathways 25 . ...
Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent electric dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. Here we report the detection of 2-cyanopyrene and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in the Taurus molecular cloud (TMC-1). Our detections of the cyanopyrene isomers suggest that small PAHs like pyrene must be either formed in or transported to the cold interstellar medium, challenging assumptions about the origin and fate of PAHs in space.
... Most PAHs have a small or zero dipole moment, so cannot be readily observed using radio astronomy, but their presence can be indirectly inferred by searching for chemically related molecules. Laboratory experiments have shown that CN-functionalized aromatics can be used as efficient proxies when searching for their pure hydrocarbon counterparts that do not possess permanent dipole moments (23,24). Pyrene is the smallest PAH in which all rings are connected to at least two others (termed compact or pericondensed). ...
Polycyclic aromatic hydrocarbons (PAHs) are organic molecules containing adjacent aromatic rings. Infrared emission bands show that PAHs are abundant in space, but only a few specific PAHs have been detected in the interstellar medium. We detect 1-cyanopyrene, a cyano-substituted derivative of the related four-ring PAH pyrene, in radio observations of the dense cloud TMC-1 using the Green Bank Telescope. The measured column density of 1-cyanopyrene is ∼ 1 .52×10 12 cm − 2 , from which we estimate that pyrene contains up to 0.1% of the carbon in TMC-1. This abundance indicates that interstellar PAH chemistry favors the production of pyrene. We suggest that some of the carbon supplied to young planetary systems is carried by PAHs that originate in cold molecular clouds.
... A non-zero intercept value was observed and is common in kinetic measurements made using the CRESU technique. 46,52,[104][105][106] Propagation of error is discussed extensively in Sec. SIX of the supplementary material. ...
We present the development of a new astrochemical research tool, HILTRAC, the Highly Instrumented Low Temperature ReAction Chamber. The instrument is based on a pulsed form of the CRESU (Cinétique de Réaction en Écoulement Supersonique Uniforme, meaning reaction kinetics in a uniform supersonic flow) apparatus, with the aim of collecting kinetics and spectroscopic information on gas phase chemical reactions important in interstellar space or planetary atmospheres. We discuss the apparatus design and its flexibility, the implementation of pulsed laser photolysis followed by laser induced fluorescence, and the first implementation of direct infrared frequency comb spectroscopy (DFCS) coupled to the uniform supersonic flow. Achievable flow temperatures range from 32(3) to 111(9) K, characterizing a total of five Laval nozzles for use with N2 and Ar buffer gases by impact pressure measurements. These results were further validated using LIF and direct frequency comb spectroscopy measurements of the CH radical and OCS, respectively. Spectroscopic constants and linelists for OCS are reported for the 10⁰1 band near 2890–2940 cm⁻¹ for both OC³²S and OC³⁴S, measured using DFCS. Additional peaks in the spectrum are tentatively assigned to the OCS-Ar complex. The first reaction rate coefficients for the CH + OCS reaction measured between 32(3) and 58(5) K are reported. The reaction rate coefficient at 32(3) K was measured to be 3.9(4) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ and the reaction was found to exhibit no observable temperature dependence over this low temperature range.
... An approach using the CRESU (Cinetique de Reáction en Ecoulement Supersonique Uniforme) method showed that formation of benzonitrile in a reaction of the cyano radical with benzene is fast and barrierless. 42 Another class of important gas-phase reactions relevant for low-temperature PAH chemistry is ion−molecule reactions. To explain the abundance of heavy ionic species, reactions of ions with small and abundant hydrocarbon building blocks such as acetylene, 43 ethylene and hydrogen cyanide are proposed. ...
Aromatic molecules play an important role in the chemistry of astronomical environments such as the cold interstellar medium (ISM) and (exo)planetary atmospheres. The observed abundances of (polycyclic) aromatic hydrocarbons such as benzonitrile and cyanonaphthalenes are, however, highly underestimated by astrochemical models. This demonstrates the need for more experimentally verified reaction pathways. The low-temperature ion–molecule reaction of benzonitrile•+ with acetylene is studied here using a multifaceted approach involving kinetics and spectroscopic probing of the reaction products. A fast radiative association reaction via an in situ experimentally observed prereactive complex shows the importance of noncovalent interactions in steering the pathway during cold ion–molecule reactions. Product structures of subsequent reactions are unambiguously identified using infrared action spectroscopy and reveal the formation of nitrogen-containing, linked bicyclic structures such as phenylpyridine•+ and benzo-N-pentalene+ structures. The results, contradicting earlier assumptions on the product structure, demonstrate the importance of spectroscopic probing of reaction products and emphasize the possible formation of linked bicyclic molecules and benzo-N-pentalene+ structures in astronomical environments.
... Indeed, a number of recent studies (Ferrero et al. 2020;Tinacci et al. 2022;Bovolenta et al. 2022) and reviews (Zamirri et al. 2019) have highlighted the importance of accurate binding energies for astrochemical simulations of smaller adsorbed molecules. Furthermore, a suitable radio proxy for benzene has been identified as cyanobenzene (Cooke et al. 2020), due to its rapid reaction with CN even at low cold ISM temperatures. Indeed cyanobenzene was first detected using in TMC-1 by McGuire et al. (2018) and has now also been further confirmed in pre-stellar sources (Burkhardt et al. 2021). ...
In this paper we provide a highly accurate value for the binding energy of benzene to proton-ordered crystalline water ice (XIh), as a model for interstellar ices. We compare our computed value to the latest experimental data available from temperature programmed desorption (TPD) experiments and find that our binding energy value agrees well with data obtained from binding to either crystalline or amorphous ice. Importantly, our new value is lower than that used in most astrochemical networks by about nearly half its value. We explore the impact of this revised binding energy value for both an AGB outflow and a protoplanetary disk. We find that the lower value of the binding energy predicted here compared with values used in the literature (4050 K versus 7587 K) leads to less depletion of gas-phase benzene in an AGB outflow, and leads to a shift outwards in the benzene snowline in the midplane of a protoplanetary disk. Using this new value, the AGB model predicts lower abundances of benzene in the solid phase throughout the outflow. The disk model also predicts a larger reservoir of gas-phase benzene in the inner disk, which is consistent with the recent detections of benzene for the first time in protoplanetary disks with JWST.
... Finally, phenol can also arise from the reaction of >6 eV electrons with benzene-water films since above this energy both OH and phenyl radicals (Fenzlaff & Illenberger, 1984) are produced. The production of aromatic species in space arises from complex chemistry, and benzonitrile has been proposed to be a tracer of benzene (e.g., CN + C 6 H 6 → C 6 H 5 CN + H (Cooke et al., 2020)). The present results also show the ability of ballistic free electrons with energies below 5 eV to trigger the reaction of benzene formation as an alternative route to multi-collision processes (Jones et al., 2011) or the irradiation of alcohol (Sivaraman et al., 2015). ...
Aromatic compounds are present in the interstellar space and on carbonaceous meteorites where they intriguingly represent the most important constituents along with the water ice. Exposed to the cosmic radiation, more complex species are generated via processes that are still far from being understood. The present study aims at studying the chemistry of benzonitrile in water ice environment when exposed to the ballistic low energy free electrons produced abundantly along the radiation tracks. Here we show that phenol and benzene are synthetized at the electron energy above but also below 6 eV via different routes, which can be selectively activated by tuning the electron energy. The reactivity presented in this work may contribute to a better understanding of the time evolution, via models and simulations, of species present in interstellar space ices and meteorites.
... Recently, in a theoretical study performed with the G3//B3LYP method, McCarthy et al. ( 2021 ) have reported the formation of the detected c yanoc yclopentadiene from the reaction between CN radical and cyclopentadiene. Furthermore, Cooke et al. ( 2020 ) studied the C 6 H 6 + CN reaction in the temperature range 15-295 K, using the CRESU technique (Reaction Kinetics in Uniform Supersonic Flow) combined with pulsed laser photolysis-laser-induced fluorescence. These authors concluded that the reaction remains rapid at temperatures rele v ant to the cold ISM. ...
N-heterocycles are of special relevance in astrobiology but at present no nitrogen-containing heterocycles have been detected in the interstellar medium (ISM). Thus far, the simplest N-heterocyclic compound, 2H-Azirine (c-C2H3N), has not been conclusively identified, despite being searched for. Recently, several cyano and ethynyl derivatives of unsaturated hydrocarbons have been discovered in the cold pre-stellar core Taurus Molecular Cloud 1 (TMC-1). The purpose of this work is to assess the feasibility of the possible formation of cyano and ethynyl derivatives of azirine (c-C2H2N-CN, c-C2H2N-CCH) under interstellar conditions and provide high-level theoretical spectroscopic parameters of the most relevant cyano- and ethynyl-azirine isomers to facilitate their experimental identification. Six isomers are located for each, cyano- and ethynyl-azirine derivatives, and their interconversion processes are studied. The reactions of 2H-azirine with the CN or CCH radicals in the gas phase are explored as possible formation routes of cyano and ethynyl azirine. We found that the formation processes of the most stable isomers, namely 3-cyano-2H-azirine, 2-cyano-2H-azirine, 3-ethynyl-2H-azirine, and 2-ethynyl-2H-azirine, are exothermic and barrier free. Thus, these compounds stand out as potential targets to be searched for in space. Based on the newly determined rotational spectroscopic parameters, which also account for 14N hyperfine splittings, we compile a line catalogue for each system up to 50 GHz as a preliminary and required step to characterize these molecules experimentally, whether in the laboratory or directly in the ISM.
... Their theoretical results reveal that the barrierless additiondissociation pathways with exit barriers are well below the energy of the reactant, showing great advantages under lowtemperature conditions. In contrast to the consistent experimental results of the CN + benzene reaction with rate coefficients of ∼4 × 10 −10 cm 3 s −1 (Balucani et al. 1999;Trevitt et al. 2010;Cooke et al. 2020), the discrepancy between the two studies on the CN + toluene reaction reached a factor of 3. The lack of product yields of the CN + toluene reaction raises questions on the structure effect of the aromatic molecule on its reaction mechanism for forming CN-substituted compounds, which motivate us to study it from the view of kinetic theories. ...
Reactions between cyano radicals and aromatic hydrocarbons are believed to be important pathways for the formation of aromatic nitriles in the interstellar medium (ISM) including those identified in the Taurus molecular cloud (TMC-1). Aromatic nitriles might participate in the formation of polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) in Titan's atmosphere. Here, ab initio kinetic simulations reveal a high efficiency of ∼10 ⁻¹⁰ cm ³ s ⁻¹ and the competition of the different products of the CN + toluene reaction at 30–1800 K and 10 ⁻⁷ –100 atm. In the star-forming region of the TMC-1 environment, the product yields of benzonitrile and tolunitriles for CN reacting with toluene are approximately 17% and 83%, respectively. Detections of the main products, tolunitriles, can serve as proxies for the undetected toluene in the ISM due to their much larger dipole moments. Competition between bimolecular and unimolecular products is extremely intense in the warmer and denser PANH-forming region of Titan's stratosphere. Computational results show that the fractions of tolunitriles, adducts, and benzonitrile are 19%–68%, 15%–64%, and 17%, respectively, at 150–200 K and 0.0001–0.001 atm (Titan's stratosphere). Then, benzonitrile and tolunitriles may contribute to the formation of PANHs by consecutive C 2 H additions. The kinetic information of aromatic nitriles for the CN + toluene reaction calculated here helps to explain the formation mechanism of polycyclic aromatic hydrocarbons or PANHs under different interstellar environments and constrains corresponding astrochemical models.
... However, the formation of the precursor ions, such as C 5 H 6 N + , is not well understood. Alternatively, laboratory studies have shown that CN radicals react efficiently with hydrocarbons to form the CN-substituted nitriles under low-temperature conditions (Sims et al. 1993;Cooke et al. 2020), as has been proposed for many CN-substituted hydrocarbons discovered in TMC-1. In this study, we assume the barrierless neutral-neutral reaction of CH 2 CHCHCH 2 and the CN radical to be the only formation path for the two new species, i.e., ...
We report the detection of the lowest-energy conformer of E -1-cyano-1,3-butadiene ( E -1- C 4 H 5 CN ), a linear isomer of pyridine, using the fourth data reduction of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We perform velocity stacking and matched-filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1 σ level. We derive a total column density of 3.8 − 0.9 + 1.0 × 10 10 cm ⁻² , which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of 2.34 × 10 ¹⁰ cm ⁻² for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by one to four orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitude. We discuss the implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.
... Their theoretical results revealed that the barrierless addition-dissociation pathways with exit barriers are well below the energy of the reactant show great advantages under low temperature conditions. In contrast to the consistent experimental results of the CN + benzene reaction with rate coefficients of ∼ 4 × 10 −10 cm 3 s −1 (Trevitt et al. 2010;Balucani et al. 1999;Cooke et al. 2020), the discrepancy between the two studies on the CN + toluene reaction reached a factor of 3. The lack of product yields of CN +toluene reaction raises questions on the structure effect of the aromatic molecule on its reaction mechanism for forming CN-substitute compounds, which motivates us to study it from the view of kinetic theories. ...
Reactions between cyano radical and aromatic hydrocarbons are believed to be important pathways for the formation of aromatic nitriles in the interstellar medium (ISM) including those identified in the Taurus molecular cloud (TMC-1). Aromatic nitriles might participate in the formation of polycyclic aromatic nitrogen containing hydrocarbons (PANHs) in Titan's atmosphere. Here, ab initio kinetics simulations reveal a high efficiency of and the competition of the different products of 30-1800 K and -100 atm of the CN + toluene reaction. In the star-forming region of TMC-1 environment, the product yields of benzonitrile and tolunitriles for CN reacting with toluene may be approximately 17 and 83, respectively. The detection of main products, tolunitriles, can serve as proxies for the undetected toluene in the ISM due to their much larger dipole moments. The competition between bimolecular and unimolecular products is extremely intense under the warmer and denser PANH forming region of Titan's stratosphere. The computational results show that the fractions of tolunitriles, adducts, and benzonitrile are 19-68, 15-64 and 17, respectively, at 150-200 K and 0.0001-0.001 atm (Titan's stratosphere). Then, benzonitrile and tolunitriles may contribute to the formation of PANHs by consecutive additions. Kinetic information of aromatic nitriles for the CN + toluene reaction calculated here helps to explain the formation mechanism of polycyclic aromatic hydrocarbons (PAHs) or PANHs under different interstellar environments and constrains corresponding astrochemical models.
... These C1 to C6 hydrocarbons-among them simple radicals such as methylidyne (CH; Suutarinen et al. 2011) and ethynyl (C 2 H; Tucker et al. 1974) along with closed shell species like vinylacetylene (C 4 H 4 ; Cernicharo et al. 2021b)-have emerged as fundamental molecular building blocks to form aromatic molecules, e.g., benzene (C 6 H 6 ; Jones et al. 2011) and naphthalene (C 10 H 8 ; Figure 1; Parker et al. 2012). These aromatics were detected astronomically through their cyano derivatives cyanobenzene (C 6 H 5 CN; McGuire et al. 2018) and 1-/2-cyanonapthalene (C 10 H 9 CN; McGuire et al. 2021) as products of, e.g., barrierless (Cooke et al. 2020) and exoergic reactions with cyano radicals (CN; Balucani et al. 1999;Kaiser & Balucani 2001) with recent astrochemical models replicating their fractional abundances in TMC-1 quantitatively (Kaiser et al. 2022). However, whereas molecular beams studies along with electronic structure calculations provided fundamental knowledge of the elementary reaction mechanisms leading to aromatics in molecular clouds (Kaiser & Hansen 2021), an understanding of the inherent reaction pathways to their C1-C6 hydrocarbon precursors has not yet been fully accomplished. ...
The recent astronomical detection of ethynylbutatrienylidene (HCCCHCCC)—a high-energy isomer of triacetylene (HCCCCCCH) and hexapentaenylidene (H 2 CCCCCC)—in TMC-1 puzzled the laboratory astrophysics community since proposed reaction pathways could not synthesize the ethynylbutatrienylidene (HCCCHCCC) under cold molecular cloud conditions. Exploiting a retrosynthesis coupled with electronic structure calculations and astrochemical modeling, we reveal that observed fractional abundance of ethynylbutatrienylidene (HCCCHCCC) of 1.3 ± 0.2 × 10 ⁻¹¹ can be quantitatively replicated though the barrierless and exoergic reaction of tricarbon (C 3 ) with the resonantly stabilized propargyl radical (C 3 H 3 ) after a few 10 ⁵ yr—typical ages of cold molecular clouds. Our study provides persuasive evidence that previously assumed “dead” reactants such as tricarbon (C 3 ) and the propargyl radical (C 3 H 3 ) provide fundamental molecular building blocks in molecular mass growth processes leading to exotic, high-energy isomers of hydrocarbons: ethynylbutatrienylidene (HCCCHCCC).
... Beyond the solar system, Infrared Space Observatory (ISO) confirmed benzene in the protoplanetary nebula CRL618 (Cernicharo et al. 2001). The suggested proxy for benzene, benzonitrile, was detected in the cold core of the Taurus Molecular Cloud (Cooke et al. 2020), which marked the first radioastronomical detection of an aromatic species in the interstellar medium (McGuire et al. 2018). ...
Benzene is a simple neutral aromatic compound found in molecular clouds, comets, and planetary atmospheres. It has been confirmed on Jupiter, Saturn, Titan, and is expected on exoplanets. In this paper, the decomposition of benzene in a simulated asteroid or comet impact into an N 2 -dominated atmosphere was investigated. The impact plasma was simulated with laser-induced dielectric breakdown and the gas phase decomposition products were observed using high-resolution Fourier transform infrared spectroscopy. The gas phase decomposition products involve mainly HCN, C 2 H 2 , and smaller amounts of CH 4 with yields of 3.1%–24.0%, 0–11.7%, and 0.5%–3.3%, respectively. Furthermore, in presence of water, benzene also produces CO and CO 2 with yields of 2.4%–35.1% and 0.01%–4.8%, respectively. The oxidation state of the product mixture is proportional to the water content. Apart from that, a black-brownish solid phase is formed during the experiments, which makes up about 60% of the original carbon content. Our results therefore show that in anoxic N 2 -dominated planetary atmospheres, impacts might lead to the depletion of benzene and the formation of HCN, C 2 H 2 , and CH 4 and, in the presence of water, to the formation of CO and CO 2 .
... A significant example is provided by the observation of benzonitrile (C 6 H 5 CN) (McGuire et al., 2018), whose detection in the ISM has been undoubtedly related to the presence of benzene in the same environment. Indeed, different experiments have demonstrated the effectiveness of the reaction between the CN radical and benzene C 6 H 6 (Lee et al., 2019;Cooke et al., 2020). Similarly, the observation of cyanonaphthalene towards TMC-1 (McGuire et al., 2021) suggests the presence of naphthalene in this cold core. ...
Radioastronomy is a powerful tool for the discovery of molecules in space but it requires molecular species to be polar. The observation of apolar species can be however enabled by protonation, which occurs from reaction with the abundant H 3 + ion whenever the proton affinity of the species under consideration is greater than that of H2. This property can be easily investigated by computational chemistry and, in this work, it has been used to asses the potential protonation of simple homo diatomics, such as Cl2, P2, and Si2, as well as apolar species containing two equivalent CN moieties, such as diisocyanogen (CNNC) and (E)-1,2-dicyanoethene. Quantum chemistry has also been exploited to investigate the mechanisms of three protonation reactions of H 3 + with Cl2, P2, and CNNC. To support laboratory measurements and astronomical observations of the resulting transient species, their rotational spectroscopic parameters were accurately computed together with fundamental vibrational frequencies. For this purpose, we have employed CCSD(T)-based computational methodologies, which provide equilibrium structures with errors smaller than 0.001 Å and 0.1° for bond distances and angles, respectively. Such an accuracy is expected to lead to rotational constants predicted, in relative terms, with uncertainties better than 0.2%. Instead, the expected accuracy on vibrational frequencies is ∼10 cm−1, thus being well suited to guide band assignments.
... Recently ethynyl cyclopropenylidene (c-C 3 HC 2 H) has been identified in the interstellar medium (Cernicharo et al. 2021d ) and theoretical studies have proposed the formation of monosubstituted c yclopropen ylidene deri v ati ves through gas-phase additions of X onto c-C 3 H 2 (being X a radical, such as CN, OH, F, or NH 2 ) (Fortenberry 2021 ;Flint & Fortenberry 2022 ). In a similar way, the formation of the detected benzonitrile (McGuire et al. 2018 ) and c yanoc yclopentadiene (McCarthy et al. 2021 ) from the reaction between CN radical and benzene or cyclopentadiene have been proposed (Cooke et al. 2020 ;McCarthy et al. 2021 ) as possible ways of formation. In this regard, we have considered the gas phase reaction between OH and 2H-azirine as a potential generation route of hydroxy azirine. ...
Hydroxy-azirine (C2H3NO) is a -OH derivative of azirine (C2H3N), molecule that has been the subject of several unfruitful searches in space. Hydroxy-azirine is an isomer of the detected prebiotic species methyl isocyanate, CH3NCO, and glycolonitrile, HOCH2CN, as well as the yet undetected imine acetaldehyde, NHCHCHO. However, the lack of preliminary spectroscopic data on hydroxy-azirine has prevented its astronomical search. The aim of this study is to provide high-level theoretical spectroscopic signatures of the most stable hydroxy-azirine isomers to enable their eventual interstellar search. Twelve isomers have been characterized for hydroxy-azirine and their isomerization processes have been analysed at the CCSD(T)-F12/cc-pVTZ-F12 level. The most stable structures are 3-hydroxy-2H-azirine (I) and 2-hydroxy-2H-azirine (II) in their syn- and anti-configurations, which are suggested as the most relevant candidates for laboratory and interstellar detection. To ease their identification by means of rotational spectroscopy, we report a set of the required spectroscopic parameters using state-of-the-art composite and coupled-cluster approaches. For astronomical purposes, we provide a complete line list for I-syn and I-anti hydroxy-azirine up to 50 GHz, which takes the hyperfine structure into account, and will be essential to hunt for these interstellar candidates experimentally. In addition, anharmonic vibrational frequencies and intensities are reported to predict a trustworthy vibrational spectra and to estimate the vibrational partition function. Finally, we analyse the possibility of formation of hydroxy-azirine from the reaction of azirine with the hydroxyl radical in the gas-phase and on the surface of ices, finding for the latter a feasible formation route under interstellar conditions.
... 8−10 Although para-DCNB does not possess a permanent dipole moment due to its symmetry, the two cyano moieties make ortho-and meta-DCNB have relatively large dipole moments, making them visible by radio astronomy. 11 Kinetic studies involving CN + toluene (C 6 H 5 CH 3 ) by Messinger et al. have shown that the addition of a CN group to a substituted benzene, such as C 6 H 5 CH 3 , has a similar reaction rate efficiency as that produced with the addition of CN directly to the benzene ring itself to form benzonitrile. 12,13 Based on the Messinger et al. kinetic study, Chitarra et al., 14 despite failing to detect DCNBs in the GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM), estimated that the column densities of ortho-and meta-DCNB could be as high as 2 × 10 10 cm −2 , which is below the benzonitrile column density of 17.3 −1.00 +0.85 × 10 12 cm −2 , to yield a DCNB/C 6 H 5 CN abundance ratio of ∼1:100. 14 Studies involving DCNBs range from experimental and theoretical vibrational spectroscopic investigations aimed at accurate assignment of the vibrational structures in the ground electronic state 9,10,15−22 to high-resolution rotational spectroscopy in the centimeter-and millimeter-wave domains, done in conjunction with high-level ab initio quantum-mechanical calculations. ...
The negative ion photoelectron spectra of 1,2-dicyanobenzene (o-DCNB), 1,3-dicyanobenzene (m-DCNB), and 1,4-dicyanobenzene (p-DCNB) radical anions (DCNB·-), acquired through the computation of Frack-Condon (FC) factors, are presented. The FC calculations utilize harmonic frequencies and normal mode vectors derived from density functional theory at the B3LYP/aug-cc-pVQZ basis set. All the totally symmetric vibrational modes are treated with Duschinsky rotations to yield neutral DCNBs in their singlet (So) and lowest triplet (T1) states, following an electron removal from the doublet anionic ground state. For the So state, the adiabatic electron affinities (EAs) for o-, m-, and p-DCNB are 1.179, 1.103, and 1.348 eV. The EAs for the lowest T1 state in o-, m-, and p-DCNB are 4.151, 4.185, and 4.208 eV, resulting in an So-T1 energy difference (ΔEST) of 2.973, 3.082, and 2.860 eV. A vibrational analysis reveals evidence of FC activity involving ring distortion, C-N bending, and ring C═C stretching vibrational progressions in both the So and T1 states. With the detection of cyanonaphthalene (C10H7CN) and cyanoindene (C9H7CN) in the interstellar medium (ISM), our results highlight the extent to which replacing a single hydrogen on an aromatic molecule with a cyano group, C≡N, can alter the vibrational structure of the molecule/radical anion. As such, dicyano-polyaromatic hydrocarbons may be reasonably robust in the ISM, making it appealing to search for them in future interstellar detection missions.
... However, the recent discovery of benzonitrile (c C H CN 6 5 ) in a nearby molecular cloud (TMC-1) marks the first radio detection of an aromatic species in the ISM (McGuire et al. 2018). It is suggested thatc C H CN 6 5 can be a proxy of benzene (c − C 6 H 6 ) at a low-temperature (Cooke et al. 2020), which is the simplest aromatic molecule and plays a significant role in interstellar chemistry. Furthermore, the phenyl radical (−C 6 H 5 ) derived from the benzene by a hydrogen abstraction could be a possible progenitor of the formation of other new aromatic species. ...
Phenol, which belongs to the C 6 H 6 O isomeric group, is the most simplistic molecule in the family of alcohol of the aromatic series. Although phenol is yet to be detected in the interstellar medium, a tentative identification was reported toward Orion KL hot core using IRAM-30 m line survey. To explore some more species of this isomeric group, we consider ten species to study the fate of their astronomical detection. It is noticed that phenol is the most energetically favorable isomer of this group. In contrast, propargyl ether is the least favorable (having relative energy ∼ 103 kcal/mol compared to phenol) species of this group. So far, the studies associated with the formation of phenol are heavily concentrated on combustion chemistry. Here, we suggest few key reactions (C 6 H 6 + OH → C 6 H 5 + H 2 O, C 6 H 6 + O → C 6 H 5 OH, C 6 H 6 + H → C 6 H 5 + H 2 , and C 6 H 5 + OH → C 6 H 5 OH + hν) for the formation of phenol. All these pathways are included in a large gas-grain chemical network to study its formation in high mass star-forming regions and dark cloud environments. It is noticed that the phenyl (−C 6 H 5 ) formation by the ice-phase hydrogen abstraction reaction of benzene (i.e. C 6 H 6 + OH → C 6 H 5 + H 2 O if allowed at ∼ 10 K) could serve as the starting point for the formation of phenol in the gas phase by radiative association reaction C 6 H 5 + OH → C 6 H 5 OH + hν. The gas-phase reaction C 6 H 6 + O → C 6 H 5 OH significantly contributes to the formation of phenol, when the ice-phase reaction C 6 H 6 + OH → C 6 H 5 + H 2 O is not considered at low temperature. Band 4 ALMA archival data of a hot molecular core, G10.43+0.03, is analyzed. It yields an upper limit of phenol abundance of 5.19 × 10 ⁻⁹ . Our astrochemical model delivers an upper limit of phenol abundance of ∼ 2.20 × 10 ⁻⁹ in the hot molecular core, whereas its production in the dark cloud is not satisfactory.
The incorporation of nitrogen atoms into cyclic compounds is essential for terrestrial life; nitrogen-containing (N-)heterocycles make up DNA and RNA nucleobases, several amino acids, B vitamins, porphyrins, and other components of biomolecules. The discovery of these molecules on meteorites with non-terrestrial isotopic abundances supports the hypothesis of exogenous delivery of prebiotic material to early Earth; however, there has been no detection of these species in interstellar environments, indicating that there is a need for greater knowledge of their astrochemical formation and destruction pathways. Here, we present results of simulations of gas-phase pyrrole and pyridine formation from an ab initio nanoreactor, a first-principles molecular dynamics simulation method that accelerates reaction discovery by applying non-equilibrium forces that are agnostic to individual reaction coordinates. Using the nanoreactor in a retrosynthetic mode, starting with the N-heterocycle of interest and a radical leaving group, then considering the discovered reaction pathways in reverse, a rich landscape of N-heterocycle-forming reactivity can be found. Several of these reaction pathways, when mapped to their corresponding minimum energy paths, correspond to novel barrierless formation pathways for pyridine and pyrrole, starting from both detected and hypothesized astrochemical precursors. This study demonstrates how first-principles reaction discovery can build mechanistic knowledge in astrochemical environments as well as in early Earth models such as Titan’s atmosphere where N-heterocycles have been tentatively detected.
While the nitrile group is by far the most prevalent one among interstellar molecules, the existence of interstellar dinitriles (molecules containing two CN groups) has recently been proven. Here we report the discovery of two new dinitriles in the cold dense cloud TMC-1 . These newly identified species are malononitrile, CH(CN), and maleonitrile, the Z isomer of NCCH=CHCN, which can be seen as the result of substituting two H atoms with two CN groups in methane and ethylene, respectively. These two molecules were detected using data from the ongoing QUIJOTE line survey of TMC-1 that is being carried out with the Yebes\,40m telescope. We derive column densities of 1.8\,times \,1011 for malononitrile and maleonitrile, respectively. This means that they are eight and three times less abundant than HCCCHCN and (E)-HCCCH=CHCN, respectively, which are analog molecules detected in TMC-1 in which one CN group is converted into a CCH group. This is in line with previous findings in which CCH derivatives are more abundant than the CN counterparts in TMC-1 . We examined the potential chemical pathways to these two dinitriles, and we find that while maleonitrile can be efficiently formed through the reaction of CN with CHCHCN, the formation of malononitrile is not clear because the neutral-neutral reactions that could potentially form it are not feasible under the physical conditions of TMC-1
In this paper, we provide a highly accurate value for the binding energy of benzene to proton-ordered crystalline water ice (XIh), as a model for interstellar ices. We compare our computed value to the latest experimental data available from temperature-programmed desorption experiments and find that our binding energy value agrees well with data obtained from binding to either crystalline or amorphous ice. Importantly, our new value is lower than that used in most astrochemical networks by about nearly half its value. We explore the impact of this revised binding energy value for both an asymptotic giant branch (AGB) outflow and a protoplanetary disc. We find that the lower value of the binding energy predicted here compared with values used in the literature (4050 K versus 7587 K) leads to less depletion of gas-phase benzene in an AGB outflow, and leads to a shift outwards in the benzene snowline in the mid-plane of a protoplanetary disc. Using this new value, the AGB model predicts lower abundances of benzene in the solid phase throughout the outflow. The disc model also predicts a larger reservoir of gas-phase benzene in the inner disc, which is consistent with the recent detections of benzene for the first time in protoplanetary discs with JWST.
Benzonitrile (BN, C6H5CN) has been detected in the cold molecular cloud Taurus Molecular Cloud-1 (TMC-1) in 2018, which is suggested to be a precursor to form more complex nitrogen-containing aromatic...
Anharmonic quantum chemical computations reveal a strong, narrow (width = 0.075 m) band in the 4.3–4.5 m region of the absorption spectra of the cyano-substituted polycyclic aromatic hydrocarbons (CN-PAHs) cyanonaphthalene, cyanoanthracene, cyanophenanthrene, and cyanopyrene. This narrow window with intense IR lines implies that CN-PAHs of various shapes and sizes offer little variation in both wavelength and intensity in this region. Subsequently, this band can be used as a tracer for CN-PAHs. The distinct features making up the band are assigned to mixed vibrational states consisting of the CN stretch fundamental and various combination bands, including in-plane CH bends, CC skeletal bends, and CC skeletal breathing motions. The extraordinarily large intrinsic intensity of the fundamental CN stretch is redistributed to nearby states via anharmonic coupling, which is readily captured when using second order vibrational perturbation theory with resonance polyad matrices. This redistribution of intensity leads to a complex spectrum. The intense bands in this wavelength region may contribute to the baseline continuum and undulating macroscopic structure seen in recent JWST NIRSpec observations.
In various astronomical environments such as the interstellar medium or (exo)planetary atmospheres, an interplay of bottom-up growth and top-down destruction processes of (polycyclic) aromatic hydrocarbons (PAHs) takes place. To get more insight into the interplay of both processes, we disentangle the fragmentation and formation processes that take place upon dissociative ionization of benzonitrile. We build on previous spectroscopic detections of the ionic fragmentation products of benzonitrile and use these as reactants for low-temperature bottom-up ion–molecule reactions with acetylene. By combining kinetics and infrared action spectroscopy, we reveal exothermic pathways to various (polycyclic) aromatic molecules, including the pentalene and phenylacetylene radical cations. We determine the reaction rate coefficients and unambiguously assign the structures of the reaction products. The data is supplemented by potential energy surface calculations and the analysis of non-covalent interactions. This study shows the unexpected formation of a linked four- and six-membered ring structure (phenylcyclobutadiene radical cation) with molecular formula C10H8˙+, and not the commonly observed isomer naphthalene˙+. All observed reactions proceed via radiative association processes and are relevant for the chemistry in (cold) astrochemical environments.
Context . Detailed astrochemical models are a key component to interpret the observations of interstellar and circumstellar molecules since they allow important physical properties of the gas and its evolutionary history to be deduced.
Aims . We update one of the most widely used astrochemical databases to reflect advances in experimental and theoretical estimates of rate coefficients and to respond to the large increase in the number of molecules detected in space since our last release in 2013.
Methods . We present the sixth release of the UMIST Database for Astrochemistry (UDfA), a major expansion of the gas-phase chemistry that describes the synthesis of interstellar and circumstellar molecules. Since our last release, we have undertaken a major review of the literature which has increased the number of reactions by over 40% to a total of 8767 and increased the number of species by over 55% to 737. We have made a particular attempt to include many of the new species detected in space over the past decade, including those from the QUIJOTE and GOTHAM surveys, as well as providing references to the original data sources.
Results . We use the database to investigate the gas-phase chemistries appropriate to both O-rich and C-rich conditions in TMC-1 and to the circumstellar envelope of the C-rich AGB star IRC+10216 and identify successes and failures of gas-phase only models.
Conclusions . This update is a significant improvement to the UDfA database. For both the dark cloud and C-rich circumstellar envelope models, calculations match around 60% of the abundances of observed species to within an order of magnitude. There are a number of detected species, however, that are not included in the model either because their gas-phase chemistry is unknown or because they are likely formed via surface reactions on icy grains. Future laboratory and theoretical work is needed to include such species in reaction networks.
Recent detections of aromatic species in dark molecular clouds suggest that formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC-1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by 2 orders of magnitude compared to its observed value. Predicted abundances for the chemically related C 4 H 3 N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in polycyclic aromatic hydrocarbon formation.
Interstellar aromatic molecules such as polycyclic aromatic hydrocarbons and polycyclic nitrogen and oxygen bearing molecules are thought to be abundant in the interstellar medium. In this class of molecules, benzonitrile (c-C6H5CN) plays an important role as a proxy for benzene. It has been detected through rotational emission in several astrophysical sources and is one of the simplest N-bearing polar aromatic molecules. Even in the cold ISM, the population of the rotational levels of benzonitrile might not be at equilibrium. Consequently, modeling its detected emission lines requires a prior computation of its quenching rate coefficients by the most abundant species in the ISM (He or H2). In this paper, we focus on the excitation of c-C6H5CN by collision with He. We compute the first potential energy surface (PES) using the explicitly correlated coupled cluster method in conjunction with large basis sets. The PES obtained is characterized by a potential well depth of -97.2 cm−1 and an important anisotropy. Scattering computations of the rotational (de-)excitation of c-C6H5CN by He atoms are performed by means of the coupled states approximation that allow to obtain collisional rates for rotational states up to j = 9 and temperatures up to 40 K. These rate coefficients are then used to examine the effect of C6H5CN excitation induced by collisions with para-H2 in molecular clouds by carrying out simple radiative transfer calculations of the excitation temperatures and show that non-equilibrium effects can be expected for H2 densities up to 105-106 cm−3.
The threshold photoionization and dissociative ionization of benzonitrile (C 6 H 5 CN) were studied using double imaging photoelectron photoion coincidence ( i ² PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source...
We report the detection of large hydrocarbon cycles toward several cold dense clouds. We observed four sources (L1495B, Lupus-1A, L483, and L1527) in the Q band (31−50 GHz) using the Yebes 40 m radiotelescope. Using the line stack technique, we find statistically significant evidence of benzonitrile (C 6 H 5 CN) in L1495B, Lupus-1A, and L483 at levels of 31.8 σ , 15.0 σ , and 17.2 σ , respectively, while there is no hint of C 6 H 5 CN in the fourth source, L1527. The column densities derived are in the range (1.7−3.8) × 10 ¹¹ cm ⁻² , which is somewhat below the value derived toward the cold dense cloud TMC-1. When we simultaneously analyze all the benzonitrile abundances derived toward cold clouds in this study and in the literature, a clear trend emerges in that the higher the abundance of HC 7 N, the more abundant C 6 H 5 CN is. This indicates that aromatic cycles are especially favored in those interstellar clouds where long carbon chains are abundant, which suggests that the chemical processes that are responsible for the formation of linear carbon chains are also behind the synthesis of aromatic rings. We also searched for cycles other than benzonitrile, and found evidence of indene (C 9 H 8 ), cyclopentadiene (C 5 H 6 ), and 1-cyano cyclopentadiene (1-C 5 H 5 CN) at levels of 9.3 σ , 7.5 σ , and 8.4 σ , respectively, toward L1495B, which shows the strongest signal from C 6 H 5 CN. The relative abundances between the various cycles detected in L1495B are consistent – within a factor of three – with those previously found in TMC-1. It is therefore likely that not only C 6 H 5 CN but also other large aromatic cycles are abundant in clouds rich in carbon chains.
The 100 m Green Bank Telescope detected ketenimine (CHCNH) in absorption towards the star-forming region Sagittarius B2(N) by means of three rotational transitions: 7–8 at 41.5 GHz, 8–9 at 23.2 GHz and 9–10 at 4.9 GHz. This information was recently brought to light by Atacama Large Millimeter/submillimeter Array (ALMA). Below 50 GHz, the rotational spectrum of ketenimine is sparse. In this context, we present the 1-pyrroline rotational spectra for the same frequency range. For spectroscopic parameter calculations, we used quantum chemistry. The PGOPHER program has been used to replicate the species’ pure rotational spectrum. This molecule’s rotating spectrum makes it a viable candidate for upcoming astronomical detections because the radio lines can be estimated with a high degree of precision in mm/sub-mm wave region.
Context. Energy redistribution after a chemical reaction is one of the few mechanisms that can explain the diffusion and desorption of molecules which require more energy than the thermal energy available in quiescent molecular clouds (10 K). This energy distribution can be important in phosphorous hydrides, elusive yet fundamental molecules for interstellar prebiotic chemistry.
Aims . Our goal with this study is to use state-of-the-art methods to determine the fate of the chemical energy in the simplest phosphorous hydride reaction.
Methods . We studied the reaction dynamics of the P + H → PH reaction on amorphous solid water, a reaction of astrophysical interest, using ab initio molecular dynamics with atomic forces evaluated by a neural network interatomic potential.
Results . We found that the exact nature of the initial phosphorous binding sites is less relevant for the energy dissipation process because the nascent PH molecule rapidly migrates to sites with higher binding energy after the reaction. Non-thermal diffusion and desorption after reaction were observed and occurred early in the dynamics, essentially decoupled from the dissipation of the chemical reaction energy. From an extensive sampling of on-site reactions, we constrained the average dissipated reaction energy within the simulation time (50 ps) to be between 50 and 70%. Most importantly, the fraction of translational energy acquired by the formed molecule was found to be mostly between 1 and 5%.
Conclusions . Including these values, specifically for the test cases of 2% and 5% of translational energy conversion, in astrochemical models, reveals very low gas-phase abundances of PH x molecules and reflects that considering binding energy distributions is paramount to correctly merging microscopic and macroscopic modelling of non-thermal surface astrochemical processes. Finally, we found that PD molecules dissipate more of the reaction energy. This effect can be relevant for the deuterium fractionation and preferential distillation of molecules in the interstellar medium.
In our quest for the presence of large complex molecules containing a majority of carbon in the interstellar medium (ISM), the search for graphene plays a central role due to its nature in making other carbon structures. Although the ingredients for graphene synthesis are present in the ISM, conclusive laboratory evidence of such formation is lacking. Therefore, in our laboratory experiments simulating the cold ISM conditions, we subjected icy mantles of benzonitrile, an aromatic with a cyanide side chain that has recently been detected in the interstellar medium, to vacuum ultraviolet photon irradiation. The irradiated ice was observed to leave a residue upon warming to room temperature. The residue was removed from the substrate and placed on a Quantifoil grid for electron microscopy analysis. Transmission electron microscopy showed quantum dots (QD) and nitrogen-doped graphene (N-Graphene) sheets. Diffraction and energy-dispersive X-ray spectroscopy revealed the crystalline nature and carbon–nitrogen composition, of the observed graphene sheet. This is the first evidence of QD and N-graphene synthesis in ice irradiation at interstellar temperatures.
Benzonitrile ( c -C 6 H 5 CN) has been recently detected in cold and dense regions of the interstellar medium, where it has been used as a signpost of a rich aromatic organic chemistry that might lead to the production of polycyclic aromatic hydrocarbons. One possible origin of this benzonitrile is interstellar ice chemistry involving benzene ( c -C 6 H 6 ) and nitrile molecules (organic molecules containing the −C≡N group). We have addressed the plausibility of this c -C 6 H 5 CN formation pathway through laboratory experiments using our new setup SPACE TIGER. The SPACE TIGER experimental setup is designed to explore the physics and chemistry of interstellar ice mantles using laser-based ice processing and product detection methods. We have found that c -C 6 H 5 CN is formed upon irradiation of c -C 6 H 6 :CH 3 CN binary ice mixtures with 2 keV electrons and Ly α photons at low temperatures (4−10 K). Formation of c -C 6 H 5 CN was also observed when c -C 6 H 6 and CH 3 CN were embedded in a CO ice matrix, but it was efficiently quenched in a H 2 O ice matrix. The results presented in this work imply that interstellar ice chemistry involving benzene and nitrile molecules could contribute to the formation of the observed benzonitrile only if these species are present on top of the ice mantles or embedded in the CO-rich ice layer, instead of being mixed into the H 2 O-rich ice layer.
Valence-bound anions with polar neutral cores can have diffuse dipole-bound excited states just below the electron detachment threshold. Because of the similarity in geometry and vibrational frequencies between the dipole-bound states (DBSs) and the corresponding neutrals, DBSs have been exploited as intermediate states to conduct resonant photoelectron spectroscopy (PES), resulting in highly non-Franck-Condon photoelectron spectra via vibrational autodetachment and providing much richer vibrational information than conventional PES. Here, we report a photodetachment and high-resolution photoelectron imaging study of the 2-cyanopyrrolide anion, cooled in a cryogenic ion trap. The electron affinity of the 2-cyanopyrrolyl radical is measured to be 3.0981 ± 0.0006 eV (24 988 ± 5 cm-1). A DBS is observed for 2-cyanopyrrolide at 240 cm-1 below its detachment threshold using photodetachment spectroscopy. Twenty-three above-threshold vibrational resonances (Feshbach resonances) of the DBS are observed. Resonant PES is conducted at each Feshbach resonance, yielding a wealth of vibrational information about the 2-cyanopyrrolyl radical. Resonant two-photon PES confirms the s-like dipole-bound orbital and reveals a relatively long lifetime of the bound zero-point level of the DBS. Fundamental frequencies for 19 vibrational modes (out of a total of 24) are obtained for the cyanopyrrolyl radical, including six out-of-plane modes. The current work provides important spectroscopic information about 2-cyanopyrrolyl, which should be valuable for the study of this radical in combustion or astronomical environments.
Polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocycles (PANHs) are important and ubiquitous species in space. However, their accurate structural and spectroscopic characterization is often missing. To fill this gap, we exploit the so-called "Lego brick" approach [Melli et al., J. Phys. Chem. A, 2021, 125, 9904] to evaluate accurate rotational constants of some astrochemically relevant PAHs and PANHs. This model is based on the assumption that a molecular system can be seen as formed by smaller fragments for which a very accurate equilibrium structure is available. Within this model, the "template molecule" (TM) approach is employed to account for the modifications occurring when going from the isolated fragment to the molecular system under investigation, with the "linear regression" model being exploited to correct the linkage between different fragments. In the present work, semi-experimental equilibrium structures are used within the TM model. The performance of the "Lego brick" approach has been first tested for a set of small PA(N)Hs for which experimental data are available, thus leading to the conclusion that it is able to provide rotational constants with a relative accuracy well within 0.1%. Subsequently, it has been extended to the accurate prediction of the rotational constants for systems lacking any spectroscopic characterization.
Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C6H4(CN)-) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C6H5(CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C6H4(CN)● radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck-Condon (FC) factors give rise to o-, m-, and p-C6H4(CN)- negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm-1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and p-positions in C6H5(CN) results in calculated gas-phase acidity values (ΔacidH298Ko) of 383.9, 385.7, and 385.3 kcal mol-1, respectively. The calculated ΔacidH298Ko is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol-1. The computed ΔacidH298Ko and EAs are utilized to estimate the bond dissociation energy (DH298(H-C6H4CN)) associated with the formation o-, m-, and p-C6H4(CN)● using the negative ion thermochemical cycle: DH298(C6H5CN) = ΔacidH298Ko (H-C6H4(CN) + EA (C6H5CN)● - IP(H). The respective values of DH298(H-C6H4CN) corresponding to the formation of ortho, meta, and para C6H4(CN) radicals are 114.15, 113.11, and 113.51 kcal mol-1.
We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.
The formation and dissociation mechanisms of polycyclic aromatic hydrocarbons (PAHs) as well as their reactivity with other interstellar molecules are elusive. In this work, we have investigated the electrical discharge chemistry of the PAH naphthalene and acetonitrile, a molecule known to be present in interstellar environments, using a combination of mass-selective IR-UV ion dip spectroscopy with the free electron laser FELIX in the mid-IR frequency region (550 – 1800 cm⁻¹) and quantum chemical calculations. In addition to the species known to be produced in the electrical discharge of pure naphthalene, –CH3 and –CN substituted unsaturated hydrocarbons have been identified. Most of them, in particular those containing a nitrogen atom in the molecular framework, such as benzo[7]annulene‐6‐carbonitrile, have a substantial dipole moment and, therefore, can be considered as potential candidates for radio astronomical searches. Among the species observed, the two isomers 1- and 2-cyanonaphthalene, which have been recently detected in the TMC-1, have been identified in our experiment, thus continuing to highlight the use of electrical discharge sources as a valuable tool to produce astronomically relevant species.
Context. This study reports the index of aromaticity calculated by numerical integration of the magnetically-induced current density for cyclic hydrocarbon molecules both known to exist in astrophysical media as well as those proposed to exist.
Aims. This study promotes the ring current strength (RCS) value for quantifying aromaticity as a means of predicting astrophysical detectability.
Methods. Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVTZ level provide optimized structures and the wave-functions needed to provide the RCS values for the molecules analyzed.
Results. The known interstellar molecules examined c -C 3 H 2 , c -(O)C 3 H 2 , c -C 3 HC 2 H, o -benzyne, benzonitrile, 1-cyano and 2-cyanonaphthalene all have RCS values of 9.9 nA T ⁻¹ (nanoampere per Tesla) or above. The known antiaromatic species have RCS values of less than 0.0 nA T ⁻¹ as expected. Several proposed interstellar molecules likely will not persist if they form due to low RCS values including c -(C)C 3 H 2 . Other species such as p -benzyne and c -HCNN ⁺ have high RCS values of 19.9 nAT ⁻¹ and 14.4nAT ⁻¹ , respectively.
Conclusions. Cyclic hydrocarbons previously observed in astrophysical media have high RCS values. Those with low or negative RCS values have yet to be observed implying that such a metric can indicate astrophysical significance.
Naphthalene (C10H8) is the simplest polycyclic aromatic hydrocarbon (PAH) and an important component in a series of astrochemical reactions involving hydrocarbons. Its molecular charge state affects the stability of its isomeric structures, which is specially relevant in ionised astrophysical environments. We thus perform an extensive computational search for low-energy molecular structures of neutral, singly, and multiply charged naphthalene and its isomers with charge states +q = 0−4 and investigate their geometric properties and bonding situations. We find that isomerisation reactions should be frequent for higher charged states and that open chains dominate their low-energy structures. We compute both the scaled-harmonic and anharmonic infrared spectra of selected low-energy species and provide the calculated scaling factors for the naphthalene neutral, cation, and dication global minima. All simulated spectra reproduce satisfactorily the experimental data and, thus, are adequate for aiding observations. Moreover, the potential presence of these species in the emission spectra of the circumnuclear regions of active galactic nuclei (AGNs), with high energetic X-ray photon fluxes, is explored using the experimental value of the naphthalene photodissociation cross-section, σph − d, to determine its half-life, t1/2, at a photon energy of 2.5 keV in a set of relevant sources. Finally, we show that the computed IR bands of the triply and quadruply charged species are able to reproduce some features of the selected AGN sources.
Radicals are abundant in a range of important gas-phase environments. They are prevalent in the atmosphere, in interstellar space, and in combustion processes. As such, understanding how radicals react is essential for the development of accurate models of the complex chemistry occurring in these gas-phase environments. By controlling the properties of the colliding reactants, we can also gain insights into how radical reactions occur on a fundamental level. Recent years have seen remarkable advances in the breadth of experimental methods successfully applied to the study of reaction dynamics involving paramagnetic species-from improvements to the well-known crossed molecular beams approach to newer techniques involving magnetically guided and decelerated beams. Coupled with ever-improving theoretical methods, quantum features are being observed and interesting insights into reaction dynamics are being uncovered in an increasingly diverse range of systems. In this highlight article, we explore some of the exciting recent developments in the study of chemical dynamics involving paramagnetic species. We focus on low-energy reactive collisions involving neutral radical species, where the reaction parameters are controlled. We conclude by identifying some of the limitations of current methods and exploring possible new directions for the field.
We report the detection of two isomers of ethynyl cyclopentadiene ( c -C 5 H 5 CCH), namely 1- and 2-ethynyl-1,3-cyclopentadiene, in the direction of TMC-1. We derive column densities of (1.4 ± 0.2) × 10 ¹² cm ⁻² and (2.0 ± 0.4) × 10 ¹² cm ⁻² , respectively, for these two cyclopentadiene derivatives, which imply that they are about ten times less abundant than cyclopentadiene. We also report the tentative detection of ethynyl benzene (C 6 H 5 CCH), for which we estimate a column density of (2.5 ± 0.4) × 10 ¹² cm ⁻² . We derived abundances for the corresponding cyano derivatives of cyclopentadiene and benzene and found values significantly lower than previously reported. The rotational temperature of the ethynyl and cyano derivatives of these cycles is about 9 K, that is, very close to the gas kinetic temperature of the cloud. The abundance ratio of the 1- and 2-isomers of ethynyl cyclopentadiene is 1.4 ± 0.5, while for the two isomers of cyano cyclopentadiene it is 2.4 ± 0.6. The relative abundances of CCH over CN derivatives is 7.7 ± 2.2 for cyclopentadiene, which probably reflects the abundance ratio of the radicals CCH and CN; this ratio is only 2.1 ± 0.5 for benzene, which suggests that additional reactions besides cyano radicals with benzene are involved in the formation of benzonitrile. The formation of these cycles is reasonably well accounted for through a chemical scheme based on neutral-neutral reactions. It is predicted that benzene should be as abundant as cyclopentadiene in TMC-1.
We report the first detection in interstellar space of the 3-cyano propargyl radical (CH 2 C 3 N). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. A total of seven rotational transitions for both ortho-and para-CH 2 C 3 N species were observed in the 31.0-50.4 GHz range. We derive a total column density of (1.6±0.4)×10 11 cm −2 and an ortho/para ratio of 2.4±1.2, which implies an abundance ratio CH 2 C 3 N/CH 3 C 3 N ∼ 0.1, in sharp contrast with the smaller analogues, in which case CH 2 CN/CH 3 CN ∼ 3. This indicates that the chemistry of the cyanides CH 2 C 3 N and CH 3 C 3 N behaves differently to that of the smaller analogues CH 2 CN and CH 3 CN. According to our chemical model calculations, the radical CH 2 C 3 N is mostly formed through the neutral-neutral reactions C + CH 2 CHCN, C 2 + CH 3 CN, and CN + CH 2 CCH together with the dissociative recombination of the CH 3 C 3 NH + ion with electrons. The neutral-neutral reaction N + C 4 H 3 could also lead to CH 2 C 3 N, although its role is highly uncertain. The identified radical CH 2 C 3 N could play a role in the synthesis of large organic N-bearing molecules, such as benzonitrile (c-C 6 H 5 CN) or nitrogen heterocycles.
Context. The chemical pathways linking the small organic molecules commonly observed in molecular clouds to the large, complex, polycyclic species long suspected of being carriers of the ubiquitous unidentified infrared emission bands remain unclear.
Aims. To investigate whether the formation of mono- and polycyclic molecules observed in cold cores could form via the bottom-up reaction of ubiquitous carbon-chain species with, for example, atomic hydrogen, a search is made for possible intermediates in data taken as part of the GOTHAM (GBT Observations of TMC-1: Hunting for Aromatic Molecules) project.
Methods. Markov chain Monte Carlo (MCMC) source models were run to obtain column densities and excitation temperatures. Astrochemical models were run to examine possible formation routes, including (a) a novel grain-surface pathway involving the hydrogenation of C 6 N and HC 6 N, (b) purely gas-phase reactions between C 3 N and both propyne (CH 3 CCH) and allene (CH 2 CCH 2 ), and (c) via the reaction CN + H 2 CCCHCCH.
Results. We report the first detection of cyanoacetyleneallene (H 2 CCCHC 3 N) in space toward the TMC-1 cold cloud using the Robert C. Byrd 100 m Green Bank Telescope. Cyanoacetyleneallene may represent an intermediate between less-saturated carbon chains, such as the cyanopolyynes, that are characteristic of cold cores and the more recently discovered cyclic species, such as cyanocyclopentadiene. Results from our models show that the gas-phase allene-based formation route in particular produces abundances of H 2 CCCHC 3 N that match the column density of 2 × 10 ¹¹ cm ⁻² obtained from the MCMC source model, and that the grain-surface route yields large abundances on ices that could potentially be important as precursors for cyclic molecules.
The recent wave of detections of interstellar aromatic molecules has sparked interest in the chemical behavior of aromatic molecules under astrophysical conditions. In most cases, these detections have been made through chemically related molecules, called proxies, that implicitly indicate the presence of a parent molecule. In this study, we present the results of the theoretical evaluation of the hydrogenation reactions of different aromatic molecules (benzene, pyridine, pyrrole, furan, thiophene, silabenzene, and phosphorine). The viability of these reactions allows us to evaluate the resilience of these molecules to the most important reducing agent in the interstellar medium, the hydrogen atom (H). All significant reactions are exothermic and most of them present activation barriers, which are, in several cases, overcome by quantum tunneling. Instanton reaction rate constants are provided between 50 K and 500 K. For the most efficiently formed radicals, a second hydrogenation step has been studied. We propose that hydrogenated derivatives of furan, pyrrole, and specially 2,3-dihydropyrrole, 2,5-dihydropyrrole, 2,3-dihydrofuran, and 2,5-dihydrofuran are promising candidates for future interstellar detections.
The photochemical processes at work in the atmosphere of Titan are very complex and lead to a great variety of compounds with aerosols as an end-product. One of the most complex molecules detected so far is benzene (C6H6). In the present work, we have updated and improved the chemistry of aromatics in order to better understand the main chemical pathways leading to the production of benzene and determine what other aromatics could be produced efficiently in the atmosphere. This new chemical scheme has been incorporated in our 1D photochemical model corresponding to mean conditions. We confirm the importance of ionic chemistry for benzene production in the upper atmosphere and we have found that excited benzene is an important intermediate in benzene production due to the exothermicity of many production reactions. Among the 24 aromatics included in the model, neutral aromatics like toluene (C6H5CH3) and ethylbenzene (C6H5C2H5) are relatively abundant, suggesting in particular that toluene could be detectable in the infrared, and eventually microwave wavelength ranges. However, we obtained large uncertainties on model results highlighting the need for more experiments and theoretical studies to improve the chemistry of aromatics.
A specific interstellar aromatic molecule
Aromatic molecules such as polycyclic aromatic hydrocarbons (PAHs) are known to exist in the interstellar medium owing to their characteristic infrared emission features. However, the infrared emission only indicates the general class of molecule, and identifying which specific molecular species are present is difficult. McGuire et al. used radio astronomy to detect rotational transitions of benzonitrile emitted from a well-known nearby cloud of interstellar gas (see the Perspective by Joblin and Cernicharo). This molecule may be a precursor to more complex PAHs. The identification of benzonitrile sheds light on the composition of aromatic material within the interstellar medium—material that will eventually be incorporated into new stars and planets.
Science , this issue p. 202 ; see also p. 156
Chemical models used to study the chemical composition of the gas and the
ices in the interstellar medium are based on a network of chemical reactions
and associated rate coefficients. These reactions and rate coefficients are
partially compiled from data in the literature, when available. We present in
this paper kida.uva.2014, a new updated version of the kida.uva public
gas-phase network first released in 2012. In addition to a description of the
many specific updates, we illustrate changes in the predicted abundances of
molecules for cold dense cloud conditions as compared with the results of the
previous version of our network, kida.uva.2011.
We report on the detection with the Infrared Space Observatory (ISO), for the first time in the circumstellar medium, of the polyacetylenic chains C4H2 and C6H2 and of benzene (C6H6) in the direction of the proto-planetary nebula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C2H2. Benzene is 40 times less abundant than acetylene. We suggest that the chemistry in CRL 618 has been strongly modified by the UV photons coming from the hot central star and by the shocks associated with its high-velocity winds. All the infrared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense torus that surrounds the central star. C4H2 and C6H2 have also been detected in CRL 2688, so it seems that C-rich proto-planetary nebulae are the best organic chemistry factories in space.
The chemical reaction dynamics to form cyanobenzene C6H5CN(X 1A1), and perdeutero cyanobenzene C6D5CN(X 1A1) via the neutral-neutral reaction of the cyano radical CN(X 2Sigma+), with benzene C6H6(X 1A1g) and perdeutero benzene C6D6(X 1A1g), were investigated in crossed molecular beam experiments at collision energies between 19.5 and 34.4 kJ mol-1. The laboratory angular distributions and time-of-flight spectra of the products were recorded at mass to charge ratios m/e=103-98 and 108-98, respectively. Forward-convolution fitting of our experimental data together with electronic structure calculations (B3LYP/6-311+G**) indicate that the reaction is without entrance barrier and governed by an initial attack of the CN radical on the carbon side to the aromatic pi electron density of the benzene molecule to form a Cs symmetric C6H6CN(C6D6CN) complex. At all collision energies, the center-of-mass angular distributions are forward-backward symmetric and peak at pi/2. This shape documents that the decomposing intermediate has a lifetime longer than its rotational period. The H/D atom is emitted almost perpendicular to the C6H5CN plane, giving preferentially sideways scattering. This experimental finding can be rationalized in light of the electronic structure calculations depicting a H-C-C angle of 101.2° in the exit transition state. The latter is found to be tight and located about 32.8 kJ mol-1 above the products. Our experimentally determined reaction exothermicity of 80-95 kJ mol-1 is in good agreement with the theoretically calculated one of 94.6 kJ mol-1. Neither the C6H6CN adduct nor the stable iso cyanobenzene isomer C6H5NC were found to contribute to the scattering signal. The experimental identification of cyanobenzene gives a strong background for the title reaction to be included with more confidence in reaction networks modeling the chemistry in dark, molecular clouds, outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as Saturn's moon Titan. This reaction might further present a barrierless route to the formation of heteropolycyclic aromatic hydrocarbons via cyanobenzene in these extraterrestrial environments as well as hydrocarbon rich flames.
Using data obtained by the DIRBE instrument on the COBE spacecraft, we present the mean 3.5-240 μm spectrum of high-latitude dust. Combined with a spectrum obtained by the FIRAS instrument, these data represent the most comprehensive wavelength coverage of dust in the diffuse interstellar medium, spanning the 3.5-1000 μm wavelength regime. At wavelengths shorter than ~60 μm the spectrum shows an excess of emission over that expected from dust heated by the local interstellar radiation field and radiating at an equilibrium temperature. The DIRBE data thus extend the observations of this excess, first detected by the IRAS satellite at 25 and 12 μm, to shorter wavelengths. The excess emission arises from very small dust particles undergoing temperature fluctuations. However, the 3.5-4.9 μm intensity ratio cannot be reproduced by very small silicate or graphite grains. The DIRBE data strongly suggest that the 3.5-12 μm emission is produced by carriers of the ubiquitous 3.3, 6.2, 7.7, 8.6, and 11.3 μm solid state emission features that have been detected in a wide variety of astrophysical objects. The carriers of these features have been widely identified with polycyclic aromatic hydrocarbons (PAHs).
Our dust model consists of a mixture of PAH molecules and bare astronomical silicate and graphite grains with optical properties given by Draine & Lee. We obtain a very good fit to the DIRBE spectrum, deriving the size distribution, abundances relative to the total hydrogen column density, and relative contribution of each dust component to the observed IR emission. At wavelengths above 140 μm the model is dominated by emission from T ≈ 17-20 K graphite and 15-18 K silicate grains. The model provides a good fit to the FIRAS spectrum in the 140-500 μm wavelength regime but leaves an excess Galactic emission component at 500-1000 μm. The nature of this component is still unresolved.
We find that (C/H) is equal to (7.3 ± 2.2) × 10⁻⁵ for PAHs and equal to (2.5 ± 0.8) × 10⁻⁴ for graphite grains, requiring about 20% of the cosmic abundance of carbon to be locked up in PAHs, and about 70% in graphite grains [we adopt (C/H)☉ = 3.6 × 10⁻⁴]. The model also requires all of the available magnesium, silicon, and iron to be locked up in silicates. The power emitted by PAHs is 1.6 × 10⁻³¹ W per H atom, by graphite grains 3.0 × 10⁻³¹ W per H atom, and by silicates 1.4 × 10⁻³¹ W per H atom, adding up to a total infrared intensity of 6.0 × 10⁻³¹ W per H atom, or ~2 L☉M.
The [C II] 158 μm line emission detected by the FIRAS provides important information on the gas phase abundance of carbon in the diffuse ISM. The 158 μm line arises predominantly from the cold neutral medium (CNM) and shows that for typical CNM densities and temperatures C⁺/H = (0.5-1.0) × 10⁻⁴, which is ~14%-28% of the cosmic carbon abundance. The remaining carbon abundance in the CNM, which must be locked up in dust, is about equal to that required to provide the observed IR emission, consistent with notion that most (75%) of this emission arises from the neutral component of the diffuse ISM.
The model provides a good fit to the general interstellar extinction curve. However, at UV wavelengths it predicts a larger extinction. The excess extinction may be the result of the UV properties adopted for the PAHs. If real, the excess UV extinction may be accounted for by changes in the relative abundances of PAHs and carriers of the 2200 Å extinction bump.
In the Python world, NumPy arrays are the standard representation for numerical data and enable efficient implementation of numerical computations in a high-level language. As this effort shows, NumPy performance can be improved through three techniques: vectorizing calculations, avoiding copying data in memory, and minimizing operation counts.
Carbon-rich evolved stars from the asymptotic giant branch to the planetary
nebula phase are characterized by a rich and complex carbon chemistry in their
circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC
11, whose Spitzer-IRS spectrum shows remarkable and diverse molecular
absorption bands. To study how the molecular composition in this object
compares to our current understanding of circumstellar carbon chemistry, we
modeled this molecular absorption. We find high abundances for a number of
molecules, perhaps most notably benzene. We also confirm the presence of
propyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is
evident; we can detect at best a marginal presence of HCN. From comparisons to
various chemical models, we can conclude that SMP LMC 11 must have an unusual
circumstellar environment (a torus rather than an outflow).
Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C(2)H + H(2)CCHCHCH(2) → C(6)H(6) + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.
The technique of excimer laser excitation/Lyman alpha H atom laser induced fluorescence was used to investigate the formation of H atoms from the 248 nm photoexcitation of benzene and toluene. The H atom signal dependence on laser excitation energy demonstrated that it is produced from two photon photolysis of the aromatics; absorption of the first photon populates the bound (1)B(2u) level followed by absorption from this level to a dissociative level, which produces H atoms, among other potential channels. Analysis of the data yields the second photon absorption cross section to produce H and is equal to 1.0 and 5.2x10(-19) cm(2) for benzene and toluene, respectively. In addition, the yield of H atoms was observed to be pressure dependent. This is because at sufficiently high pressures the nanosecond lifetime of the (1)B(2u) state can be pressure quenched and hence may compete with the absorption of the second photon. The yields of H atoms were determined as a function of pressure for a range of the laser energies and with various collider gases. The analysis of these data allowed the total absorption cross section for the second photon to be determined and is equal to 2.8 and 1.7x10(-17) cm(2) for benzene and toluene, respectively. In addition, the rate constants for quenching (1)B(2u) with various gases (He, Ar, N(2), and O(2)) were determined. This large absorption coefficient for the second photon implies that with a pulsed laser source of 248 nm it is difficult to avoid aromatic photodissociation. We highlight a few previous studies that may need to be reevaluated in the light of the results from this study.
We have observed an evolved star with a rare combination of spectral features, MSX SMC 029, in the Small Magellanic Cloud (SMC) using the low-resolution modules of the Infrared Spectrograph on the Spitzer Space Telescope. A cool dust continuum dominates the spectrum of MSX SMC 029. The spectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs) and absorption at 13.7 micron from C2H2, a juxtaposition seen in only two other sources, AFGL 2688 and IRAS 13416-6243, both post-asymptotic giant branch (AGB) objects. As in these sources, the PAH spectrum has the unusual trait that the peak emission in the 7-9 micron complex lies beyond 8.0 micron. In addition, the 8.6 micron feature has an intensity as strong as the C-C modes which normally peak between 7.7 and 7.9 micron. The relative flux of the feature at 11.3 micron to that at 8 micron suggests that the PAHs in MSX SMC 029 either have a low ionization fraction or are largely unprocessed. The 13-16 micron wavelength region shows strong absorption features similar to those observed in the post-AGB objects AFGL 618 and SMP LMC 11. This broad absorption may arise from the same molecules which have been identified in those sources: C2H2, C4H2, HC3N, and C6H6. The similarities between MSX SMC 029, AFGL 2688, and AFGL 618 lead us to conclude that MSX SMC 029 has evolved off the AGB in only the past few hundred years, making it the third post-AGB object identified in the SMC. Comment: 4 figures, Fig. 4 color; to appear in the 20 November 2006 Astrophysical Journal Letters
Methanol (CH3OH) is considered by astronomers to be the simplest complex organic molecule (COM) and has been detected in various astrophysical environments, including protoplanetary disks, comets, and the interstellar medium (ISM). Studying the reactivity of methanol at low temperatures will aid our understanding of the formation of other complex, and potentially prebiotic molecules. A major destruction route for many neutral COMs, including methanol, is via their reactions with radicals such as CN, which is ubiquitous in space. Here, we study the kinetics of the reaction between methanol and the CN radical using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with Pulsed Laser Photolysis – Laser-Induced Fluorescence (PLP-LIF). Electronic structure calculations were also performed to identify the exothermic channels through which this reaction can proceed. Our results for the rate coefficient, are represented by the modified Arrhenius equation, k(T) = 1.26 × 10⁻¹¹(T / 300 K)−0.7exp(−5.4 K / T) and display a negative temperature dependence over the temperature range 16.7—296 K, which is typical of what has been seen previously for other radical-neutral reactions that do not possess potential energy barriers. The rate coefficients obtained at room temperature strongly disagree with a previous kinetics study, which is currently available in the Kinetics Database for Astrochemistry (KIDA) and therefore used in some astrochemical models.
The field of astrochemistry concerns the formation and abundance of molecules in the interstellar medium, star-forming regions, exoplanets and solar system bodies. These astrophysical objects contain the chemical material from which new planets and solar systems are formed. Around 200 molecules have thus far been observed in the interstellar medium; almost half containing six or more atoms and considered ``complex'' by astronomical standards. All of these complex molecules consist of at least one carbon atom and thus the term complex organic molecules (COMs) has been coined by the astrochemical community. In order to understand the formation and destruction of these COMs under the extreme conditions of star-forming regions, three kinds of activity are involved: (1) the astronomical identification of complex molecules present in the ISM; (2) the construction of astrochemical models that attempt to explain the formation routes of the observed molecules; and (3) laboratory measurements and theoretical calculations of critical kinetic parameters that are included in the models. In the following review, we present recent laboratory efforts to produce quantitative kinetic data for gas-phase reactions at low temperatures. We discuss the use of the CRESU technique, a French acronym standing for \textit{Cin\'{e}tique de R\'{e}action en Ecoulement Supersonique Uniforme}, which means reaction kinetics in uniform supersonic flow, to measure reactions of astrochemical importance. In particular, we highlight recent and future advances in the measurement of product-specific reaction kinetics at low temperatures.
The recent astronomical detection of benzonitrile (C6H5CN) in the cold, starless cloud TMC-1 demonstrates that aromatic chemistry is efficient even in the primordial stages of star and planet formation. C6H5CN may serve as a convenient observational proxy for benzene, which is otherwise challenging to detect in space, provided the chemistry linking these two molecules is tightly constrained. Here we present a high-resolution microwave spectroscopic study in combination with a highly accurate thermochemical treatment of the formation chemistry of C6H5CN and benzene. We demonstrate that C6H5CN is a highly useful tracer for benzene in the presence of CN radical, either in space or in the laboratory, and by inference, that the reaction C2H + CH2(CH)2CH2 yields benzene, along with its high-energy polar isomer fulvene. By exploiting -CN tagging, formation pathways to produce benzene using a variety of acyclic hydrocarbon precursors are then explored. A robust, self-consistent, and chemically accurate theoretical treatment has also been undertaken for several key reactions. The results are discussed both in the context of aromatic molecule synthesis and astrochemistry.
A new chirped-pulse/uniform flow (CPUF) spectrometer has been developed and used to determine product branching in a multichannel reaction. With this technique, bimolecular reactions can be initiated in a cold, thermalized, high-density molecular flow and a broadband microwave spectrum acquired for all products with rotational transitions within a chosen frequency window. In this work, the CN + CH3CCH reaction was found to yield HCN via a direct H-abstraction reaction, whereas indirect addition/ elimination pathways to HCCCN, CH3CCCN, and CH2CCHCN were also probed. From these observations, quantitative branching ratios were established for all products as 12(5)%, 66(4)%, 22(6)%, and 0(8)% into HCN, HCCCN, CH3CCCN, and CH2CCHCN, respectively. The values are consistent with statistical calculations based on new ab initio results at the CBS-QB3 level of theory. This work is a demonstration of CPUF as a powerful technique for quantitatively determining the branching into polyatomic products from a bimolecular reaction.
Bimolecular reactions of phenyl-type radicals with the C4 and C5 hydrocarbons vinylacetylene and (methyl-substituted) 1,3-butadiene have been found to synthesize polycyclic aromatic hydrocarbons (PAHs) with naphthalene and 1,4-dihydronaphthalene cores in exoergic and entrance barrierless reactions under single-collision conditions. The reaction mechanism involves the initial formation of a van der Waals complex and addition of a phenyl-type radical to the C1 position of a vinyl-type group through a submerged barrier. Investigations suggest that in the hydrocarbon reactant, the vinyl-type group must be in conjugation with a -C≡CH or -HC=CH2 group to form a resonantly stabilized free radical intermediate, which eventually isomerizes to a cyclic intermediate followed by hydrogen loss and aromatization (PAH formation). The vinylacetylene-mediated formation of PAHs might be expanded to more complex PAHs, such as anthracene and phenanthrene, in cold molecular clouds via barrierless reactions involving phenyl-type radicals, such as naphthyl, which cannot be accounted for by the classical hydrogen abstraction-acetylene addition mechanism. Expected final online publication date for the Annual Review of Physical Chemistry Volume 66 is March 31, 2015. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.
Many materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains, we analyze the observed aromatic (3.28, 6.2 μm) and aliphatic (3.4 μm) hydrocarbon absorption features in the diffuse ISM along the line of sight toward the Galactic center Quintuplet Cluster. Observationally, sp
2 bonds can be measured in astronomical spectra using the 6.2 μm CC aromatic stretch feature, whereas the 3.4 μm aliphatic feature can be used to quantify the fraction of sp
3 bonds. The fractional abundance of these components allows us to place the Galactic diffuse ISM hydrocarbons on a ternary phase diagram. We conclude that the Galactic hydrocarbon dust has, on average, a low H/C ratio and sp
3 content and is highly aromatic. We have placed the results of our analysis within the context of the evolution of carbon dust in the ISM. We argue that interstellar carbon dust consists of a large core of aromatic carbon surrounded by a thin mantle of hydrogenated amorphous carbon (a-C:H), a structure that is a natural consequence of the processing of stardust grains in the ISM.
Using a continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus, rate coefficients have been measured for the reactions of the cyanogen (CN) and ethynyl (C2H) radicals with allene (CH2CCH2) and methyl acetylene (CH3CCH) at temperatures from 295 down to 15 K for the reactions of CN and down to 63 K for those of C2H. All four reactions occur at rates close to the collision-determined limit. The results are compared with those obtained earlier for the reactions of other alkenes and alkynes, and, in the accompanying Letter by Vakhtin et al., with results for C2H+CH2CCH2 and C2H+CH3CCH obtained at 103 K using a pulsed Laval apparatus. The implications of these latest results for the chemistry of interstellar clouds and planetary atmospheres are discussed.
The association of benzene molecules to form dimers has been studied at temperatures between 15 and 123 K in a CRESU (Cinétique de Réactions en Ecoulement Supersonique Uniforme) apparatus with helium as the buffer gas. Second-order rate coefficients (k2) for the formation of dimers have been determined for temperatures between 15 and 88 K. The effective third-order rate coefficients (no study of fall off behavior has been undertaken) obtained by dividing the values of k2 by [He] show a strong negative temperature dependence. Further evidence for the formation of dimers at critical concentrations of benzene has been obtained from a study of the CH+C6H6 reaction and from LIF spectra. Modelling calculations indicate that any systematic errors in the results arising from the formation of higher oligomers are small. Based on a model in which it is assumed that the benzene molecules in the dimer are free to rotate, third-order rate coefficients have been calculated for dimerization and are in good agreement with the experimental values for T>=30 K, where effects of falloff should not be too great.
A KrF excimer laser (248 nm) is used to dissociate a low pressure (5–10 mTorr) sample of cyanogen iodide (ICN) and the resulting CN X 2Σ+ fragments are probed by laser‐induced fluorescence (LIF) via various vibrational sequences of the B 2Σ+–X 2Σ+ transition. In addition to measuring relative vibrational and rotational populations in the CN X 2Σ+ photofragment, the alignment of rotational angular momentum in this fragment is determined from the variation in LIF intensity as the direction of linear polarization of the probe laser is rotated with respect to that of the photolysis laser. A unifying model is proposed for the continuum photodissociation dynamics which is able to account for present and previous experimental measurements characterizing both the I and CN photofragments.
Flash photolysis of gaseous benzene has been carried out with a KrF laser (248.4 nm) as an excitation source. Two new transients have been detected, one of which has a structured absorption in the wavelength region from 235 to 260 nm. It has a rise time of 80 ns and a long decay time (≳25 μs). The precursor of this transient has also been detected. It appears as a shoulder at 225 nm and has a lifetime of 80 ns. This species is postulated to be a highly excited vibrational state of the ground electronic state, namely, a hot benzene molecule. This species is quenched by ground state benzene as well as by 12 different foreign gases, including oxygen and nitrogen, which have similar quenching rates. The decay of the hot benzene is accompanied by an increase in the temperature of the sample, which leads in turn to formation of the species with the structured absorption. The temperature of the equilibrated sample was determined by comparing the thermalized spectra in the nanosecond time scale with the steady-state absorption spectra at higher temperatures. The temperature rises by about 50° when 20 Torr of benzene are excited with an energy of 33 mJ/cm2. This temperature increase indicates that the yield of hot benzene is 0.6±0.2. The Tn←T1 absorption spectrum has been measured and the intersystem crossing yield is estimated to be 0.2±0.1. The major nonradiative channel in benzene is internal conversion.
An entirely new experimental method is described which enables the rate constants of neutral–neutral gas‐phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser‐induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well‐matched by the expression k1(T)=(2.49±0.17)×10−11 (T/298)(−0.63±0.04) cm3 molecule−1 s−1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10−11 (T/298)(−1.14±0.15) cm3 molecule−1 s−1. The kinetic data are discussed in terms of the latest quantum chemical and reaction rate theories for these systems.
Infrared-ultraviolet double resonance (IRUVDR) experiments have been
implemented in the ultra-cold environment provided by a CRESU
(Cinétique de Réaction en Ecoulement Supersonique
Uniforme) apparatus. With this technique rate coefficients of two kinds
have been measured for rotational energy transfer in collisions between
NO and He, Ar and N2: (a) rate coefficients for total removal
from specific states of NO(X 2Π1/2 v=3; J=0.5,
3.5 or 6.5) and (b) state-to-state rate coefficients for rotational
energy transfer from these levels to specific final states. Using
different Laval nozzles, results have been obtained at several different
temperatures: for He as collision partner, 295, 149, 63, 27, 15 and 7 K;
for Ar, 139, 53, 44 and 27 K; and for N2, 86 and 47 K. The
thermally averaged cross-sections for total removal show remarkably
little variation, either with temperature or with initial rotational
state. The variation of state-to-state rate coefficients with ΔJ
shows three general features: (i) a decrease with increasing ΔJ
(ii) a propensity to favor even ΔJ transitions over odd ΔJ
changes; and (iii) at lower temperatures, decreases in J are
increasingly favored over increases in J and the distribution of rate
coefficients against ΔJ becomes narrower. The experimental rate
coefficients for collisions with He and Ar are compared with those from
both close coupled and coupled states calculations based on potential
energy surfaces determined within the coupled electron pair
approximation (CEPA) with a large atomic orbital basis set. The
agreement between theory and experiment of both the total and the
state-to-state rate coefficients is excellent over the complete range of
temperatures covered in the experiments.
The electronic absorption spectra of the cyanogen halides, XCN where X = Cl, Br, and I, have been investigated in the range 3,100-1,050 {angstrom}. The spectra are analyzed in terms of vibronic structure, oscillator strengths, and effective quantum numbers. The spectra of the cyanogen halides exhibit no regular Rydberg structure. The absence of regularity is shown to be a direct consequence of the presence of intravalence excitations and Rydberg/valence interactions. In confirmation of the above, the intravalence transitions arising from the 2{pi} {yields} (5{sigma}, 3{pi}, and 6{sigma}) configurational excitations are observed and assigned.
We show that the physical conditions in CRL 618 are such that efficient formation of benzene, C6H6, occurs. A combination of high temperatures, high densities, and high ionization rates drives an efficient ion-molecule chemistry involving condensation reactions of acetylene and its derivatives, rather than reactions involving atomic hydrogen, as was suggested for the interstellar synthesis of benzene. We find a column density of benzene within a factor of 2 of that observed providing that the material is trapped in a long-lived reservoir of gas in the disk around CRL 618. We note that the chemistry can give rise to other carbon chain molecules as well as a large abundance of benzonitrile, C6H5CN.
Crossed molecular beam experiments of cyano radicals, CN(X 2Σ+, ν = 0), in their electronic and vibrational ground state reacting with unsaturated hydrocarbons acetylene, C2H2(X 1Σ), ethylene, C2H4(X 1Ag), methylacetylene, CH3CCH(X 1A1), allene, H2CCCH2(X 1A1), dimethylacetylene, CH3CCCH3(X 1A1'), and benzene, C6H6 (X 1A1g), were performed at relative collision energies between 13.3 and 36.4 kJ mol-1 to unravel the formation of unsaturated nitriles in the outflows of late-type AGB carbon stars and molecular clouds. In all reactions, the CN radical was found to attack the π electron density of the hydrocarbon molecule with the radical center located at the carbon atom; the formation of an initial addition complex is a prevalent pathway on all the involved potential energy surfaces. A subsequent carbon-hydrogen bond rupture yields the nitriles cyanoacetylene, HCCCN (X 1Σ+), vinylcyanide, C2H3CN (X 1A'), 1-methylcyanoacetylene, CH3CCCN (X 1A1), cyanoallene, H2CCCH(CN) (X 1A'), 3-methylcyanoacetylene, HCCCH2CN(X 1A'), 1,1-cyanomethylallene, H2CCC(CN)(CH3) (X 1A'), and cyanobenzene, C6H5CN (X 1A1). In case of acetylene and ethylene, a second reaction channel involves a [1, 2]-H atom shift in the initial HCCHCN and H2CCH2CN collision complexes prior to a hydrogen atom release to form cyanoacetylene, HCCCN (X 1Σ+), and vinylcyanide, C2H3CN (X 1A'). Since all these radical-neutral reactions show no entrance barriers, have exit barriers well below the energy of the reactant molecules, and are exothermic, the explicit identification of this CN versus H atom exchange pathway under single collision conditions makes this reaction class a compelling candidate to synthesize unsaturated nitriles in interstellar environments holding temperatures as low as 10 K. This general concept makes it even feasible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in the interstellar medium. Here HCCCN, C2H3CN, and CH3CCCN have been already observed; since CH3CCH is the common precursor to H2CCCH(CN)/CH3CCCN and the latter isomer has been assigned unambiguously toward TMC-1 and OMC-1, H2CCCH(CN) is strongly expected to be present in both clouds as well. The formation of isonitrile isomers was not observed in our experiments. Since all reactions to HCCNC, C2H3NC, CH3CCNC, H2CCCH(NC), H2CCC(NC)(CH3), and C6H5NC are either endothermic or the exit barrier is well above the energy of the reactants, neutral-neutral reactions of cyano radicals with closed shell unsaturated hydrocarbons cannot synthesize isonitriles in cold molecular clouds. However, in outflow of carbon stars, the enhanced translational energy of both reactants close to the photosphere of the central star can compensate this endothermicity, and isonitriles might be formed in these hotter environments as well.
This paper describes the solution of several problems associated with the fitting and prediction of vibration-rotation spectra with multiple spin interactions. In order to regularize arithmetic with complex operators, a modified Wang basis function is proposed which has the property of making all operators which are even powers of angular momentum pure real and all odd powers pure imaginary. Next, a generalized direction cosine operator is described, which can be calculated in a Wang basis using a spherical tensor formalism. Finally, the problem of assigning quantum numbers is addressed for the case when there are more than two interacting states. A robust algorithm for assignment and sorting of eigenvectors is presented.
Density functional theory calculations at the B3LYP/6-31+G∗∗ level were employed to characterize the critical points for adducts, isomers, products, and intervening transition states for the reactions between benzene and the ethynyl (C2H) or cyano (CN) radicals. Both addition reactions were found to have no barriers in their entrance channels, making them efficient at the low temperature and pressure conditions that prevail in the haze-forming region of Titan’s atmosphere as well as in the dense interstellar medium (ISM). The dominant products are ethynylbenzene (C6H5C2H) and cyanobenzene (C6H5CN). Hydrogen abstraction reactions were also characterized but found to be non-competitive. Trajectory calculations based on potentials fit to about 600 points calculated at the ROMP2/6-31+G∗∗ level for each interaction surface were used to determine reaction rates. The rates incorporated any necessary corrections for back reactions as ascertained from a multiwell treatment used to determine outcome distributions over the range of temperatures and pressures pertinent to Titan and the ISM and are in good agreement with the limited available experimental data.
Crossed molecular beam experiments of ground state cyano radicals, CN(X2Σ+), with hydrocarbons acetylene (C2H2), ethylene (C2H4), methylacetylene (CH3CCH), allene (H2CCCH2), dimethylacetylene (CH3CCCH3), and benzene (C6H6,) were performed to investigate the formation of unsaturated nitriles in Titan’s atmosphere. These radical–neutral reactions have no entrance barrier, depict an exit barrier well below the energy of the reactant molecules, and are all exothermic. The CN radical attacks the π electron density at the olefine, alkyne, and aromatic molecules; the formation of an initial addition complex is a common pathway on the involved potential energy surfaces for all reactions. A subsequent carbon–hydrogen bond rupture yields the unsaturated nitriles HCCCN, C2H3CN, CH3CCCN, H2CCCH(CN), H2CCCH2CN, and C6H5CN as detected in our experiments. The explicit identification of this CN vs H atom exchange pathway under single collision, makes this reaction-class a compelling candidate to synthesize unsaturated nitriles in Titan’s atmosphere. This versatile concept makes it even possible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in Titan. Here, the C2H2 as well as cyanoacetylene, HCCCN, have been already identified unambiguously in Titan’s troposphere. Those nitriles as sampled in our crossed beam experiments resemble an ideal challenge to be detected in the framework of the NASA–ESA Cassini–Huygens mission to Titan.
Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9-4.9) x 10(-10) cm(3) molecule(-1) s(-1). These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 x 10(-10) cm(3) molecule(-1) s(-1) at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C(6)H(5)CN) is the only product recorded with no detectable evidence for a C(6)H(5) + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC(6)H(4)CH(3)) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the approximately 40 meV photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C(6)H(5)CH(2)) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn's moon Titan ( approximately 100 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).
The specific action of UV on the reversion of the ochre allele cycl-9, in which 21 out of 23 revertants have been shown to arise from A-T-to-G.C transitions at position one in the UAA codon, was found to depend on the function of the RAD6 gene, since cycl-9 reversion occurred by a variety of single-base-pair substitutions in a strain carrying the rad6-1 allele.
Polycyclic aromatic hydrocarbons (PAHs) have long been postulated as constituents of the interstellar gas and circumstellar disks. Observational infrared emission spectra have been plausibly interpreted in support of this hypothesis, but the small (or zero) dipole moments of planar, unsubstituted PAHs preclude their definitive radio astronomical identification. Polar PAHs, such as corannulene, thus represent important targets for radio astronomy because they offer the possibilities of confirming the existence of PAHs in space and revealing new insight into the chemistry of the interstellar medium. Toward this objective, the high-resolution rotational spectrum of corannulene has been obtained by Fourier transform microwave spectroscopy, and the dipole moment (2.07 D) of this exceptionally polar PAH has been measured by exploiting the Stark effect.
The first direct measurement of the reaction rate constant of a polycyclic aromatic hydrocarbon in the gas phase in the temperature range 58-470 K is reported. The reaction is OH+ anthracene and the experiment has been performed in a continuous flow Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus, which had to be modified for this purpose. Pulsed laser photolysis of H(2)O(2) has been used to generate OH radicals and laser-induced fluorescence to observe the kinetic decay of the radicals and hence determine the rate coefficients. The reaction is found to be fast, and the rate constant increases monotonically as the temperature is lowered. The rate coefficients match the expression k(cm(3) molecules(-1) s(-1))=1.12 x 10(-10)(T/300)(-0.46).
The reaction of the C2H radical with benzene is studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A2Delta) chemiluminescence from the C2H + O2 reaction. Measurements at very low photolysis energy are performed using CF3C2H as the C2H precursor to study the influence of benzene photodissociation on the rate coefficient. Rate coefficients are obtained over a temperature range between 105 and 298 K. The average rate coefficient is found to be five times greater than the estimated value presently used in the photochemical modeling of Titan's atmosphere. The reaction exhibits a slight negative temperature dependence which can be fitted to the expression k(cm3 molecule(-1) s(-1)) = 3.28(+/-1.0) x 10(-10) (T/298)(-0.18(+/-0.18)). The results show that this reaction has no barrier and may play an important role in the formation of large molecules and aerosols at low temperature. Our results are consistent with the formation of a short lifetime intermediate that decomposes to give the final products.
The rate coefficient of the reaction of the methylidine radical CH with anthracene has been studied over a wide temperature range (58-470 K) in a dedicated "Cinétique de Réaction en Ecoulement Supersonique Uniforme" (Reaction Kinetics in Uniform Supersonic Flow) apparatus. The reaction exhibits a slight positive temperature dependence, which can be fitted to the expression k(T) = (3.32 +/- 1.00) x 10(-10)(T/298)((0.46+/-0.14)) cm3 molecule(-1) s(-1). Even at the lowest temperature, the reaction remains very fast indicating that the kinetics are probably driven by a capture process.
New developments and recent applications of pulsed and miniaturised Laval nozzle technology allowing many gas-phase molecular processes to be studied at very low temperatures are highlighted. In the present Minireview we focus on molecular energy transfer and reactions of molecular radicals (e.g. OH) with neutral molecules. We show that with the combination of pulsed laser photolysis and sensitive laser-induced fluorescence detection a large number of fast reactions of radicals with more or less complex neutral molecules can be measured in Laval nozzle expansions nowadays. It is also demonstrated that collisional energy transfer of neutral molecules can be measured via kinetically controlled selective fluorescence (KCSF) excitation down to 58 Kelvin. Finally, we show that even the primary steps in the oxidation of biomolecules or biomolecular building blocks initiated by OH radicals can be followed at low temperatures. The temperature dependence of the measured rate constants is the key for an understanding of the underlying molecular mechanisms and the Laval nozzle expansion provides a unique environment for these measurements. The experimental finding that many reactions between radicals and neutral species can be rapid at low temperatures are discussed in terms of pre-reactive complexes formed in the overall complex forming bimolecular reactions.
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