Article

# Astronomical searches for nitrogen heterocycles

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## Abstract

We have conducted extensive astronomical searches for the N-bearing ring molecules pyridine, quinoline and isoquinoline towards the circumstellar envelopes of carbon-rich stars, and for interstellar pyrimidine in hot molecular cores. Here we report the derived upper limits on the column densities of these molecules, and summarize the current status of these observations.

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... It has been suggested that polycyclic aromatic nitrogen heterocycles (PANHs) are present in a wide variety of space and planetary settings, including the galactic interstellar medium (ISM) and extragalactic objects ( Bernstein et al. 2005;Charnley et al. 2005;Bauschlicher et al. 2009), and they have also been identified in carbonaceous globules in meteorites (Saito & Kimura 2009;Bonnet et al. 2015). PANHs have been predicted to be major components of the atmosphere and the surface of Titan ( Imanaka et al. 2004;McCord et al. 2006;Quirico et al. 2008;Kim & Kaiser 2009;Landera & Mebel 2010) within the category of material referred to as tholins, which are believed to be present across the outer solar system and responsible for the varying reddish hues of bodies such as Titan, Callisto, Pluto, and Charon with the exact composition of the tholin determining the exact color (Dalle Ore et al. 2011;Cruikshank et al. 2015). ...
... It has been observed previously that the infrared spectra of the PAH anthracene and the PANH acridine, despite the obvious differences in atomic structure, molecular, and crystal symmetry, are functionally indistinguishable (Fialkovskaya & Nefedov 1968;Langhoff et al. 1998;Hudgins et al. 2005). Although additional small peaks are observed in the spectra of acridine owing to symmetry splitting in the C-C stretching and C-H bending regions, these are small and difficult to observe without a high signal-to-noise source such as synchrotron infrared light ( Langhoff et al. 1998;Charnley et al. 2005). Comprehensive comparative vibrational assignments have been made of both molecular crystals (Fialkovskaya & Nefedov 1968;Brigodiot & Lebas 1972). ...
... Owing to the use of synchrotron-source infrared with a high signal-to-noise ratio, these weak acridine vibrations are observable in our data, but would not be detectable using a laboratory source or during observational infrared spectroscopy. The spectroscopic difficulties of identifying PANHs in the ISM have been summarized in other works ( Charnley et al. 2005;Boersma et al. 2013). Changes to band positions indicate differences in atomic positions, often relating to the breaking of molecular symmetry. ...
Article
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The influence of polycyclic aromatic nitrogen heterocycles (PANHs), which have been suggested as contributors to the interstellar IR emission bands, on interstellar emission features is difficult to constrain because their infrared characteristics are strongly similar to those for polycyclic aromatic hydrocarbons (PAHs). One possible solution is to seek a means of visualizing the presence of PANHs that provides information that is distinct from that for PAHs. Although PANHs and PAHs have similar infrared characteristics in many settings, this relationship may not be universally maintained. We have used in situ high-pressure synchrotron-source Fourier transform infrared spectroscopy to determine that the responses of two representative molecules, acridine and anthracene, differ at high pressures (>ca. 1 GPa). Because there are a number of high-pressure environments that can be remotely observed by infrared spectroscopy, they represent a potential to glimpse the distribution of PANHs across the cosmos. © 2016. The American Astronomical Society. All rights reserved.
... If not shielded, lifetimes are proposed to be only hours, as N-heterocycles are highly susceptible to UV photolysis. 7 Searches for N-heterocycles in a variety of interstellar environments have been unsuccessful: pyridine, quinoline, and isoquinoline have been searched for in circumstellar shells, 8 pyrimidine in hot molecular cores, 8,9 malonitrile, quinoline, isoquinoline, and pyrimidine in the molecular cloud TMC-1, 10 and uracil in molecular cloud Sgr B2(N). 11 These non-detections suggest that if N-heterocycles are produced in these regions, they are depleted rapidly by deposition or further chemical processing. ...
... If not shielded, lifetimes are proposed to be only hours, as N-heterocycles are highly susceptible to UV photolysis. 7 Searches for N-heterocycles in a variety of interstellar environments have been unsuccessful: pyridine, quinoline, and isoquinoline have been searched for in circumstellar shells, 8 pyrimidine in hot molecular cores, 8,9 malonitrile, quinoline, isoquinoline, and pyrimidine in the molecular cloud TMC-1, 10 and uracil in molecular cloud Sgr B2(N). 11 These non-detections suggest that if N-heterocycles are produced in these regions, they are depleted rapidly by deposition or further chemical processing. ...
Article
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A fundamental question in the field of astrochemistry is whether the molecules essential to life originated in the interstellar medium (ISM) and, if so, how they were formed. Nitrogen-containing heterocycles are of particular interest because of their role in biology, but to date no N-heterocycle has been detected in the ISM and it is unclear how and where such species might form. Recently, the β-cyanovinyl radical (HCCHCN) was implicated in the low temperature gas-phase formation of pyridine. While neutral vinyl cyanide (H2CCHCN) has been rotationally characterized and detected in the ISM, HCCHCN has not. Here we present the first theoretical study of all three cyanovinyl isomers at the CCSD(T)/ANO1 level of theory and the experimental rotational spectra of cis-and trans-HCCHCN, as well as those of their ¹⁵N isotopologues, from 5 to 75 GHz. The observed spectra are in good agreement with calculations, and provide a basis for further laboratory and astronomical investigations of these radicals.
... If HCN is also present, it can substitute for one or more acetylene molecules during the polymerization process, resulting in the formation of N-heterocycles (Frenklach and Feigelson, 1989;Ricca et al., 2001;Hamid et al., 2014). Despite the plausibility of their formation, to date no small aromatic N-heterocycles have been unambiguously observed in the gas phase in space (Simon and Simon, 1973;Kuan et al., 2004;Charnley et al., 2005;Brünken et al., 2006). ...
Article
Nucleobases are the informational subunits of RNA and DNA and are essential to all known forms of life. The nucleobases can be divided into two groups of molecules: the pyrimidine-based compounds that include uracil, cytosine, and thymine, and the purine-based compounds that include adenine and guanine. Previous work in our laboratory has demonstrated that uracil, cytosine, thymine, and other nonbiological, less common nucleobases can form abiotically from the UV photoirradiation of pyrimidine in simple astrophysical ice analogues containing combinations of H2O, NH3, and CH4. In this work, we focused on the UV photoirradiation of purine mixed with combinations of H2O and NH3 ices to determine whether or not the full complement of biological nucleobases can be formed abiotically under astrophysical conditions. Room-temperature analyses of the resulting photoproducts resulted in the detection of adenine, guanine, and numerous other functionalized purine derivatives. Key Words: Pyrimidine-Nucleobases-Interstellar; Ices-Cometary; Ices-Molecular processes-Prebiotic chemistry. Astrobiology 17, xxx-xxx.
... × 10 14 cm −2 could be derived from these observations [71]. More generally, no N-heterocyclic molecules have been unequivocally detected so far in the ISM [72], although there are observational indications that they may be present [73]. However, purines have been found in a large number of carbonaceous chondrites including Murchison, Murray, and Orgueil [74 -77]. ...
... Only upper limits of pyrimidines, pyridine, quinoline, and isoquinolines were detected in the gas phase of astrophysical environments [67][68][69][70], which may be explained by their low stability against UV radiation in these environments [71,72]. Laboratory work has demonstrated that uracil, cytosine, and thymine may be formed by UV photoirradiation of astrophysical ice analogues containing pyrimidine [73][74][75][76]. ...
Article
Full-text available
Carbonaceous chondrites are very primitive meteorites that are rich in carbon. They contain many soluble organic compounds, including nitrogen heterocycles. These play a crucial role in present-day living organisms as they are components of the genetic material and of the co-factors of enzymes. This review outlines the nitrogen heterocycle content of carbonaceous meteorites. The potential mechanisms of formation of these molecules are also described. Measurements of the compound-specific carbon and hydrogen isotopic compositions are mentioned as a way of establishing the origin of the nitrogen heterocycles detected in meteorites.
... Additionally, although purine was not specifically identified, it was demonstrated that N-heterocycles can also form from the ultraviolet (UV) irradiation of small homocyclic aromatic hydrocarbons (e.g., benzene, naphthalene) in ices containing H 2 O and NH 3 (Materese et al. 2015). To date, no small N-heterocycles have been firmly identified in the gas phase in space (Simon & Simon 1973;Kuan et al. 2004;Charnley et al. 2005;Brünken et al. 2006), but several of them have been detected in a number of carbonaceous chondrites including Murchison, Murray, Orgueil, Allan Hills 83100, Lewis Cliff 90500, and Lonewolf Nunataks 94102 ( Folsome et al. 1971;Hayatsu et al. 1975;Stoks & Schwartz 1979;Callahan et al. 2011). Isotopic measurements taken for some of these compounds suggest an extraterrestrial origin rather than terrestrial contamination (Martins et al. 2008). ...
Article
The aromatic nitrogen heterocyclic compound purine is the core structural framework of many important biomolecules, particularly nucleobases. Purine and purine derivatives have been observed in carbonaceous chondrites, and it has been hypothesized that the exogenous delivery of these compounds, along with many other biologically relevant compounds, may have played a role in the emergence of life. Numerous experiments in our laboratory have demonstrated that the nucleobases used by life to encode genetic material could have been produced abiotically under astrophysically relevant conditions. Specifically, the UV photoprocessing of pyrimidine and purine in simple ices of astrophysical interest has resulted in the production of all five biological nucleobases, namely, uracil (RNA), cytosine (RNA and DNA), thymine (DNA), adenine (RNA and DNA), and guanine (RNA and DNA). Additionally, follow-up work has examined the photochemistry of pyrimidine in more complex astrophysical ice mixtures to better understand the formation of these compounds under realistic conditions. In this work, we examine the photochemistry of purine in more complex ices of astrophysical interest and compare our results with those from simpler ice mixtures. We also examine the effects of competing parallel synthesis of organic compounds in the ices (unrelated to purine). Finally, we discuss the astrophysical and astrobiological implications of our findings. © 2018. The American Astronomical Society. All rights reserved.
... It may be possible that the alkylated N-containing cyclic compounds could be formed by alkyl radical addition to parental N-cyclic compounds (e.g., pyridine), in interstellar molecular clouds under very cold conditions. However, the pyridine molecule has not been found in spite of extensive astronomical searches for Nbearing ring molecules (Charnley et al., 2005). Alternatively, alkylated N-heterocycles could be produced via hexamethylenetetramine (HMT), which is a major product of photochemical reactions of interstellar ice analogs containing C1 molecules (CO, CH 4 or CH 3 OH) and ammonia (e.g., Bernstein et al. 1995;Oba et al., 2017), but HMT has not been detected in meteorites. ...
Article
Very complex mixtures of organic compounds occur in extraterrestrial materials such as carbonaceous meteorites. These intricate signatures of meteoritic organic matter can provide clues to elucidate chemical evolution processes in space. Previously, these complex organic molecules have not been well-resolved in primitive meteorites, so the formation mechanisms of extraterrestrial organics remain largely conjectural. In this study, the occurrence and abundance of soluble CHN organic compounds were examined in CM vs. CR meteorites to investigate possible chemical processes associated with the different parent bodies. Hydrogenated alkylpiperidines (CnH2n+1N) are more abundant in the CR chondrite, in contrast to more abundant aromatic alkylpyridines (CnH2n–5N) in the CM chondrites. Both alkylpyridines and alkylpiperidines are most likely synthesized from simple aldehydes and ammonia on meteorite parent bodies, but the differences between the distribution of N-cyclic compounds are consistent with different redox conditions of the parent bodies which influenced the organic molecular evolution processes in extraterrestrial materials.
... Polycyclic aromatic nitrogen heterocycles (PANHs) are species with one or more CH groups substituted by a nitrogen atom in PAHs [5]. Since nitrogen is the fourth most abundant element in space, it has been proposed that PANHs should also be present in the ISM [2]. ...
Article
Stability of naphthalene and two of its nitrogen containing derivatives namely, quinoline and isoquinoline are investigated under high energy electron impact. Astro-biologically important statistical dissociation channels are probed using the delayed extraction time-of-flight mass spectrometry technique. The experimental configuration was designed to avoid yield variation due to the thermal velocities for 5 μs or more after the collision. The statistical decay channels show an increase in the yield of daughter ions as a function of extraction delay in the range of 10⁶ s⁻¹. On the other hand, the yields due to fast dissociation channels were observed to decrease as a function of extraction delay. On the basis of projectile beam energy dependence of the yields, the effect of plasmon excitation in quinoline and isoquinoline is shown for the first time. A strong dependence of the statistical dissociation yield on the location of the nitrogen atom in the two molecules is observed. An attempt is made to correlate these observations with the various structure parameters obtained using structure calculations.
... However, despite the extensive search for such N-heterocycles in astrophysical environments in the gas phase, they have not been found to date, 117−120 and only an upper limit for the abundance of pyrimidine could be derived from these observations. 119 Nevertheless, because of their abundant presence in meteorites, several mechanisms for their formation in astrophysical environments have been proposed, both experimentally and theoretically. One proposed mechanism involves a pathway similar to that for the formation of PAHs, i.e., the condensation of three acetylene (C 2 H 2 ) molecules in the gas phase, 515,516 in which one or several of these C 2 H 2 have been replaced with HCN or HNC. ...
Article
Despite the generally hostile nature of the environments involved, chemistry does occur in space. Molecules are seen in environments that span a wide range of physical and chemical conditions and that clearly were created by a multitude of chemical processes, many of which differ substantially from those associated with traditional equilibrium chemistry. The wide range of environmental conditions and processes involved with chemistry in space yields complex populations of materials, and because the elements H, C, O, and N are among the most abundant in the universe, many of these are organic in nature, including some of direct astrobiological interest. Much of this chemistry occurs in "dense" interstellar clouds and protostellar disks surrounding forming stars because these environments have higher relative densities and more benign radiation fields than in stellar ejectae or the diffuse interstellar medium. Because these are the environments in which new planetary systems form, some of the chemical species made in these environments are expected to be delivered to the surfaces of planets where they can potentially play key roles in the origin of life. Because these chemical processes are universal and should occur in these environments wherever they are found, this implies that some of the starting materials for life are likely to be widely distributed throughout the universe.
... 26−30 Additionally, these nitriles are acyclic isomers of the aromatic heterocycle, pyridine, which has yet to be identified in the ISM. 31 Detection of any of these nitriles would provide additional motivation to detect pyridine and other aromatic heterocycles. The existence of organic nitriles in the ISM is relevant not only to our understanding of the chemical processes in the ISM but also to our understanding of the origin of amino acids, nucleotides, and other prebiotic compounds that are critical for life on Earth. ...
Article
Four cyanobutadiene isomers of considerable interest to the organic chemistry, molecular spectroscopy, and astrochemistry communities were synthesized in good yields and isolated as pure compounds: (E)-1-cyano-1,3-butadiene (E-1), (Z)-1-cyano-1,3-butadiene (Z-1), 4-cyano-1,2-butadiene (2), and 2-cyano-1,3-butadiene (3). A diastereoselective synthesis was developed to generate (E)-1-cyano-1,3-butadiene (1) (10:1 E/Z) via tandem SN2 and E2' reactions. The potential energy surfaces of the E2' reactions leading to (E)- and (Z)-1-cyano-1,3-butadiene (1) were analyzed by density functional theory calculations, and the observed diastereoselectivity was rationalized in the context of the Curtin-Hammett principle. The preparation of pure samples of these reactive compounds enables measurement of their laboratory rotational spectra, which are the critical data needed to search for these species in space by radioastronomy.
... However, H 2 O ice has also a detrimental effect on the formation of methyl-bearing pyrimidine derivatives in general, and thymine in particular, as the presence of OH radicals will favor oxidation of pyrimidine and its derivatives, and inhibit their methylation. Our calculations were compared to recent experiments of UV irradiation of pyrimidine in ices containing H 2 O and a methyl group donor, either CH 3 OH or CH 4 . In these experiments, the formation of thymine was shown to be inefficient, occurring only when ices were subjected to a photon dose higher than the dose needed to form detectable amounts of uracil and cytosine under the same conditions. ...
Article
Nucleobases are the carriers of the genetic information in ribonucleic acid and deoxyribonucleic acid (DNA) for all life on Earth. Their presence in meteorites clearly indicates that compounds of biological importance can form via non-biological processes in extraterrestrial environments. Recent experimental studies have shown that the pyrimidine-based nucleobases uracil and cytosine can be easily formed from the ultravioletirradiation of pyrimidine in H2O-rich ice mixtures that simulate astrophysical processes. In contrast, thymine, which is found only in DNA, is more difficult to form under the same experimental conditions, as its formation usually requires a higher photon dose. Earlier quantum chemical studies confirmed that the reaction pathways were favorable provided that several H2O molecules surrounded the reactants. However, the present quantum chemical study shows that the formation of thymine is limited because of the inefficiency of the methylation of pyrimidine and its oxidized derivatives in an H2O ice, as supported by the laboratory studies. Our results constrain the formation of thymine in astrophysical environments and thus the inventory of organic molecules delivered to the early Earth and have implications for the role of thymine and DNA in the origin of life.
... 3,4,[13][14][15][16][17][18][19] Because of their abundance on Earth, these nucleobases and their building blocks such as Pym have been intensively searched for in extraterrestrial media to understand their possible delivery to Earth. [20][21][22][23] Although such investigations have failed so far, the presence of these aromatic N-heterocycles in the interstellar medium has received widespread acceptance. [24][25][26][27][28][29][30][31][32] For example, the 6.2 mm feature in the unidentified infrared emission spectrum was suggested to originate from polycyclic aromatic hydrocarbons with at least one N atom in the aromatic skeleton. ...
Article
Full-text available
Pyrimidine (Pym, 1,3-diazine, 1,3-diazabenzene) is an important N-heterocyclic building block of nucleobases. Understanding the structures of its fragment and precursor ions provides insight into its prebiotic and abiotic synthetic route. The long-standing controversial debate about the structures of the primary fragment ions of the Pym+ cation (C4H4N2+, m/z 80) resulting from loss of HCN, C3H3N+ (m/z 53), is closed herein with the aid of a combined approach utilizing infrared photodissociation (IRPD) spectroscopy in the CH and NH stretch ranges (νCH/NH) and density functional theory (DFT) calculations. IRPD spectra of cold Ar/N2-tagged fragment ions reveal that the C3H3N+ population is dominated by cis-/trans-HCCHNCH+ ions (~90%) along with a minor contribution of the most stable H2CCCNH+ and cis-/trans-HCCHCNH+ isomers (~10%). We also spectroscopically confirm that the secondary fragment resulting from further loss of HCN, C2H2+ (m/z 26), is the acetylene cation (HCCH+). The spectroscopic characterization of the identified C3H3N+ isomers and their hydrogen-bonded dimers with Ar and N2 provides insight into the acidity of their CH and NH groups. Finally, the vibrational properties of Pym+ in the 3 μm range are probed by IRPD of Pym+-(N2)1-2 clusters, which shows a high π-binding affinity of Pym+ toward a nonpolar hydrophobic ligand. Its νCH spectrum confirms the different acidity of the three nonequivalent CH groups.
... Although several plausible mechanisms for their formation have been proposed, no N-heterocycles have been unequivocally detected in astronomical environments to date via the detection of their rotation lines, and only upper limits have been placed on their abundances in the gas phase (Simon & Simon 1973;Kuan et al. 2004;Charnley et al. 2005;Brünken et al. 2006). Peeters et al. (2005) showed that small N-heterocyles are much more susceptible to photodestruction than their homocyclic counterparts, which could explain a lower abundance of these molecules in astrophysical environments. ...
Article
Aromatic heterocyclic molecules are an important class of molecules of astrophysical and biological significance that include pyridine, pyrimidine, and their derivatives. Such compounds are believed to exist in interstellar and circumstellar environments, though they have never been observed in the gas phase. Regardless of their presence in the gas phase in space, numerous heterocycles have been reported in carbonaceous meteorites, which indicates that they are formed under astrophysical conditions. The experimental work described here shows that N- and O-heterocyclic molecules can form from the ultraviolet (UV) irradiation of the homocyclic aromatic molecules benzene (C6H6) or naphthalene (C10H8) mixed in ices containing H2O and NH3. This represents an alternative way to generate aromatic heterocycles to those considered before and may have important implications for astrochemistry and astrobiology.
... However, the aromatic molecule benzene has been observed in circumstellar environments [13,14] and is considered as a building block for PAH formation. Spectral searches for other small aromatic building blocks, such as pyridine and pyrimidine have been unsuccessful up to this day [15,16]. Recently, the radio-astronomical detection of the aromatic molecule benzonitrile in the interstellar medium rekindled the interest in a possible link between small nitrogen containing organic molecules and larger nitrogen containing (polycyclic) aro-matic hydrocarbons (N-PAHs) [17]. ...
Article
Full-text available
Cold ion action spectroscopy is applied to record the gas phase infrared fingerprint spectra of mass-selected C5H5N•+ and C5H5NH⁺ cations using an infrared free electron laser. The structures of the cations are deduced from the experimental spectra using anharmonic vibrational frequencies from density functional theory calculations. A very good agreement between experimental and theoretical infrared frequencies is observed. The dominant structure of the C5H5N•+ cation is assigned to the aromatic pyridine radical cation form. Additionally, a minor contribution of the lower energetic α-distonic isomer is observed. The C5H5NH⁺ cation is ascribed to the aromatic pyridinium cation where protonation has taken place on the nitrogen atom. The rare-gas tag, used in the action spectroscopic method, has a negligible effect on the vibrational frequencies. The observed species with now accurately determined vibrational frequencies are good candidates for future rotational spectroscopic studies and infrared observations in astronomical sources such as interstellar clouds or Titan’s atmosphere.
... The structures of these molecules are simple enough that it is possible to search for them via their rotational transitions. However, searches to date have yielded only negative results [pyrrole: Kutner et al. (1980), furan: Dickens et al. (2001), imidazole: Irvine et al. (1981), pyrimidine: Charnley et al. (2005)]. ...
Article
Full-text available
Recent observational and experimental evidence for the presence of complex organics in space is reviewed. Remote astronomical observations have detected $$\sim$$200 gas-phased molecules through their rotational and vibrational transitions. Many classes of organic molecules are represented in this list, including some precursors to biological molecules. A number of unidentified spectral phenomena observed in the interstellar medium are likely to have originated from complex organics. The observations of these features in distant galaxies suggests that organic synthesis had already taken place during the early epochs of the Universe. In the Solar System, almost all biologically relevant molecules can be found in the soluble component of carbonaceous meteorites. Complex organics of mixed aromatic and aliphatic structures are present in the insoluble component of meteorites. Hydrocarbons cover much of the surface of the planetary satellite Titan and complex organics are found in comets and interplanetary dust particles. The possibility that the early Solar System, or even the early Earth, have been enriched by interstellar organics is discussed.
... Following the expected presence of PANHs in the ISM, astronomical searches for pyridine, pyrimidine, quinoline, and isoquinoline were conducted (Kuan et al. 2003;Charnley et al. 2005); however, no positive assignments were made. The upper limit on the column density for pyridine and quinoline in the post-AGB object GL 618 is about 3 × 10 13 cm −2 for an adopted temperature of 200 K. Furthermore, PANHs might be subject to photo destruction by ultraviolet radiation, which was recently demonstrated experimentally (Peeters et al. 2005). ...
Article
Full-text available
Meteorites contain bio-relevant molecules such as vitamins and nucleobases, which consist of aromatic structures with embedded nitrogen atoms. Questions remain over the chemical mechanisms responsible for the formation of nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) in extraterrestrial environments. By exploiting single collision conditions, we show that a radical mediated bimolecular collision between pyridyl radicals and 1,3-butadiene in the gas phase forms nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) 1,4-dihydroquinoline and to a minor amount 1,4-dihydroisoquinoline. The reaction proceeds through the formation of a van der Waals complex, which circumnavigates the entrance barrier implying it can operate at very low kinetic energy and therefore at low temperatures of 10 K as present in cold molecular clouds such as TMC-1. The discovery of facile de facto barrierless exoergic reaction mechanisms leading to PANH formation could play an important role in providing a population of aromatic structures upon which further photo-processing of ice condensates could occur to form nucleobases.
... The neutral quinoline molecule has been extensively studied computationally [2][3][4][5][6][7][8][9] and experimentally in the gas phase by measuring the dielectric constant to determine the dipole moment [10], photoionisation [11][12][13], UV-Vis absorbance [14][15][16], microwave [17] and infrared [18] spectroscopy. Quinoline has also general astrophysical interest [19][20][21]. Nevertheless, few experimental negative ion gas-phase studies exist. ...
Article
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We report a joint photoelectron spectroscopic and theoretical study on the molecular anion, quinoline-. Analysis of the vibrationally resolved photoelectron spectrum found the adiabatic electron affinity, EAa(C9H7N), to be 0.16 ± 0.05 eV. These findings were supported by density functional theory calculations. Our experimental and computational results demonstrate the unusual electrophilicity for a polycyclic aromatic heterocycle.
... Among them, pyridine (C 5 H 5 N) and pyrimidine (C 4 H 4 N 2 ) are prototypical aromatic molecules, in which the benzene ring is substituted by one and two nitrogen atoms, respectively. These molecules have also been the subject of intense radioastronomical surveys towards the interstellar-medium (ISM), although their detection was unsuccessful [1]. ...
Conference Paper
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Ion desorption measurements induced by the impact of electrons onto pyridine and pyrimidine ices will be presented. The data reported here is of relevance for chemical evolution models of astrophysical objects.
Article
The ability to selectively control of chemical reactions related to biology, combustion, and catalysis has recently attracted much attention. In particular, the hydrogen atom relocation may be used to manipulate bond-breaking and new bond-forming processes and may hold promise for far-reaching applications. Thus, the hydrogen atom migration preceding fragmentation of the gas-phase pyridine molecules by the H+, H2+, He+, He++ and O+ impact has been studied experimentally in the energy range of 5-2000 eV using collision-induced luminescence spectroscopy. Formation of the excited NH(A3Π) radicals was observed among the atomic and diatomic fragments. The structure of the pyridine molecule is lacking of the NH group, therefore observation of its A3Π→X3Σ‾ emission bands is an evidence of the hydrogen atom relocation prior to the cation-induced fragmentation. The NH(A3Π) emission yields indicate that formation of the NH radicals depends on the type of selected projectile and can be controlled by tuning its velocity. The plausible collisional mechanisms as well as fragmentation channels for NH formation in pyridine are discussed.
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Two homologous series of alkylpyridines (CnH2n-5N and CnH2n-7N) were identified in the methanol extract of the Murchison meteorite by liquid chromatography/high-resolution mass spectrometry. The wide range of saturated- and unsaturated-alkylated (C1 to C21) pyridines is more diverse than previously found, and could be produced by reactions of aldehyde condensation and aldehydes with NH3 by imine formation in Solar or pre-solar environments. This finding implies a high aldehyde activity under alkaline conditions in the presence of ammonia during the chemical evolution of soluble organic matters detected in a CM2-type carbonaceous meteorite.
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We present fast proton impact induced fragmentations of pyrimidine and pyridazine as an experimental resource to investigate isomeric signatures. Major isomeric imprints are identified for few fragment ions and differences of more than an order of magnitude for the cross sections of fragments of the same mass were measured. The observation of the molecular structure of these isomers gives no apparent indication for the reasons for such substantial differences. It is verified that the simple displacement of the position of one nitrogen atom strongly inhibits or favors the production of some ionic fragment species. The dependency of the fragmentation cross sections on the proton impact energy, investigated by means of time of flight mass spectroscopy and of a model calculation based in first order perturbation theory, allows us to disentangle the complex collision dynamics of the ionic fragments. The proton-induced fragmentation discriminates rather directly the association between a molecular orbital ionization and the fragment-ions creation and abundance, as well as how the redistribution of the energy imparted to the molecules takes place, triggering not only single but also double vacancy and leads to specific fragmentation pathways.
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We report the synthesis of complex organic compounds including nicotinic and quinolinic acid, two members involved in the nicotinamide adenine dinucleotide (NAD) biosynthetic pathway, in irradiated astrophysical ice analogs. If delivered to Earth by meteorites and comets, these compounds may have contributed to the origin and early evolution of life.
Chapter
The interstellar medium (ISM) is a complex physical and chemical environment which gives rise to a variety of spectral features. This chapter gives a brief overview on the diversity of chemical species that are observed in diffuse interstellar clouds, tenuous entities of gas, and dust scattered throughout the galaxy and beyond. The growing evidence for the existence of large organic, prebiotic molecules in these astrophysical environments is discussed.
Article
Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium, yet the formation mechanisms of even their simplest prototypes—quinoline and isoquinoline—remain elusive. Here, we reveal a novel concept that under high temperature conditions representing circumstellar envelopes of carbon stars, (iso)quinoline can be synthesized via the reaction of pyridyl radicals with two acetylene molecules. The facile gas phase formation of (iso)quinoline in circumstellar envelopes defines a hitherto elusive reaction class synthesizing aromatic structures with embedded nitrogen atoms that are essential building blocks in contemporary biological-structural motifs. Once ejected from circumstellar shells and incorporated into icy interstellar grains in cold molecular clouds, these NPAHs can be functionalized by photo processing forming nucleobase-type structures as sampled in the Murchison meteorite.
Article
The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6-benzoquinoline and 7,8-benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48-87 GHz) and Fourier Transform Microwave (FT-MW) spectroscopy (2-26 GHz) of a supersonic rotationally cold molecular jet. Guided by theoretical molecular orbital predictions, spectral analysis of mm-wave spectra, and higher resolution FT-MW spectroscopy provided accurate rotational and centrifugal distortion constants together with (14)N nuclear quadrupole coupling constants for both species. The tricyclic frames of these species undergo low energy out-of-plane zero-point vibrations resulting in deviations from the moments of inertia that the rigid structure would exhibit. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent in general for planar species with a number of pronounced out-of-plane vibrations. The resultant constants for the benzoquinolines are of sufficient accuracy to be used in astrophysical searches for planar aromatic heterocycles.
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Polycyclic aromatic hydrocarbons (PAHs) are highly relevant for astrophysics as possible, though controversial, carriers of the unidentified infrared emission bands that are observed in a number of different astronomical objects. In support of radio-astronomical observations, high resolution laboratory spectroscopy has already provided the rotational spectra in the vibrational ground state of several molecules of this type, although the rotational study of their dense infrared (IR) bands has only recently become possible using a limited number of experimental set-ups. To date, all of the rotationally resolved data have concerned unperturbed spectra. We presently report the results of a high resolution study of the three lowest vibrational states of quinoline C9H7N, an N-bearing naphthalene derivative. While the pure rotational ground state spectrum of quinoline is unperturbed, severe complications appear in the spectra of the ν 45 and ν 44 vibrational modes (located at about 168 cm−1 and 178 cm−1, respectively). In order to study these effects in detail, we employed three different and complementary experimental techniques: Fourier-transform microwave spectroscopy, millimeter-wave spectroscopy, and Fourier-transform far-infrared spectroscopy with a synchrotron radiation source. Due to the high density of states in the IR spectra of molecules as large as PAHs, perturbations in the rotational spectra of excited states should be ubiquitous. Our study identifies for the first time this effect and provides some insights into an appropriate treatment of such perturbations.
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The matrix-isolated, mid-infrared spectra of seven acridine-based polycyclic aromatic nitrogen heterocycles (PANHs) have been measured and compared to their non-nitrogen containing parent molecule. The acridine species investigated include acridine, benz[a]acridine, benz[c]acridine, dibenz[a,j]acridine, dibenz[c,h]acridine, dibenz[a,h]acridine and dibenz[a,c]acridine. The previously reported results for 1 and 2-azabenz[a]anthracenes are included for comparison. The experimentally determined band frequencies and intensities are compared with their B3LYP/6–31G(d) values. The overall agreement between experimental and theoretical values is good and in line with our previous investigations. Shifts, typically to the blue, are noted for the C–H out-of-plane (CHoop) motions upon insertion of a nitrogen atom. The formation of a bay region upon addition of additional benzene rings to the anthracene/acridine structure splits the solo hydrogen motions into a bay region solo and an external solo hydrogen, with the bay region solo hydrogen coupling to the quartet hydrogen motions and the external solo hydrogen coupling with the duo hydrogen motions resulting in an extreme decrease in intensity for the CHoop solo hydrogen band when the external hydrogen is replaced by a nitrogen atom. The C–C and C–H in-plane region of this acridine series exhibits the characteristic two fold increase in intensity, noted previously for PANHs. The strong ≈1400 cm −1 band, which was identified in the previous PANH study, is noted in several molecular species as well as another strong PANH feature between 1480 and 1515 cm −1 for several molecules. The presence of these strong bands appear to be primarily responsible for the two-fold increase in the CH-inplane region's (1100–1600 cm −1) intensity. The C–H stretching region can be characterized by contributions from the solo (bay or external), duo and quartet hydrogens, similar to what was observed in the dibenzopolyacene compounds.
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The rotational spectrum of cyanopyrazine (2-pyrazinecarbonitrile, p-C4H3N2-CN) has been obtained from 130 to 500 GHz. Rotational transitions of cyanopyrazine have been measured, assigned, and least-squares fit for the first time. Over 7000 transitions of the ground vibrational state have been least-squares fit to partial octic, A- and S-reduced Hamiltonians with low statistical uncertainty (σfit = 34 kHz). Similar to other cyanoarenes, the first two fundamental modes are the out-of-plane (ν27, A′′) and in-plane (ν19, A′) nitrile bending modes, which form an a- and b-axis Coriolis-coupled dyad. Greater than 5800 transitions from each of these vibrational modes were fit to a partial octic, A-reduced Hamiltonian (σfit = 38 kHz), and the analysis reveals the precise energy separation, ΔE27,19, between the coupled vibrational states, as well as values for eight a- and b-type Coriolis-coupling coefficients, Ga, GaJ, GaK, GaJJ, FbcK, Gb, GbJ, and Fac. Cyanopyrazine is a strongly polar derivative of pyrazine, thus cyanopyrazine can serve as a potential tracer molecule for its nonpolar parent compound in extraterrestrial environments. The transition frequencies and spectroscopic constants provided in this work, combined with theoretical or experimental nuclear quadrupole coupling constants, provide the foundation for future radioastronomical searches for cyanopyrazine.
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We have quantum chemically analyzed how the stability of small and larger polycyclic aromatic hydrocarbons (PAHs) is determined by characteristic patterns in their structure using density functional theory at the BLYP/TZ2P level. In particular, we focus on the effect of the nonbonded H···H interactions that occur in the bay region of kinked (or armchair) PAHs, but not in straight (or zigzag) PAHs. Model systems comprise anthracene, phenanthrene, and kekulene as well as derivatives thereof. Our main goals are: (1) to explore how nonbonded H•••H interactions in armchair configurations of kinked PAHs affect the geometry and stability of PAHs and how their effect changes as the number of such interactions in a PAH increases; (2) to understand the extent of stabilization upon the substitution of a bay C–H fragment by either C• or N; and (3) to examine the origin of such stabilizing/destabilizing interactions.
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The 1 H- and 2 H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1 H-1,2,3-triazole and sixteen 2 H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium ( r e SE ) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a “best theoretical estimate” (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1 H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2 H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and r e SE values is discussed.
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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.
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N-heterocycles are suspected to play an important role in the chemical origin of life. Despite their detection in meteorites and in Titan’s atmosphere, their extra-terrestrial chemical formation networks remain elusive. Furthermore N-heterocyclics are undetected in the interstellar medium. This paper assesses the photostability of protonated N-hetero(poly)acenes after UV and VUV excitation. It provides information on their ability to retain the N atom into the cycle to generate larger N-containing species or functionalized N-heterocyles. Protonated N-hetero(poly)acenes were generated using electrospray ionization and injected into a linear ion trap where they were irradiated by radiation of 4.5 to 10 eV using the DESIRS beamline at the synchrotron SOLEIL. The photodissociation action spectra of protonated pyridine, quinoline, isoquinoline, and acridine were measured by recording the photofragment yields as a function of photon energy. The four systems exhibit dissociation channels associated with H2 and HCN/HNC loss but with different branching ratios. The results indicate that increasing the size of the N-hetero(poly)acenes increases the chance of retaining the N atom in the larger fragment ion after photodissociation but it remains that all the protonated N-hetero(poly)acenes studied lose their N atom at part of a small neutral photofragment, with high propensity. Therefore, protonated N-hetero(poly)acenes in interstellar space are unlikely precursors to form larger N-containing species. However, protonated pyridine, quinoline, isoquinoline, and acridine are most likely to retain their N atoms in planetary atmospheres where UV radiation at the planet’s surface is typically restricted to wavelengths greater than 200 nm – suggesting such environments are possible substrates for prebiotic chemistry.
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The interactions of ions with molecules and the determination of their dissociation patterns are challenging endeavors of fundamental importance for theoretical and experimental science. In particular, the investigations on bond-breaking and new bond-forming processes triggered by the ionic impact may shed light on the stellar wind interaction with interstellar media, ionic beam irradiations of the living cells, ion-track nanotechnology, radiation hardness analysis of materials, and focused ion beam etching, deposition, and lithography. Due to its vital role in the natural environment, the pyridine molecule has become the subject of both basic and applied research in recent years. Therefore, dissociation of the gas phase pyridine (C5H5N) into neutral excited atomic and molecular fragments following protons (H+) and dihydrogen cations (H2+) impact has been investigated experimentally in the 5–1000 eV energy range. The collision-induced emission spectroscopy has been exploited to detect luminescence in the wavelength range from 190 to 520 nm at the different kinetic energies of both cations. High-resolution optical fragmentation spectra reveal emission bands due to the CH(A2Δ → X2Πr; B2Σ+ → X2Πr; C2Σ+ → X2Πr) and CN(B2Σ+ → X2Σ+) transitions as well as atomic H and C lines. Their spectral line shapes and qualitative band intensities are examined in detail. The analysis shows that the H2+ irradiation enhances pyridine ring fragmentation and creates various fragments more pronounced than H+ cations. The plausible collisional processes and fragmentation pathways leading to the identified products are discussed and compared with the latest results obtained in cation-induced fragmentation of pyridine.
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The analysis of syn-2-cyano-1,3-butadiene (C5H5N, μa = 3.2 D, μb = 2.3 D) in its ground vibrational state and two lowest-energy excited vibrational states, ν27 (A″, 144 cm⁻¹) and ν19 (A′, 163 cm⁻¹), in the 130–360 GHz frequency region has been completed. Nearly 4200 rotational transitions have been measured in the ground vibrational state for the first time, resulting in the determination of the spectroscopic constants for a complete octic Hamiltonian with low error. Analysis of the two lowest-energy, Coriolis-coupled fundamentals reported herein, each containing circa 3000 transitions, yielded two possible least-squares fitting solutions. Both solutions address perturbation between the two vibrational states, including resonances and several nominal interstate transitions, using four a-type and five b-type Coriolis coupling terms (Ga, GaJ, GaK, Fbc, Gb, GbK, Fac, and FacK, with or without FacJ). The energy separation between the two states, ΔE27,19 = 12.307065 (2) cm⁻¹, agrees between the two solutions within their statistical uncertainties, giving confidence that this value is accurate despite the differing Coriolis-coupling terms. The precise rotational and distortion constants determined in this work provide the foundation for an astronomical search for syn-2-cyano-1,3-butadiene across the radio band.
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The 100 m Robert C. Byrd Green Bank Telescope K-band (KFPA) receiver was used to perform a high-sensitivity search for rotational emission lines from complex organic molecules in the cold interstellar medium towards TMC-1 (cyanopolyyne peak), focussing on the identification of new carbon-chain-bearing species as well as molecules of possible prebiotic relevance. We report a detection of the carbon-chain oxide species HC$_7$O and derive a column density of $(7.8\pm0.9)\times10^{11}$~cm$^{-2}$. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C$_7$H$^-$, and/or reaction of C$_6$H$_2$$^+$ with CO (followed by dissociative electron recombination). Upper limits are given for the related HC$_6$O, C$_6$O and C$_7$O molecules. In addition, we obtained the first detections of emission from individual $^{13}$C isotopologues of HC$_7$N, and derive abundance ratios HC$_7$N/HCCC$^{13}$CCCCN = $110\pm16$ and HC$_7$N/HCCCC$^{13}$CCCN = $96\pm 11$, indicative of significant $^{13}$C depletion in this species relative to the local interstellar elemental $^{12}$C/$^{13}$C ratio of 60-70. The observed spectral region covered two transitions of HC$_{11}$N, but emission from this species was not detected, and the corresponding column density upper limit is $7.4\times10^{10}$ cm$^{-2}$ (at 95% confidence). This is significantly lower than the value of $2.8\times10^{11}$ cm$^{-2}$ previously claimed by Bell et al. (1997) and confirms the recent non-detection of HC$_{11}$N in TMC-1 by Loomis et al. (2016). Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline and pyrimidine.
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It is a fact that interstellar formation processes are thermodynamically affected. Based on this, the seven heterocycles; imidazole, pyridine, pyrimidine, pyrrole, quinoline, isoquinoline and furan that have been searched for from different astronomical sources with only upper limits of their column density determined without any successful detection remain the best candidates for astronomical observation with respect to their isomers. These molecules are believed to be formed on the surface of the interstellar dust grains and as such, they are susceptible to interstellar hydrogen bonding. In this study, a two way approach using ab initio quantum chemical simulations is considered in optimizing the searches for these molecules in interstellar medium. Firstly, these molecules and their isomers are subjected to the effect of interstellar hydrogen bonding. Secondly, the deuterated analogues of these heterocycles are examined for their possible detectability. From the results, all the heterocycles except furan are found to be strongly bonded to the surfaces of the interstellar dust grains thereby reducing their abundances, thus contributing to their unsuccessful detection. Successful detection of furan remains highly feasible. With respect to their D-analogues, the computed Boltzmann factor indicates that they are formed under the dense molecular cloud conditions where major deuterium fractionation dominates implying very high D/H ratio above the cosmic D/H ratio which suggests the detectability of these deuterated species.
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The growth mechanisms of organic molecules in an ionizing environment such as the interstellar medium are not completely understood. Here we examine by means of ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) computations the possibility of bond formation and molecular growth upon ionization of Van der Waals clusters of pure HCN clusters, and mixed clusters of HCN and HCCH, both of which are widespread in the interstellar medium. Ionization of van der Waals clusters can potentially lead to growth in low temperature and low-density environments. Our results show, that upon ionization of the pure HCN clusters, strongly bound stable structures are formed that contain NH bonds, and growth beyond pairwise HCN molecules is seen only in a small percentage of cases. In contrast, mixed clusters, where HCCH is preferentially ionized over HCN, can grow up to 3 or 4 units long with new carbon-carbon and carbon-nitrogen covalent bonds. Moreover, cyclic molecules formed, such as the radical cation of pyridine, which is a prebiotic molecule. The results presented here are significant as they provide a feasible pathway for molecular growth of small organic molecules containing both carbon and nitrogen in cold and relatively denser environments such as in dense molecular clouds but closer to the photo-dissociation regions, and protoplanetary disks. In the mechanism we propose, first, a neutral van der Waals cluster is formed. Once the cluster is formed it can undergo photoionization which leads to chemical reactivity without any reaction barrier.
Preprint
The growth mechanisms of organic molecules in an ionizing environment such as the interstellar medium are not completely understood. Here we examine by means of ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) computations the possibility of bond formation and molecular growth upon ionization of Van der Waals clusters of pure HCN clusters, and mixed clusters of HCN and HCCH, both of which are widespread in the interstellar medium. Ionization of van der Waals clusters can potentially lead to growth in low temperature and low-density environments. Our results show, that upon ionization of the pure HCN clusters, strongly bound stable structures are formed that contain NH bonds, and growth beyond pairwise HCN molecules is seen only in a small percentage of cases. In contrast, mixed clusters, where HCCH is preferentially ionized over HCN, can grow up to 3 or 4 units long with new carbon-carbon and carbon-nitrogen covalent bonds. Moreover, cyclic molecules formed, such as the radical cation of pyridine, which is a prebiotic molecule. The results presented here are significant as they provide a feasible pathway for molecular growth of small organic molecules containing both carbon and nitrogen in cold and relatively denser environments such as in dense molecular clouds but closer to the photo-dissociation regions, and protoplanetary disks. In the mechanism we propose, first, a neutral van der Waals cluster is formed. Once the cluster is formed it can undergo photoionization which leads to chemical reactivity without any reaction barrier.
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The pure rotational spectrum of pyrimidine (m-C4H4N2), the meta-substituted dinitrogen analog of benzene, has been studied in the millimeter-wave region from 235 GHz to 360 GHz. The rotational spectrum of the ground vibrational state has been assigned and fit to yield accurate rotational and distortion constants. Over 1700 distinct transitions were identified for the normal isotopologue in its ground vibrational state and least-squares fit to a partial sextic S-reduced Hamiltonian. Transitions for all four singly substituted ¹³C and ¹⁵N isotopologues were observed at natural abundance and were likewise fit. Deuterium-enriched samples of pyrimidine were synthesized, giving access to all eleven possible deuterium-substituted isotopologues, ten of which were previously unreported. Experimental values of rotational constants and computed values of vibration–rotation interaction constants and electron-mass corrections were used to determine semi-experimental equilibrium structures (reSE) of pyrimidine. The reSE structure obtained using coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] corrections shows exceptional agreement with the re structure computed at the CCSD(T)/cc-pCV5Z level (≤0.0002 Å in bond distance and ≤0.03° in bond angle). Of the various computational methods examined, CCSD(T)/cc-pCV5Z is the only method for which the computed value of each geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The exceptionally high accuracy and precision of the structure determination is a consequence of the large number of isotopologues measured, the precision and extent of the experimental frequency measurements, and the sophisticated theoretical treatment of the effects of vibration–rotation coupling and electron mass. Taken together, these demanding experimental and computational studies establish the capabilities of modern structural analysis for a prototypical monocyclic aromatic compound.
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VUV photons from a synchrotron source were used to record the gas-phase absorption spectrum of isoquinoline over the range 3.5 to 10.7 eV. The rich spectrum exhibits both broad and sharp features, of varying intensities, that are analyzed into eight valence and eight Rydberg transitions. Previous data on the valence transitions of isoquinoline were essentially limited to solution spectra up to 5.4 eV. Our study increases their number considerably. The features in the 3.96 eV region are discussed in terms of vibronic coupling between the nπ∗ 1 ¹ A′′ and ππ∗ 2 ¹ A′ valence electronic states. The intensities of some spectral features are augmented by collective π-electron modes considered to be of plasmon-type. Assignments of the valence transitions were facilitated by our DFT calculations and by earlier Pariser-Parr-Pople MO calculations. The calculation results are compared and their relative value is discussed. The DFT calculations reproduce very well a number of experimentally determined properties of the ground state of isoquinoline, in particular its bond distances and angles, rotational constants, vibrational frequencies and dipole moment. No Rydberg series of isoquinoline have previously been observed. Three of the newly observed Rydberg series converge to the D 0 electronic ground state of the ion, while two converge to the D 1 and three to the D 3 excited electronic states of the cation. Astrophysical applications of the VUV absorption spectrum of isoquinoline, in particular the measured absorption cross-sections, are briefly discussed. A comparison between the absorption spectra of isoquinoline and quinoline highlights their similarities and differences, related to their respective molecular orbitals.
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The first ππ * transition for protonated 2-, 3-, and 4-formylpyridine (FPH ⁺ ) ( m/z 108) is investigated by mass spectrometry coupled with photodissociation action spectroscopy at room temperature and 10 K. The photoproduct ions are detected over 35000-43000 cm ⁻¹ and the major product channel for 3-FPH ⁺ and 4-FPH ⁺ is the loss of CO forming protonated pyridine at m/z 80. For 2-FPH ⁺ the CO loss product is present but a more abundant photoproduct arises from loss of CH 2 O to form m/z 78. Plausible potential energy pathways that lead to dissociation are mapped out and comparisons are made to products arising from collision-induced dissociation. Although in all cases the elimination of CO is the overwhelming thermodynamically-preferred pathway, the protonated 2-FPH ⁺ results suggest that the CH 2 O product is kinetically driven and competitive with CO loss. Also, for each isomer, radical photoproduct ions are detected at lower abundances. SCS-CC2/aug-cc-pVTZ Franck-Condon simulations assist with assignment of vibrionic structure and adiabatic energies (0-0) for 2-FPH ⁺ at 36560 cm ⁻¹ , 37430 cm ⁻¹ for 3-FPH ⁺ , and 36140 cm ⁻¹ for 4-FPH ⁺ , yielding an accurate prediction, on average, within 620 cm ⁻¹ .
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The absorption spectrum of quinoline was measured in the gas phase between 3.5 and 10.7 eV using a synchrotron photon source. A large number of sharp and broad spectral features were observed, some of which have plasmon-type collective π-electron modes contributing to their intensities. Eight valence electronic transitions were assigned, considerably extending the number of π-π* transitions previously observed mainly in solution. The principal factor in solution red-shifts is found to be the Lorentz-Lorenz polarizability parameter. Rydberg bands, observed for the first time, are analysed into eight different series, converging to the D0 ground and two excited electronic states, D3 and D4, of the quinoline cation. The R1 series limit is 8.628 eV for the first ionization energy of quinoline, a value more precise than previously published. This value, combined with cation electronic transition data provides precise energies, respectively 10.623 eV and 11.355 eV, for the D3 and D4 states. The valence transition assignments are based on DFT calculations as well as on earlier Pariser-Parr-Pople SCF LCAO MO results. The relative quality of the P-P-P and DFT data is discussed. Both are far from spectroscopic accuracy concerning electronic excited states but were nevertheless useful for our assignments. Our time-dependent DFT calculations of quinoline are excellent for its ground state properties such as geometry, rotational constants, dipole moment and vibrational frequencies, which agree well with experimental observations. Vibrational components of the valence and Rydberg transitions mainly involve C-H bend and C=C and C=N stretch modes. Astrophysical applications of the VUV absorption of quinoline are briefly discussed.
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Recently, several cm-wave transitions of benzonitrile have been detected in the interstellar medium (TMC-1). In this work, the mm-wave spectrum of benzonitrile (C6H5CN, C2v, μa = 4.5 D), a planar asymmetric rotor molecule, is reported in the 103–360 GHz frequency region. Over 3000 rotational transitions have been newly measured for the ground state and were combined with previous data in a global fit, which allows predictions of both low-temperature hyperfine resolved and high-frequency unresolved rotational spectra. Rotational transitions in the lowest fundamentals, ν22 and ν33 (141 and 163 cm⁻¹, respectively), have been observed for the first time, and were complemented by identification of several nominal interstate transitions resulting from strong interstate mixing. Application of a two-state, Coriolis-coupled model accounted, to within experimental precision, for many identified interstate perturbations resulting in ΔE22,33 = 19.108185(7) cm⁻¹ and |ζ22,33a| = 0.841(7). This study provides insight into the generic problem affecting the lowest excited vibrational states of this molecular class and describes the techniques allowing its successful analysis. The new data provide the foundation for future remote detection applications of benzonitrile.
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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.
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We reanalysed the 14N hyperfine structure in the rotational spectra of pyridine and [4-D]-pyridine with higher precision and determined the D quadrupole coupling. It is intended to provide a reference for comparison with substituted pyridines.
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Amino acid analyses using HPLC of pristine interior pieces of the CI carbonaceous chondrites Orgueil and Ivuna have found that beta-alanine, glycine, and gamma-amino-n-butyric acid (ABA) are the most abundant amino acids in these two meteorites, with concentrations ranging from approximately 600 to 2,000 parts per billion (ppb). Other alpha-amino acids such as alanine, alpha-ABA, alpha-aminoisobutyric acid (AIB), and isovaline are present only in trace amounts (<200 ppb). Carbon isotopic measurements of beta-alanine and glycine and the presence of racemic (D/L approximately 1) alanine and beta-ABA in Orgueil suggest that these amino acids are extraterrestrial in origin. In comparison to the CM carbonaceous chondrites Murchison and Murray, the amino acid composition of the CIs is strikingly distinct, suggesting that these meteorites came from a different type of parent body, possibly an extinct comet, than did the CM carbonaceous chondrites.
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During the encounters with comet Halley, PICCA on GIOTTO measured the gas phase organic ion composition of the coma, and PUMA on VEGA 1 measured the dust composition. Joining those results a consistent picture of the parent organic matter from which dust and gas is produced can be obtained. One recognizes a complex unsaturated polycondensate, which splits during coma-formation into the more refractory C=C,C-N-containing dust part, and the more volatile C=C,C-O-containing gas part. The responsible exothermal chemical reactions, triggered by the sun light may play a major role in the dynamics of coma formation. The latent heat and reactivity may cause problems regarding a sample return mission.
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Stellar nucleosynthesis of heavy elements such as carbon allowed the formation of organic molecules in space, which appear to be widespread in our Galaxy. The physical and chemical conditions—including density, temperature, ultraviolet (UV) radiation and energetic particles—determine reaction pathways and the complexity of organic molecules in different space environments. Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. During the protostellar collapse, interstellar organic molecules in gaseous and solid phases are integrated into protostellar disks from which planets and smaller solar system bodies form. After the formation of the planets 4.6 billion years ago, our solar system, including the Earth, was subjected to frequent impacts for several hundred million years. Life on Earth may have emerged during or shortly after this heavy bombardment phase, perhaps as early as 3.90–3.85 billion years ago, but the exact timing remains uncertain. A prebiotic reducing atmosphere, if present, predicts that building blocks of biopolymers—such as amino acids, sugars, purines and pyrimidines—would be formed in abundance. Recent modelling of the Earth's early atmosphere suggests, in contrast, more neutral conditions (e.g. H2O, N2, CO2), thus, precluding the formation of significant concentrations of prebiotic organic compounds. Moreover, even if the Earth's atmosphere were reducing, the presence of UV photons would readily destroy organic compounds unless they were quickly sequestered away in rocks or in the prebiotic ocean. Other possible sources of organic compounds would be high temperature vent chemistry, although the stability of such compounds (bases, amino acids) in these environments remains problematic. Finally, organic compounds may have been delivered to the Earth by asteroids, comets and smaller fragments, such as meteorites and interplanetary dust particles. It is likely that a combination of these sources contributed to the building blocks of life on the early Earth. It may even have taken several starts before life surpassed the less than ideal conditions at the surface. What is certain is that once life emerged, it learned to adapt quickly taking advantage of every available refuge and energy source (e.g. photosynthesis and chemosynthesis), an attribute that eventually led to complex metabolic life and even our own existence. Current experimental research investigating the origin of life is focused on the spontaneous formation of stable polymers out of monomers. However, understanding the spontaneous formation of structure is not enough to understand the formation of life. The introduction and evolution of information and complexity is essential to our definition of life. The formation of complexity and the means to distribute and store information are currently being investigated in a number of theoretical frameworks, such as evolving algorithms, chaos theory and modern evolution theory. In this paper we review the physical and chemical processes that form and process organic matter in space. In particular we discuss the chemical pathways of organic matter in the interstellar medium, its evolution in protoplanetary disks and its integration into solar system material. Furthermore, we investigate the role of impacts and the delivery of organic matter to the prebiotic Earth. Processes that may have assembled prebiotic molecules to produce the first genetic material and ideas about the formation of complexity in chemical networks are also discussed.
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The new interstellar molecules propenal (CH2CHCHO) and propanal (CH3CH2CHO) have been detected largely in absorption toward the star-forming region Sagittarius B2(N) by means of rotational transitions observed with the 100 m Green Bank Telescope (GBT) operating in the range from 18 GHz (λ ~ 1.7 cm) to 26 GHz (λ ~ 1.2 cm). The GBT was also used to observe the previously reported interstellar aldehyde propynal (HC2CHO) in Sagittarius B2(N), which is a known source of large molecules presumably formed on interstellar grains. The presence of these three interstellar aldehydes toward Sagittarius B2(N) strongly suggests that simple hydrogen addition on interstellar grains accounts for successively larger molecular species: from propynal to propenal and from propenal to propanal. Energy sources within Sagittarius B2(N) likely permit the hydrogen addition reactions on grain surfaces to proceed. This work demonstrates that successive hydrogen addition is probably an important chemistry route in the formation of a number of complex interstellar molecules. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO).
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Interstellar glycolaldehyde (CH2OHCHO) has been detected in emission toward the Galactic center source Sagittarius B2(N) by means of millimeter-wave rotational transitions. Glycolaldehyde is an important biomarker since it is structurally the simplest member of the monosaccharide sugars that heretofore have gone undetected in interstellar clouds. There is no consensus as to how any such large complex molecules are formed in the interstellar clouds. It may be that the typical environment of dense interstellar clouds is favorable to glycolaldehyde synthesis by means of the polymerization of formaldehyde (H2CO) molecules either on grain surfaces or in the gas phase. Alternatively, we speculate that glycolaldehyde and other complex molecules may undergo assembly from functional molecular groups on grain surfaces. Utilizing common chemical precursors, a chance process could account for the high degree of isomerism observed in complex interstellar molecules (e.g., methyl formate, acetic acid, and glycolaldehyde). This work suggests that the phenomenon of isomerism be investigated further as a means of potentially constraining interstellar chemistry routes for those individual sources where the condition of good source-beam coupling can be achieved.
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We present new observations of interstellar acetone [(CH3)2CO] from both the NRAO 12 m and the BIMA array. We report NRAO 12 m detections of 13 new acetone emission features that can be assigned to 20 acetone transitions. These assignments are based on the measured and calculated frequencies in 2002 of Groner and coworkers, and they confirm the interstellar acetone identification in 1987 by Combes and coworkers. In addition, our BIMA array observations show that acetone emission is concentrated in the vicinity of the hot molecular core Sgr B2 (N-LMH). The beam-averaged column density for acetone is NT = 2.9(3) × 1016 cm-2. This value is consistent with the 1990 conclusions of Herbst, Giles, & Smith that the observed acetone abundance is too high to be explained by gas-phase synthesis reactions.
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The mid-infrared spectra of the nitrogen-containing heterocyclic polycyclic aromatic compounds 1-azabenz[a]-anthracene; 2-azabenz[a]anthracene; 1-azachrysene; 2-azachrysene; 4-azachrysene; 2-azapyrene, and 7,8 benzoquinoline in their neutral and cation forms were investigated. The spectra of these species isolated in an argon matrix have been measured. Band frequencies and intensities were tabulated and these data compared with spectra computed using density functional theory at the B3LYP level. The overall agreement between experiment and theory is quite good, in keeping with earlier results on homonuclear polycyclic aromatic hydrocarbons. The differences between the spectral properties of nitrogen bearing aromatics and non-substituted, neutral polycyclic aromatic hydrocarbons will be discussed.
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We report the results of an initial survey in a variety of Galactic sources for cyclopropenylidene (C3H2), the first interstellar hydrocarbon ring molecule. C3H2 is found to be very widespread throughout the Galaxy. This, together with its large dipole moment and many observable transitions, makes cyclopropenylidene a promising probe for physical conditions in the interstellar medium. The ortho 1(10)-1(01) transition at 18 GHz is detected in a variety of environments, including giant molecular clouds, diffuse clouds, cold dark clouds, the spiral arm clouds in the direction of distant continuum sources, and the envelope of the carbon star IRC + 10216. The 2(20)-2(11) para line at 21.6 GHz was surveyed in many sources having strong 1(10)-1(01) emission, and, when detected, it was always seen in absorption. A more limited survey of the ortho 2(12)-1(01) transition at 85.3 GHz has been conducted. In addition, the 2(11)-2(02) line of the para species at 46.8 GHz was detected in the dark clouds TMC-1 and L134N. Maps have been made of the clouds TMC-1, L134N, W51, and Orion, confirming that the C3H2 emission is extended in these objects. The data obtained thus far suggest that C3H2 is one of the more abundant organic constituents of the dense interstellar medium.
Chapter
AGB stars provide a fascinating observational laboratory for investigating a range of chemical processes important in many applications of molecular astrophysics. Figure 5.1 outlines the basic chemical structure of an AGB cir-cumstellar envelope (CSE) and identifies the processes that dominate in each region. These include chemistry in local thermodynamic equilibrium (LTE), which occurs at high density and temperature close to the stellar photosphere; the effects of shock waves driven by stellar pulsations, which expose molecules formed in LTE to new physical conditions and which can form “nonequilibrium” molecules; the nucleation of solid particles; the interaction of stellar photons with the dust particles and the subsequent interaction between the gas and the dust as the solid matter is ejected to large distances by radiation pressure; the growth of dust grains; and finally, the interaction of stellar material with the interstellar radiation field.
Article
A fragment of the Murchison (C2) carbonaceous meteorite was analyzed for basic, N -heterocyclic compounds, by dual detector capillary gas chromatography as well as capillary gas chromatography/mass spectrometry, using two columns of different polarity. In the formic acid extract 2,4,6-trimethylpyridine, quinoline, isoquinoline, 2-methylquinoline and 4-methylquinoline were positively identified. In addition, a suite of alkylpyridines and quinolines and/or isoquinolines was tentatively identified from their mass spectra. The (iso)quinolines were found to contain methyl substituents exclusively. The distribution of the pyridines observed reveals a similarity to that observed from catalytic reactions of ammonia and simple aldehydes under conditions similar to those applied in Fischer-Tropsch type reactions.
Article
The ground state rotational spectra of furan, pyrrole, and pyridine have been investigated in the millimeter-wave range. High J transitions (J<60) have been measured and accurate rotational and centrifugal distortion constants have been determined. The results of the analysis are sufficiently accurate for the prediction of all strong transitions throughout the millimeter-wave range. The experimental quartic centrifugal distortion constants are compared with the values calculated from a quantum chemical force field.
Article
The rotational spectrum of 2H-azirine has been measured in the frequency range 50-300 GHz. The a- and b-type spectra have been assigned and have allowed the determination of the rotational and centrifugal distortion constants. The quadrupole and spin-rotation coupling constants deduced from the hyperfine structure analysis are presented.
Article
The microwave spectrum of pyridine has been studied in the region from 20 000 to 40 000 Mc. Twelve low‐J R‐branch lines have been identified. Analysis of the spectrum requires that the dipole moment lie in the a axis, and leads to the following values of rotational constants: a=6039.436 Mc, b=5804.997 Mc, c=2959.210 Mc, and κ=+0.847781. The dipole moment of pyridine vapor was found to be 2.15±0.05 D from quantitative Stark effect studies.
Article
Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J″⩽128 and ν⩽234 GHz. Complementary measurements were also made on low-J, centimeter-wave spectra observed in supersonic expansion and with fully resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are μa=0.14355(19), μb=2.0146(17), μtot=2.0197(17) D for quinoline, and μa=2.3602(21), μb=0.9051(14), μtot=2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported.
Article
Radiation induced chemistry is the major source of organic matter in space. Comets as small bodies, that were kept in the cold parts of our solar system since its formation provide a unique source to study such genuine material. When the VEGA and GIOTTO spacecrafts flew by comet P/Halley in 1986 the mass-spectrometers Puma and PIA measured the composition of cometary dust particles impacting at speeds of well above 65 km s−1. Ion formation upon impact leads to mostly atomic ions. However, a small fraction of the ions measured could be related to molecules. A sophisticated analysis allowed for the first time to point to the chemical nature of cometary organics based on actual mass spectra. The next logical step for in situ cometary exploration is a rendezvous-type mission. This had been planned by NASA and the German BMFT, but was unfortunately canceled in the spring of 1992. In the meantime the European Space Agency (ESA) has dedicated its next major mission, Rosetta, to perform a comet rendezvous. A time-of-flight secondary ion mass spectrometer (CoMA) can provide much higher mass resolution up to molecule masses of some 3000 Da.
Article
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Article
Carbonaceous chondrites comprise a unique subset of meteorites. Two classes of carbonaceous chondrites, the so-called CI1 and CM2 chondrites, are particularly interesting, in part because of their relatively high carbon content and the fact that most of this carbon is present as organic matter. This material is largely macromolecular but also contains a complex mixture of organic compounds. The organic-rich CI1 and CM2 chondrites also contain an extensive clay mineralogy and other minerals that are believed to be indicative of an early episode of hydrous activity in the meteorite parent body. Recent stable isotope measurements have shown the organic matter in general, to be substantially enriched in deuterium and the discrete organic compounds to be enriched in 15N and somewhat enriched in 13C relative to terrestrial matter. These findings suggest that the organic matter is comprised of, or is closely related to, interstellar organic compounds.
Article
The matrix-isolation technique has been employed to measure the mid-infrared spectra of several polycyclic aromatic nitrogen heterocycles in both neutral and cationic forms. The species studied include:  7,8-benzoquinoline (C13H9N), 2-azapyrene (C15H9N), 1- and 2- azabenz[a]anthracene (C17H11N), and 1-, 2-, and 4-azachrysene (also C17H11N). The experimentally measured band frequencies and intensities for each molecule are tabulated and compared with their calculated values computed using density functional theory at the B3LYP/4-31G level. The overall agreement between experiment and theory is good, in keeping with previous investigations involving the parent aromatic hydrocarbons. Several interesting spectroscopic trends are found to accompany nitrogen substitution into the aromatic framework of these compounds. For the neutral species, the nitrogen atom produces a significant increase in the total integrated infrared intensity across the 1600−1100 cm-1 region and plays an essential role in the molecular vibration that underlies an uncharacteristically intense, discrete feature that is observed near 1400 cm-1 in the spectra of 7,8-benzoquinoline, 1-azabenz[a]anthracene, and 4-azachrysene. The origin of this enhanced infrared activity and the nature of the 1400 cm-1 vibrational mode are explored. As a secondary result of the computations, the computed dipole moments and rotational constants for the species under study are reported. The dipole moments calculated are significantly stronger than those of the parent PAH and in principle could facilitate interstellar detection of these PAH related compounds.
Article
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.
Article
Current photochemical models of Titan, Saturn's largest satellite, include chemical reactions producing molecules composed of up to six carbon atoms and then extrapolate straight to aerosols, such as tholins. Laboratory evidence suggests that polycyclic aromatic hydrocarbons containing nitrogen atoms are formed during simulations of Titan's atmosphere. This paper presents preliminary work on reactions that produce nitrogenated aromatic molecules containing up to 12 nonhydrogen atoms. These species would help bridge the gap between the small molecules and the aerosols in Titan's stratosphere. We have determined the barriers and reaction energies for the incorporation of nitrogen into the rings of polycyclic aromatic hydrocarbons. The presence of a nitrogen atom increases the barrier heights relative to the pure hydrocarbon species to about 15 kcal/mol. Nitrogen in the ring promotes the formation of additional hydrocarbon rings by lowering the ring closing barrier to about 4 kcal/mol. Inclusion of these species in photochemical models of Titan is indicated.
Article
Many organic compounds or their precursorsfound in meteorites originated in the interstellar or circumstellarmedium and were later incorporated intoplanetesimals during the formation of thesolar system. There they either survivedintact or underwent further processing tosynthesize secondary products on themeteorite parent body.The most distinct feature of CI and CM carbonaceouschondrites, two typesof stony meteorites, is their high carbon content(up to 3% of weight), either in theform of carbonates or of organic compounds. The bulkof the organic carbon consistsof an insoluble macromolecular material with a complexstructure. Also present is asoluble organic fraction, which has been analyzedby several separation and analyticalprocedures. Low detection limits can be achievedby derivatization of the organicmolecules with reagents that allow for analysisby gas chromatography/massspectroscopy and high performance liquidchromatography. The CM meteoriteMurchison has been found to contain more than70 extraterrestrial amino acids andseveral other classes of compounds includingcarboxylic acids, hydroxy carboxylicacids, sulphonic and phosphonic acids, aliphatic,aromatic and polar hydrocarbons,fullerenes, heterocycles as well as carbonylcompounds, alcohols, amines and amides.The organic matter was found to be enriched indeuterium, and distinct organiccompounds show isotopic enrichments of carbon andnitrogen relative to terrestrialmatter.
Article
Samples of the Murchison (C2), Murray (C2) and Orgueil (C1) carbonaceous meteorites were analyzed for nitrogen-heterocyclic compounds using gas chromatography, cation and anion exclusion liquid chromatography and mass spectrometry. The purines adenine, guanine, hypoxanthine and xanthine were identified in formic acid extracts of all samples, in concentrations ranging from 114–655 ppb. Purines have not previously been found in the Murray meteorite and adenine. hypoxanthine and xanthine have never simultaneously been detected in meteorite extracts. All four biologically significant purines, as well as the pyrimidine uracil have now been identified in these meteorites. A number of other, previously reported N-heterocyclic compounds such as certain hydroxypyrimidines and s-triazines could not be detected in any of the extracts. Laboratory data indicated that both these classes of compounds may be formed from structurally simple precursors (such as guanylurea in the case of s-triazines) during the extraction and analysis of meteorite extracts.We find that the suite of N-heterocyclic compounds identified in meteorites do not, at present, permit a clear distinction to be made between mechanisms of synthesis such as the Fischer-Tropsch type and other candidates. Secondary reactions and conversions in meteorite parent bodies, of HCN and other nitriles produced by Miller-Urey type reactions as well as by Fischer-Tropsch type reactions, must also be considered.
Article
Interstellar chemistry leads to the formation of many prebiologically important molecules and is therefore of the fundamental interest to Astrobiology. Many organics can be produced in the gas phase where they can be detected. Molecules formed by reactions on the surfaces of dust grains are also best detected by millimeter-wave observations of sources in which the products of grain-surface chemistry have been deposited into the gas phase. In this article, we present a summary of the status of several searches for potential prebiotic molecules – aziridine, 2H-azirine, pyrimidine and glycine – in the hot molecular cores Sgr B2(N-LMH), W51 e1/e2 and Orion KL. Detections of these organic molecules would strengthen the thesis that interstellar organic matter, delivered in cometary impacts, could have played an important role in the prebiotic chemistry of the early Earth.
Article
Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observational results from the ground and space, as well as laboratory simulation experiments and new methods for theoretical modeling, this review recapitulates the inventory and distribution of organic molecules in different environments. The evolution, survival, transport, and transformation of organics is monitored, from molecular clouds and the diffuse interstellar medium to their incorporation into solar system material such as comets and meteorites. We constrain gas phase and grain surface formation pathways to organic molecules in dense interstellar clouds, using recent observations with the Infrared Space Observatory (ISO) and ground-based radiotelescopes. The main spectroscopic evidence for carbonaceous compounds in the diffuse interstellar medium is discussed (UV bump at 2200 Å, diffuse interstellar bands, extended red emission, and infrared absorption and emission bands). We critically review the signatures and unsolved problemsrelated to the main organic components suggested to be present in the diffuse gas, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, diamonds, and carbonaceous solids. We also briefly discuss the circumstellar formation of organics around late-typestars. In the solar system, space missions to comet Halley and observations of the bright comets Hyakutake and Hale-Bopp have recently allowed a reexamination of the organic chemistry of dust and volatiles in long-period comets. We review the advances in this area and also discuss progress being made in elucidating the complex organic inventory of carbonaceous meteorites. The knowledge of organic chemistry in molecular clouds, comets, and meteorites and their common link provides constraints for the processes that lead to the origin, evolution, and distribution of life in the Galaxy.
Article
Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for example as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UV stability. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10–100 years, while in the Solar System at 1 AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UV flux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astronomical environments.
Article
Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for example as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UVstability. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10-100years, while in the Solar System at 1AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UVflux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astronomical environments.
Article
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.
Article
A comprehensive reinvestigation of the rotational spectrum of pyrimidine was carried out by using several different spectrometers. All singly substituted 13C- and 15N-isotopic species of pyrimidine have been measured in natural abundance with millimeter-wave free jet and waveguide Fourier transform microwave techniques, and complete rs and r0 heavy atom geometries have been determined. The ground state rotational spectrum in the centimeter-wave region was measured at sub-Doppler resolution of the cavity Fourier transform spectrometer and all elements in the inertial and principal nuclear quadrupole-coupling tensors of the nitrogen nuclei in pyrimidine have been determined. The room-temperature spectrum was measured up to 337 GHz and J = 66 with BWO-based spectrometers and sextic level centrifugal distortion constants in the rotational Hamiltonian have been determined for the ground state and three lowest vibrational fundamentals of pyrimidine. Copyright 1999 Academic Press.
Article
The rotational spectrum of ethylenimine (aziridine, c-C(2)H(4)NH) has been investigated in selected regions from 118 to 950 GHz using the Cologne terahertz spectrometer. About 320 lines have been measured spanning the quantum numbers 2 </= J </= 59 and 0 </= K(c) </= 50. All lines have been fit together with previously published data to yield refined ground state constants such as A = 22 736.19294(31) MHz, B = 21 192.46114(31) MHz, and C = 13 383.16401(30) MHz and centrifugal distortion constants that permit accurate frequency predictions. FTMW measurements were performed at the ETH Zürich to redetermine the dipole moment, to improve (14)N-quadrupole-coupling constants, as well as to determine (14)N-spin-rotation coupling constants. The most reliable values for the components of the dipole moment are µ(b) = 0.97(12) D and µ(c) = 1.357(25) D determined by the Stark- and additional intensity measurements of appropriate b- and c-type transitions. Moreover, we performed quantum chemical calculations of the dipole moment of ethylenimine that confirm these experimental results. This three-membered ring molecule is of potential astrophysical interest. Copyright 2000 Academic Press.
Article
A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.
Article
Rotational spectroscopy at millimeter wavelengths is a powerful means of investigating the chemistry of dense interstellar clouds. These regions can exhibit an interesting complement of gas phase molecules, including relatively complex organics. Here we report the tentative first astronomical detection of aziridine (ethylenimine), the possible detection of propenal (acrolein), and upper limits on the abundances of cyclopropenone, furan, hydroxyethanal (glycolaldehyde), thiohydroxylamine (NH2SH), and ethenol (vinyl alcohol) in various interstellar clouds.
Article
It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.
Article
A comprehensive study of the PAH hypothesis is presented, including the interstellar, IR spectral features which have been attributed to emission from highly vibrationally excited PAHs. Spectroscopic and IR emission features are discussed in detail. A method for calculating the IR fluorescence spectrum from a vibrationally excited molecule is described. Analysis of interstellar spectrum suggests that the PAHs which dominate the IR spectra contain between 20 and 40 C atoms. The results are compared with results from a thermal approximation. It is found that, for high levels of vibrational excitation and emission from low-frequency modes, the two methods produce similar results. Also, consideration is given to the relationship between PAH molecules and amorphous C particles, the most likely interstellar PAH molecular structures, the spectroscopic structure produced by PAHs and PAH-related materials in the UV portion of the interstellar extinction curve, and the influence of PAH charge on the UV, visible, and IR regions.
Article
We have searched three hot molecular cores for submillimetre emission from the nucleic acid building block pyrimidine. We obtain upper limits to the total pyrimidine (beam-averaged) column densities towards Sgr B2(N), Orion KL and W51 e1/e2 of 1.7 × 1014, 2.4 × 1014 and 3.4 × 1014 cm−2, respectively. The associated upper limits to the pyrimidine fractional abundances lie in the range (0.3−3) × 10−10. Implications of this result for interstellar organic chemistry, and for the prospects of detecting nitrogen heterocycles in general, are discussed briefly.
Infrared spectroscopy fo matrix-isolated polycyclic aromatic hydrocarbons and their ions. 6. Polycyclic aromatic nitrogen hetero-cycles Molecule and grain formation Asymptotic Giant Branch Stars The astrobiology of nucleobases
• A L Mattioda
• D M Hudgins
• C W Bauschlicher
• M Rosi
• L J Allaman-Dola
• T J Millar
Mattioda, A.L., Hudgins, D.M., Bauschlicher, C.W., Rosi, M., Allaman-dola, L.J. Infrared spectroscopy fo matrix-isolated polycyclic aromatic hydrocarbons and their ions. 6. Polycyclic aromatic nitrogen hetero-cycles. J. Phys. Chem. A 107, 1486–1498, 2003. Millar, T.J. Molecule and grain formation. in: Habing, H.J., Olofsson, H. (Eds.), Asymptotic Giant Branch Stars. Springer Verlag, New York, pp. 247–289, 2003. Peeters, Z., Botta, O., Charnley, S.B., Ruiterkamp, R., Ehrenfreund, P. The astrobiology of nucleobases. Astrophys. J. Lett. 593, L129–L132, 2003. Peeters, Z., Botta, O., Charnley, S.B., Kisiel, Z., Kuan, Y.-J., Ehrenfre-und, P. Formation and photostability of N-heterocycles in space – I.
Astrophysical and astrochemical insights into the origin of life Nitrogen and deuterium hyperfine structure in the rotational spectra of pyridine
• P Ehrenfreund
• W M Irvine
• L Becker
• J Blank
• J R Brucato
Ehrenfreund, P., Irvine, W.M., Becker, L., Blank, J., Brucato, J.R., et al. Astrophysical and astrochemical insights into the origin of life. Rep. Prog. Phys. 65, 1427–1487, 2002. Heineking, N., Dreizler, H., Schwartz, R. Nitrogen and deuterium hyperfine structure in the rotational spectra of pyridine and 4 D pyridine. Z. Naturforsch. 41a, 1210, 1986.
Confirmation of interstellar acetone. Astro-phys Nitrogen-heterocyclic compounds in mete-orites – Significance and mechanisms of formation. Geochim. Cosmo-chim Basic nitrogen-heterocyclic compounds in the Murchison meteorite
• M N Simon
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Simon, M.N., Simon, M. Search for interstellar acrylonitrile, pyrimidine, and pyridine. Astrophys. J. 184, 757–762, 1973. Snyder, L.E., Lovas, F.J., Mehringer, D.M., Miao, Y., Kuan, Y.-J., Hollis, J.M., Jewell, P.R. Confirmation of interstellar acetone. Astro-phys. J. 578, 245–255, 2002. Stoks, P.G., Schwartz, A.W. Nitrogen-heterocyclic compounds in mete-orites – Significance and mechanisms of formation. Geochim. Cosmo-chim. Acta 45, 563–569, 1981. Stoks, P.G., Schwartz, A.W. Basic nitrogen-heterocyclic compounds in the Murchison meteorite. Geochim. Cosmochim. Acta 46, 309–315, 1982.
Infrared Space ObservatoryÕs Discovery of C 4 H 2 , C 6 H 2 , and benzene in CRL 618 Spectroscopic diagnostics of organic chemistry in the protostellar environment Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes
• J Cernicharo
• A M Heras
• A G G M Tielens
• J R Pardo
• F Herpin
• M Guelin
• L B F M Waters
• S B Charnley
• P Ehrenfreund
• Y.-J Kuan
Cernicharo, J., Heras, A.M., Tielens, A.G.G.M., Pardo, J.R., Herpin, F., Guelin, M., Waters, L.B.F.M. Infrared Space ObservatoryÕs Discovery of C 4 H 2, C 6 H 2, and benzene in CRL 618. Astrophys. J. Lett. 546, L123–L126, 2001. Charnley, S.B., Ehrenfreund, P., Kuan, Y.-J. Spectroscopic diagnostics of organic chemistry in the protostellar environment. Spectrochim. Acta, Part A 57, 685–704, 2001. Cherchneff, I., Barker, J.R., Tielens, A.G.G.M. Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes. Astrophys. J. 401, 269–287, 1992. Chyba, C.F., Thomas, P.J., Brookshaw, L., Sagan, C. Cometary delivery of organic molecules to the early Earth. Science 249, 366–373, 1990.
A search for circumstellar quinoline and isoquinoline
• O Botta
• N Cox
• Y.-J Kuan
• D Despois
• H.-C Huang
Organic matter in meteorites: molecular and isotopic analyses of the Murchison meteorite
• Cronin
Polycyclic aromatic hydrocarbon formation in carbon-rich stellar envelopes
• Cherchneff