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Photofragment Translational Spectroscopy of 1,3-Butadiene and 1,3-Butadiene-1,1,4,4- d 4 at 193 nm
Abstract
The photodissociation dynamics of 1,3-butadiene at 193 nm have been investigated with photofragment translational spectroscopy coupled with product photoionization using tunable VUV synchrotron radiation. Five product channels are evident from this study: C(4)H(5) + H, C(3)H(3) + CH(3), C(2)H(3) + C(2)H(3), C(4)H(4) + H(2), and C(2)H(4) + C(2)H(2). The translational energy (P(E(T))) distributions suggest that these channels result from internal conversion to the ground electronic state followed by dissociation. To investigate the dissociation dynamics in more detail, further studies were carried out using 1,3-butadiene-1,1,4,4-d(4). Branching ratios were determined for the channels listed above, as well as relative branching ratios for the isotopomeric species produced from 1,3-butadiene-1,1,4,4-d(4) dissociation. C(3)H(3) + CH(3) is found to be the dominant channel, followed by C(4)H(5) + H and C(2)H(4) + C(2)H(2), for which the yields are approximately equal. The dominance of the C(3)H(3) + CH(3) channel shows that isomerization to 1,2-butadiene followed by dissociation is facile.
... A remaining problem is the low ionisation efficiency, PI cross-sections being typically about one-two orders of magnitude smaller than EI cross-sections. A variety of photodissociation studies in recent years have demonstrated the power of soft PI by synchrotron radiation [78][79][80][81][82][83][84][85][86], but there have been only very few examples of reactive scattering studies exploiting this approach, namely those on the reactions Cl þ propane and n-pentane [87][88][89]. ...
... Notwithstanding the success of those experiments, a shortcoming with the VUV PI approach is that absolute PI cross-sections are very often not known, and therefore branching ratios cannot be easily estimated. As a matter of fact, studies of photodissociation processes by soft PI using synchrotron light are usually accompanied by measurements carried out using hard EI, where many data have to be taken at all possible fragment masses in order to estimate branching ratios [78][79][80][82][83][84][85][86]. ...
... In our laboratory, we have very recently implemented in CMB reactive scattering experiments an alternative method, that is, product detection by soft EI, which is achieved by using electrons with low, tunable energy [12]. Although not affording the same degree of selectivity as VUV synchrotron radiation, the soft EI approach offers similar advantages with respect to the dissociative ionisation problem; in addition, it gives us the bonus of the possibility of determining branching ratios, since absolute EI cross-sections are often known or can be reliably estimated [86,[91][92][93]. The soft EI approach is well known in mass spectrometry and widely used, for instance, in discharge-flow mass-spectrometric kinetic studies, but it was never applied in CMB experiments prior to our recent work [76,77], mainly because of the low detection sensitivity. ...
In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C( 1 D), N( 2 D), O( 1 D) – with H 2 (D 2 ), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O( 3 P) and C( 3 P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research.
... UV excitation of these species typically deposits sufficient energy for multiple isomerization and dissociation pathways to be thermodynamically accessible, and detailed photodissociation studies can reveal, among other things, whether scrambling between the two (or more) starting isomers becomes facile at any point during the dynamics. For example, in our laboratory very different primary photochemistry from the 193 nm photodissociation of two C 4 H 6 isomers, 1,2-and 1,3-butadiene [1,2] have been observed, even though both species dissociate via internal conversion to the ground state surface. ...
... A fixed-source, rotating-detector apparatus based on EI detection was used for additional photodissociation experiments [2,31]. In general, operating conditions were similar to those described above. ...
... In this section, centre-of-mass translational energy distributions used to fit the TOF spectra obtained on the two instruments are presented. It is considerably easier to fit the data obtained on the PI instrument, owing to greatly reduced dissociative ionization, so the procedure previously used in our analysis of 1,3-butadiene [2] was followed, in which distributions are fit to the PI data and then used to obtain branching ratios from the EI data. ...
The dissociation dynamics of allene, propyne, and propyne-d 3 at 193 nm were investigated with photofragment translational spectroscopy. Products were either photoionized using tunable VUV synchrotron radiation or ionized with electron impact. Product time-of-flight data were obtained to determine centre-of-mass translational energy (P(E T)) distributions, and photoionization efficiency (PIE) curves were measured for the hydrocarbon products. The two major product channels evident from this study are atomic and molecular hydrogen loss, with a H:H 2 branching ratio of 90:10, regardless of precursor. The P(E T) distribution for each channel is also largely independent of precursor. Both channels appear to occur following internal conversion to the ground electronic state. The propyne-d 3 results show that there is extensive isotopic scrambling prior to H(D) atom loss, and that the H:D product ratio is approximately unity. The PIE curves for H(D) atom loss from allene, propyne, and propyne-d 3 indicate that the dominant corresponding C 3 H 3 product is the propargyl radical in all cases. There is some evidence from the PIE curves that the dominant C 3 H 2 products from allene and propyne are propadienylidene (H 2 CCC:) and propargylene (HCCCH), respectively.
... Other notable features are the strong peaks due to C 2 H + 3 , C 2 H + 4 , and C 3 H + 3 fragments. The latter is the main dissociation product observed in photodissociation 49 and electron impact 50 experiments with 1,3-butadiene. However, here we observe a wider range of fragment sizes than in the photodissociation experiments 49 , and more small fragments, e.g. ...
... The latter is the main dissociation product observed in photodissociation 49 and electron impact 50 experiments with 1,3-butadiene. However, here we observe a wider range of fragment sizes than in the photodissociation experiments 49 , and more small fragments, e.g. C 2 H + 3 , C 2 H + 4 , than in the electron impact experiments 50 . ...
We report on studies of collisions between 3 keV Ar⁺ projectile ions and neutral targets of isolated 1,3-butadiene (C4H6) molecules and cold, loosely bound clusters of these molecules. We identify molecular growth processes within the molecular clusters that appears to be driven by knockout processes and that could result in the formation of (aromatic) ring structures. These types of reactions are not unique to specific projectile ions and target molecules, but will occur whenever atoms or ions with suitable masses and kinetic energies collide with aggregates of matter, such as carbonaceous grains in the interstellar medium or aerosol nanoparticles in the atmosphere.
... A number of recent studies investigating dissociation dynamics are found in the literature. [1][2][3][4][5][6][7][8][9] The present research focuses on the simplest π-conjugated molecule 1,3-trans butadiene [H 2 C= =CH-CH= =CH 2 ], which is used as a prototype system for the important cis-trans transitions in photoreceptor molecules (bacteriorhodopsin) and also as a model system for conjugated polymers. ...
... Several theoretical and experimental studies focus on the dynamics of excited and ionized butadiene. [1][2][3][4][5][6]10 In an early study on fast photoisomerization kinematics, it was found that transfer of a hydrogen adjacent to the twisted C==C bond causes a potential crossing of low-lying neutral excited and ground states leading to significant H-transfer through nonadiabatic coupling. 10 The first photoelectronphotoion coincidence (PEPICO) measurements provide strong evidence that different isomeric forms of C 4 H 6 , e.g., 1,3butadiene, cyclobutyene, and 1-and 2-butyne, are formed prior to dissociation. ...
Dissociative double-photoionization of butadiene in the 25-45 eV energy range has been studied with tunable synchrotron radiation using full three-dimensional ion momentum imaging. Using ab initio calculations, the electronic states of the molecular dication below 33 eV are identified. The results of the measurement and calculation show that double ionization from π orbitals selectively triggers twisting about the terminal or central C–C bonds. We show that this conformational rearrangement depends upon the dication electronic state, which effectively acts as a gateway for the dissociation reaction pathway. For photon
energies above 33 eV, three-body dissociation channels where neutral H-atom evaporation precedes C–C charge-separation in the dication species appear in the correlation map. The fragment angular distributions support a model where the dication species is initially aligned with the molecular backbone parallel to the polarization vector of the light, indicating a high probability for double-ionization to the “gateway states” for molecules with this orientation.
... The primary photochemistry of butadiene has also been studied. The major product was found to be propargyl radical, 14 which is thought to be a major source of benzene via radical-radical recombination. 4,6 Investigations into the secondary reactions of butadiene photoproducts are currently underway. ...
... 1,3-Butadiene has a similar absorption spectrum, the breadth of which is explained by rapid internal conversion to the ground state via two conical intersections. 14 Diacetylene, by contrast, has broadened but vibronically-resolved transitions beginning around 250 nm, and the photophysics is dominated by intersystem crossing rather than internal conversion. 17,18 Furthermore, in the case of diacetylene, the lowest bond dissociation energy is above the wavelength range of its major ultraviolet absorption, so even if internal conversion occurs it does not lead to free radical products. ...
The ultraviolet photochemistry of vinylacetylene (C4H4) was studied under temperature and pressure conditions similar to Titan's atmosphere by exciting the molecule in a constrained expansion that opens into the ion source region of a time-of-flight mass spectrometer. The primary dissociation products detected by vacuum-ultraviolet ionization were found to be C4H3 and C4H2, in a ratio of 3-10 : 1. Subsequent reaction of the C4H3 radicals with the parent C4H4 produced two major secondary products: C8H6 and C6H4. The former was spectroscopically identified as phenylacetylene, confirming that photochemical reactions of C4H4 can produce aromatic molecules. The primary dissociation reaction was also studied computationally. The results were consistent with the experimental findings for C4H2 and C4H3. However, the major product is C2H2, which is undetected by 118 nm photoionization in the present experiment but should account for roughly two-thirds of the products. Simulations were also performed to confirm that the present experiment accurately represents the 220 nm photochemistry of vinylacetylene at the temperature and pressure of Titan's atmosphere, with a product yield of C2H2 : C4H2 : C4H3 of 66 : 7 : 27. Accounting for the wavelength dependent solar flux on Titan, the estimated absorption cross section of vinylacetylene in the ultraviolet, and the slightly wavelength dependent product distribution, the overall product yield predicted by the simulations for ultraviolet photolysis of vinylacetylene on Titan is C2H2 : C4H2 : C4H3 = 65 : 8 : 27. Finally, a simulation was performed under conditions of a shock tube experiment to examine the differences between thermal and photochemical dissociation. The product yield of this simulation was C2H2 : C4H2: C4H3 = 61 : 1 : 38.
... 112 The planar trans-C 4 H 6 isomer composes 95% of gaseous samples at room temperature; rotation about the central C-C bond leads to a non-planar gauche structure [ Fig. 5(a)]. 113 The ultraviolet photochemistry of C 4 H 6 has been studied for decades, [114][115][116][117] and IR-driven photoisomerization should be possible, given the ∼2000 cm −1 torsional barrier in the electronic ground state. 118 We evaluate prospects for cavity-coupling the ν 11 CH 2 wagging mode of trans-C 4 H 6 , which is the most strongly absorbing band at room temperature (Fig. 5, Table I). ...
Gas-phase molecules are a promising platform to elucidate the mechanisms of action and scope of polaritons for optical control of chemistry. Polaritons arise from the strong coupling of a dipole-allowed molecular transition with the photonic mode of an optical cavity. There is mounting evidence of modified reactivity under polaritonic conditions; however, the complex condensed-phase environment of most experimental demonstrations impedes mechanistic understanding of this phenomenon. While the gas phase was the playground of early efforts in atomic cavity quantum electrodynamics, we have only recently demonstrated the formation of molecular polaritons under these conditions. Studying the reactivity of isolated gas-phase molecules under strong coupling would eliminate solvent interactions and enable quantum state resolution of reaction progress. In this Perspective, we contextualize recent gas-phase efforts in the field of polariton chemistry and offer a practical guide for experimental design moving forward.
... Among conjugated polyenes, we are interested in 1,3butadiene (BD) because of its structural simplicity. BD has been investigated both experimentally and theoretically to understand energy relaxation processes in conjugated polyenes in a simplified manner [2][3][4][5][6][7][8][9][10][11][12][13]. In gaseous BD, the lowest optical excitation corresponds to the π → π * transition: the 1 1 A g → 1 1 B u band, with a peak absorption of approximately 210 nm (5.90 eV) [3,14]. ...
Modification of a molecular structure at the minimum-energy conical intersection (MECI) on the relaxation pathway of the smallest conjugated polyene, trans-1,3-butadiene (BD), was achieved by substituting the hydrogen atoms at the end of the molecular chain with methyl groups. At the MECI between S1 and S0, BD is known to have a pyramidalized structure; therefore, the substituted methyl groups are expected to hinder this pyramidalization and change the relaxation pathway. Here, the relaxation dynamics of three conjugated dienes: BD, trans-1,3-pentadiene (PD), and 2,5-dimethyl-2,4-hexadiene (HD), were investigated by time-resolved photoelectron spectroscopy (TRPES) using single-order high harmonic pulses. Ultrafast relaxation to the ground state with decay times of less than 100 fs was observed in all three molecules. The potential energy curves from the theoretical calculation show the common features among these dienes, which explains the similar relaxation dynamics. However, HD was found theoretically to have a transoid structure at MECI between S1 and S0, while BD and PD both had pyramidalized structures. The relaxation pathway switching was corroborated experimentally by analysis of the photoelectron spectra specifically appearing in HD after a few hundred femtoseconds upon photoexcitation. The larger twist in the transoid structure stimulates molecular vibrations that modulated the photoionization probabilities.
... In a previous C 4 H 5 + PES study, Hopkinson and co-workers [58] found that trans-buta-1,3-dien-4-yl cation could be located only at HF level, but not at MP2 or MP4 level, and it can collapse into buta-1,2-dien-4-yl cation (CH 2 CHCCH 2 + ) via low barrier. Previous investigations [20,59] about photodissociation of 1,3-butadiene have also suggested that formation of buta-1,2dien-4-yl is more favorable among the two possible channels to produce C 4 H 5 + from C 4 H 6 + . Based on the analysis above, we may get the conclusion that fragment ion of C 4 H 5 + with an observed onset at 13.20 ± 0.02 eV should be assigned to buta-1,2-dien-4-yl cation and is generated by H loss from the central carbon atom of 1,3butadiene radical cation. ...
In this work, photoionization and dissociation of cyclohexene have been studied by means of coupling a reflectron time-of-flight mass spectrometer with the tunable vacuum ultraviolet (VUV) synchrotron radiation. The adiabatic ionization energy of cyclohexene as well as the appearance energies of its fragment ions C6H9+, C6H7+, C5H7+, C5H5+, C4H6+, C4H5+, C3H5+ and C3H3+ were derived from the onset of the photoionization efficiency (PIE) curves. The optimized structures for the transition states and intermediates on the ground state potential energy surfaces related to photodissociation of cyclohexene were characterized at the ωB97X-D/6-31+g(d,p) level. The coupled cluster method, CCSD(T)/cc-pVTZ, was employed to calculate the corresponding energies with the zero-point energy corrections by the ωB97X-D/6-31+g(d,p) approach. Combining experimental and theoretical results, possible formation pathways of the fragment ions were proposed and discussed in detail. The retro-Cope rearrangement was found to play a crucial role in the formation of C4H6+, C4H5+ and C3H5+. Intramolecular hydrogen migrations were observed as dominant processes in most of the fragmentation pathways of cyclohexene. The present research provides a clear picture of the photoionization and dissociation processes of cyclohexene in the 8- to 15.5-eV photon energy region. Copyright
... However, numerous studies explored the excitation and photolysis of 1,3-butadiene in the 193–220 nm range, from which it is found that the primary dissociative channel leads to formation of C 3 H 3 + CH 3[14,57585960. In addition to C 3 H 3 + CH 3 , the following minor photolysis channels have also been reported in the 193– 220 nm energy range: C 4 H 5 + H, C 4 H 4 + H 2 , C 2 H 3 + C 2 H 3 and C 2 H 2 + C 2 H 4 with varying relative ratios[14,59,60]. The 1,3-butadiene cross-section for absorption at 248 nm is nearly two orders ofmagnitude smaller than at 193 nm and certain channels are energetically inhibited, such as formation of two vinyl radicals (C 2 H 3 ) by cleavage of the central CÀ ÀC bond. ...
Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.
... The molecular beam photodissociation instrument on which these experiments were performed has been described in detail previously [19,20]. ClN 3 was formed by passing a mixture of 5% Cl 2 in He over the surface of moist sodium azide (NaN 3 ). ...
The primary reaction products from 248-nm chlorine azide photolysis are identified in a collision-free experiment. In contrast to all previous reports, the radical channel producing Cl+N3 (95±3%) is seen to dominate the photochemistry. The molecular channel producing NCl+N2 (5±3%) was also observed.
... 7,8 In addition to the lowest energy dissociation path, molecules fragment via competitive (or parallel) and sequential paths at higher energies. Such pathways have been investigated in a few neutral systems by photofragment translational spectroscopy 9,10 in which momentum matching of neutral product pairs can provide sufficient information to model sequential dissociation steps. However, the dearth of photodissociation laser wavelengths prevents the study of such reactions at more than one or two photon energies. ...
Recent advances in threshold photoelectron photoion coincidence (TPEPICO) make possible the analysis of several parallel and sequential dissociations of energy selected ions. The use of velocity focusing optics for the simultaneous collection of threshold and energetic electrons not only improves the resolution, but also permits subtraction of coincidences associated with "hot" electrons, thereby yielding TPEPICO data with no contamination from "hot" electrons. The data analysis takes into account the thermal energy distribution of the sample and uses statistical theory rate constants and energy partitioning in dissociation reactions to model the time of flight distributions and the breakdown diagram. Examples include CH2BrCl and P(C2H5)3. Of particular interest is the ability to extract error limits for rate constants and dissociation energies.
... 1,2 It is conceivable that the higher total rate coefficients observed at higher pressure simply reflect collisional stabilization. However, 193 nm photodissociation experiments by Robinson et al. 48 and the ab initio and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations carried out by Lee et al. 49 on the C 4 H 6 system indicate that the dissociation of 1,3-butadiene to two vinyl radicals is vastly disfavored relative to other bimolecular channels, even at 620 kJ mol -1 excess energy (i.e., about 150 kJ mol -1 above the vinyl + vinyl asymptote). Substantial backdissociation from a 1,3-butadiene adduct in the vinyl + vinyl reaction is therefore exceedingly unlikely, all but ruling out any significant pressure dependence for recombination on the singlet surface. ...
The rate coefficient for the self-reaction of vinyl radicals has been measured by two independent methods. The rate constant as a function of temperature at 20 Torr has been determined by a laser-photolysis/laser absorption technique. Vinyl iodide is photolyzed at 266 nm, and both the vinyl radical and the iodine atom photolysis products are monitored by laser absorption. The vinyl radical concentration is derived from the initial iodine atom concentration, which is determined by using the known absorption cross section of the iodine atomic transition to relate the observed absorption to concentration. The measured rate constant for the self-reaction at room temperature is approximately a factor of 2 lower than literature recommendations. The reaction displays a slightly negative temperature dependence, which can be represented by a negative activation energy, (E(a)/R) = -400 K. The laser absorption results are supported by independent experiments at 298 K and 4 Torr using time-resolved synchrotron-photoionization mass-spectrometric detection of the products of divinyl ketone and methyl vinyl ketone photolysis. The photoionization mass spectrometry experiments additionally show that methyl + propargyl are formed in the vinyl radical self-reaction, with an estimated branching fraction of 0.5 at 298 K and 4 Torr.
The focus of this excellent textbook is the topic of molecular reaction dynamics. The chapters are all written by internationally recognised researchers and, from the outset, the contributors are writing with the young scientist in mind. The easy to use, stand-alone, chapters make it of value to students, teachers, and researchers alike. Subjects covered range from the more traditional topics, such as potential energy surfaces, to more advanced and rapidly developing areas, such as femtochemistry and coherent control. The coverage of reaction dynamics is very broad, so many students studying chemical physics will find elements of this text interesting and useful. Tutorials in Molecular Reaction Dynamics includes extensive references to more advanced texts and research papers, and a series of 'Study Boxes' help readers grapple with the more difficult concepts. Each chapter is thoroughly cross-referenced, helping the reader to link concepts from different branches of the subject. Worked problems are included, and each chapter concludes with a selection of problems designed to test understanding of the subjects covered. Supplementary reading material, and worked solutions to the problems, are contained on a secure website.
The focus of this excellent textbook is the topic of molecular reaction dynamics. The chapters are all written by internationally recognised researchers and, from the outset, the contributors are writing with the young scientist in mind. The easy to use, stand-alone, chapters make it of value to students, teachers, and researchers alike. Subjects covered range from the more traditional topics, such as potential energy surfaces, to more advanced and rapidly developing areas, such as femtochemistry and coherent control. The coverage of reaction dynamics is very broad, so many students studying chemical physics will find elements of this text interesting and useful. Tutorials in Molecular Reaction Dynamics includes extensive references to more advanced texts and research papers, and a series of 'Study Boxes' help readers grapple with the more difficult concepts. Each chapter is thoroughly cross-referenced, helping the reader to link concepts from different branches of the subject. Worked problems are included, and each chapter concludes with a selection of problems designed to test understanding of the subjects covered. Supplementary reading material, and worked solutions to the problems, are contained on a secure website.
The focus of this excellent textbook is the topic of molecular reaction dynamics. The chapters are all written by internationally recognised researchers and, from the outset, the contributors are writing with the young scientist in mind. The easy to use, stand-alone, chapters make it of value to students, teachers, and researchers alike. Subjects covered range from the more traditional topics, such as potential energy surfaces, to more advanced and rapidly developing areas, such as femtochemistry and coherent control. The coverage of reaction dynamics is very broad, so many students studying chemical physics will find elements of this text interesting and useful. Tutorials in Molecular Reaction Dynamics includes extensive references to more advanced texts and research papers, and a series of 'Study Boxes' help readers grapple with the more difficult concepts. Each chapter is thoroughly cross-referenced, helping the reader to link concepts from different branches of the subject. Worked problems are included, and each chapter concludes with a selection of problems designed to test understanding of the subjects covered. Supplementary reading material, and worked solutions to the problems, are contained on a secure website.
Recent experimental and computational advances have heralded huge progress in the range and the detail of the database pertaining to photoinduced C–H bond fission processes. This Perspective provides a snapshot of the current state of knowledge as determined via gas phase (i.e. isolated molecule) studies of the primary photochemistry of families of hydrocarbon molecules (alkynes, alkenes, alkanes, aromatics and selected heteroatom containing analogues) and the corresponding radicals (including saturated and unsaturated hydrocarbon radicals). Different families show different and, in many cases, understandable propensities for dissociating from an excited electronic state or following non-adiabatic coupling (i.e. internal conversion) to high vibrational levels of the ground electronic state. The Perspective seeks to emphasise the potentially vast range of behaviours (dissociation timescales, product energy disposals, etc.) that can be expected to accompany internal conversion, reflecting the extent to which the tuning coordinate (i.e. the nuclear motions that tune the energy separation between the excited and ground state) projects onto the dissociation coordinate of interest (i.e. the breaking of the C–H bond).
We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N 2 as well as new photodissociation branching ratios for CH 4 and C 2 H 2 . Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions. Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan's atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH ⁺ ) in the upper ionosphere, (ii) the low mass negative ion densities (CN ⁻ , C 3 N ⁻ /C 4 H ⁻ ) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO 2 , H 2 O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.
La atom reactions with 1-butyne and 2-butyne are carried out in a laser-vaporization molecular beam source. Both reactions yield the same La-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage and coupling of the butynes. The dehydrogenated species La(C4H4) is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of La(C4H4) produced from the two reactions exhibit two identical transitions, each consisting of a strong origin band and several vibrational intervals. The two transitions are assigned to the ionization of two isomers: La(η⁴-CH2CCCH2) (Iso A) and La(η⁴-CH2CHCCH) (Iso B). The ground electronic states are ²A1 (C2v) for Iso A and ²A (C1) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and results in a ¹A1 ion of Iso A and a ¹A ion of Iso B. The formation of Iso A from 2-butyne and Iso B from 1-butyne involves the addition of La to the C=C triple bond, the activation of two C(sp³)-H bonds, and concerted elimination of a H2 molecule. The formation of Iso A from 1-butyne and Iso B from 2-butyne involves the isomerization of the two butynes to 1,2-butadiene in addition to the concerted H2 elimination.
Time-resolved Fourier transform infra-red spectroscopy has been employed to identify vibrational modes of several transient molecular and radical species. IR emission from the vinyl radical generated by five different precursor molecules utilizing different dissociation pathways has been observed. An analytical method, two-dimensional cross spectral correlation, has been employed to verify the low energy bending modes of the vinyl radical as well as one stretching mode. Isotopic substitution has allowed several bending modes to be compared to their non-isotopically substituted analog.
Highly vibrationally excited acetylene has also been generated via photochemical reaction. IR emission is modeled according to known harmonic and anharmonic terms of the linear ground electronic state of acetylene. An unexpectedly strong combination band is found in the modeling. Through collisionally induced vibrational energy transfer, energy transfer rates, collisional mass dependence, and the vibrational energy content of acetylene can be obtained.
The production of highly vibrationally excited acetylene has presented the opportunity to study a short lived isomer of acetylene, vinylidene. The influence of vinylidene on acetylene is observed in the energy transfer rates at high energy. An enhancement in the energy transfer rate is observed above the acetylene-vinylidene isomerization barrier. An energy transfer model is used to verify this observation. A theoretical model is postulated which involves a mixing of vibrational states of acetylene and vinylidene above the isomerization barrier thus accounting for the enhancement of the energy transfer rate at high energies.
Another method involving collisions between translationally hot hydrogen atoms and room temperature acetylene molecules generate a significant amount of vibrationally hot acetylene. Possible mechanisms are proposed and tested via isotopic substitution. A mechanism proceeding through a short lived vinyl intermediate dissociating to form a highly vibrationally excited acetylene molecule was found. A combined statistical/impulsive model and classical trajectory calculation confirm the production of a short lived vinyl radical intermediate.
Thiol (SH)-terminated surfaces have been progressively gaining interest over the past years as a consequence of their widespread potential applications. Here, SH-terminated thin films have been prepared by “co-polymerizing” gas mixtures comprising ethylene (C2H4) or butadiene (C4H6) with hydrogen sulfide (H2S). This has been accomplished by either vacuum-ultraviolet (VUV) irradiation of the flowing gas mixtures with near-monochromatic radiation from a Kr lamp, or by low-pressure r.f. plasma-enhanced chemical vapor deposition (PECVD). Varying the gas mixture ratio, R, allows one to control the films’ sulfur content as well as the thiol concentration [SH]. The deposits were characterized by X-ray photoelectron spectroscopy (XPS), before and after chemical derivatization with N-ethylmaleimide, and by ATR FTIR. VUV- and plasma-prepared coatings were found to possess very similar structures and characteristics, showing chemically bonded sulfur concentrations, [S], up to 48 at% and [SH] up to 3%. All coatings remained essentially unchanged in thickness after immersion in water for 24 h.
1,3-Butadiene is an important product in combustion and pyrolysis of hydrocarbon fuels and it is also an important precursor to formpolycyclic aromatic hydrocarbons (PAHs). Currently, a variety of experimental and mechanism studies have been performed on 1,3-butadiene oxidation. However, few studies about pyrolysis mechanism of 1,3-butadiene have been done. In this work, the optimization of the geometries and the vibrational frequencies for the reactants, products, and transition states of the relevant reactions in 1,3-butadiene pyrolysis have been performed at the B3LYP/CBSB7 level. Their single point energies and the thermodynamic parameters are also calculated by using the composite CBS-QB3 method. The high-pressure limit rate constants for tight transition state reactions and barrierless reactions are obtained by transition state theory and variable reaction coordinate transition state theory, respectively. The calculated rate constants in this work are in good agreement with those available from literature. Furthermore, the mechanism of Hidaka et al. is updated with replacing the calculated rate constants of reactions in this work to simulate the shock tube experiment results of 1,3-butadiene pyrolysisand the updated mechanism consists of 45 species and 224 reactions. It can be seen that the updated mechanism can improve the concentration profiles of the main products, ethylene, 1-butylene-3-acetylene, and benzene in 1,3-butadiene pyrolysis. It can also provide reliable kinetic and thermodynamic parameters to further improve the core mechanism of C0-C4 species.
The photodissociation dynamics of the methyl perthiyl (CH3SS) radical are investigated via molecular beam photofragment translational spectroscopy, using "soft" electron ionization to detect the radicals and their photofragments. With this new capability, we have shown that CH3SS can be generated from flash pyrolysis of dimethyl trisulfide. Utilizing this source of radicals and the advantages afforded by soft electron ionization, we have reinvestigated the photodissociation dynamics of CH3SS at 248 nm, finding CH3S + S to be the dominant dissociation channel with CH3 + SS as a minor process. These results differ from previous work reported in our laboratory in which we found CH3 + SS and CH2S + SH as the main dissociation channels. The difference in results is discussed in the light of our new capabilities for characterization of radical production.
The fluorine atom reaction with trans-1,3-butadiene has been investigated by using the crossed molecular beam method. Signals at mass 72, 46 and 33 have been observed. A single reaction channel C 4H 5F + H is observed for this reaction channel. Product angular distributions and velocity distributions are determined. The experimental results indicate that the channel mainly proceeds via a long-lived complex at a collisional energy of 23.8 kj/mol. The collision complex is likely formed with the F atom attacking the delocalized electrons in the direction perpendicular to the molecular plane of trans-1,3-butadiene molecule.
The study focuses on rapid nuclear rearrangement/isomerization taking place in an early stage of butadiene fragmentation using multiple ion momentum imaging spectroscopy and tunable synchrotron radiation. In addition, ab initio calculations are performed to disentangle the nuclear dynamics preceding dissociation.
Cycloaddition reactions have been used both as diversity-generating reactions and as complexity-generating reactions, contributing to the achievement of structural diversity in the first case and of structural complexity in the second. In addition, with their ability to generate new stereocenters, cycloaddition reactions also contribute to the development of stereo structure—activity relationships during screening campaigns. Following a chronological order, this chapter presents an overview of Diels—Alder and inverse electron-demand Diels—Alder reactions, and 1,3-dipolar cycloaddition reactions, including Huisgen [3+2] cycloadditions, nitrone and nitrile oxide cycloadditions, and azomethineylide cycloadditions. Due to the fact that [4+2] cycloaddition reactions usually furnish a high degree of chemo-, regio-, and stereoselectivities, and generate up to four stereocenters in a single step, these reactions have long been adopted for diversity-oriented synthesis (DOS). The chapter then presents miscellaneous cyloaddition reactions, such as [2+2+2], [2+2], formal [4+3], and [3+3].
We have reinvestigated the photodissociation dynamics of the phenyl radical at 248 nm and 193 nm via photofragment translational spectroscopy under a variety of experimental conditions aimed at reducing the nascent internal energy of the phenyl radical and eliminating signal from contaminants. Under these optimized conditions, slower translational energy (P(ET)) distributions for H-atom loss were seen at both wavelengths than in previously reported work. At 193 nm, the branching ratio for C2H2 loss vs. H-atom loss was found to be 0.2 ± 0.1, a significantly lower value than was obtained previously in our laboratory. The new branching ratio agrees with calculated Rice-Ramsperger-Kassel-Marcus rate constants, suggesting that the photodissociation of the phenyl radical at 193 nm can be treated using statistical models. The effects of experimental conditions on the P(ET) distributions and product branching ratios are discussed.
In trans-1,3-butadiene, the ultrafast relaxation from the doubly excited state 21Ag and the corresponding recovery of the ground state 11Ag were observed simultaneously for the first time by time-resolved photoelectron spectroscopy (TRPES) using 29.5 eV high harmonic pulses. The fast recovery of 11Ag shows that the following dissociation upon photoexcitation takes place after returning to the ground state. At 427 fs after photoexcitation, only the ionization energy from the C═C σ bond was found to remain shifted. Accompanying theoretical calculations with an assumption of Koopmans’ theorem show that the ionization energy of the C═C σ bond is modulated by vibrational excitation of the antisymmetric C═C stretching mode. TRPES by high harmonics can probe the changes in the molecular structure sensitively.
Ultrafast recovery of the valence electrons to the ground state in 1,3-butadiene with time constants of 53~99 fs after photoexcitation was observed by time-resolved photoelectron spectroscopy using high harmonic pulses for the first time.
Femtosecond relaxation and picosecond photodissociation dynamics of 1,3-butadiene were investigated by time-resolved photoelectron spectroscopy with high harmonics pulses, probing the deeper molecular orbitals which are sensitive to the molecular structure.
Reactions of C2H with isobutane (k1), 1-butene (k2), isobutylene (k3), 1,3-butadiene (k4), methyl cyanide (k5), ethyl cyanide (k6), and propyl cyanide (k7) are studied at low temperature using a pulsed Laval nozzle apparatus. The C2H radical is prepared by 193-nm photolysis of acetylene, and the C2H concentration is monitored using CH(A 2Δ) chemiluminescence from the C2H + O2 reaction. The rate constants at low and high temperatures are k1 = (1.3 ± 0.3) × 10-10 and (1.0 ± 0.2) × 10-10 for isobutane, k2 = (2.1 ± 0.4) × 10-10 and (2.2 ± 0.4) × 10-10 for 1-butene, k3 = (1.4 ± 0.3) × 10-10 and (1.2 ± 0.2) × 10-10 for isobutylene, and k4 = (2.9 ± 0.6) × 10-10 and (3.3 ± 0.6) × 10-10 for 1,3-butadiene at T = 104 and 296 K, respectively (in units of cm3 molecule-1 s-1). Comparison with existing data shows a trend of a decrease in activation energy with increasing size of the hydrocarbon chain. For these reactions of hydrocarbons containing four carbon atoms, the activation energy is zero within experimental uncertainty, and the rate constants do not depend on temperature in the 104−296 K temperature range. The rate constants for C2H reactions with methyl cyanide, ethyl cyanide, and propyl cyanide are measured at three temperatures, 104, 165, and 298 K. Measured rate constants are fit to an Arrhenius expression and are k5 = (1.8 ± 0.35) × 10-11 exp(−766 ± 38/T), k6 = (1.5 ± 0.3) × 10-11 exp(−145 ± 10/T), and k7 = (2.1 ± 0.4) × 10-11 exp(−51 ± 4/T) (in units of cm3 molecule-1 s-1, T is in Kelvin). At T = 296 K, k3, k5, k6, and k7 are measured as a function of total pressure and show no pressure dependence in the 0.6−8 Torr (0.08−1.07 kPa) pressure range. Results from this work are compared with the results of previous investigations of C2H reactions at low temperature and are discussed in relation to the atmospheres of Saturn and Titan.
Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been carried out to investigate the potential energy surface for the C(3Pj) + C3H5(X 2A1) reaction. The results show that C(3Pj) can add without a barrier either to a terminal CH2 group of the allyl radical to form a metastable intermediate INT1 or between two CH2 groups to produce a bicyclic structure INT2. INT1 immediately isomerizes to buta-1,3-dien-4-yl (INT3), which can decompose to vinylacetylene + H or to the vinyl radical + acetylene. Buta-1,3-dien-4-yl (INT3) can also rearrange to 1,2-dien-4-yl (INT4), and the latter fragments by the H atom loss to vinylacetylene or butatriene. The alternative pathway from the reactants to 1,2-dien-4-yl (INT4) involves two sequential ring openings in the bicyclic intermediate INT2. 1,2-dien-4-yl (INT4) can also rearrange to but-2-yn-1-yl (INT7), which in turn decomposes to butatriene + H. The RRKM theory has been applied to compute rate constants for unimolecular reaction steps and the product branching ratios. Vinylacetylene + H and vinyl radical + acetylene are predicted to be the major reaction products, and their branching ratios strongly depend on the initial branching between the intermediates INT1 and INT2 varying between 29.8/69.8 for 100% of INT1 to 62.4/35.7 for 100% of INT2. Butatriene + H are expected to be the minor products, while the other product channels are much less favorable according to the calculated energies and rate constants.
Photodissociation of 1,3-butadiene at 157 nm has been investigated using the photofragment translational spectroscopy technique. Tunable VUV synchrotron radiation was used for product photoionization detection. Six dissociation channels have been identified including C4H5 + H, C4H4 + H2, C3H3 + CH3, C2H3 + C2H3, C2H4 + C2H2, and C4H4 + H + H. The product kinetic energy distributions for all observed dissociation channels have been determined. The relative branching ratios for different channels are also estimated. It appears that the majority of the excited 1,3-butadiene molecules go through isomerization before they dissociate, while direct bond dissociation channels are less important in comparison with 1,2-butadiene photodissociation at the same excitation wavelength.
The photoionization (PI) cross sections of allyl and 2-propenyl radicals to form C3H5+ were measured using tunable vacuum ultraviolet (VUV) synchrotron radiation coupled with photofragment translational spectroscopy. At 10 eV, the cross sections were found to be 6.2±1.2 and 5.1±1.0 Mb, respectively. Using these values, the PI efficiency curves for each radical were placed on an absolute scale from 7.75 to 10.75 eV.
Photofragment translational spectroscopy was used to study the photodissociation of the methyl perthiyl radical CH(3)SS at 248 nm. The radical was produced by flash pyrolysis of dimethyl disulfide (CH(3)SSCH(3)). Two channels were observed: CH(3) + S(2) and CH(2)S + SH. Photofragment translational energy distributions indicate that CH(3) + S(2) results from C-S bond fission on the ground state surface. The CH(2)S + SH channel can proceed through isomerization to CH(2)SSH on the ground state surface but also may involve production of electronically excited CH(2)S.
The vacuum-ultraviolet photoionization and dissociative photoionization of 1,3-butadiene in a region ∼8.5–17 eV have been investigated with time-of-flight photoionization mass spectrometry using tunable synchrotron radiation. The adiabatic ionization energy of 1,3-butadiene and appearance energies for its fragment ions, C4H5+, C4H4+, C4H3+, C3H3+, C2H4+, C2H3+, and C2H2+, are determined to be 9.09, 11.72, 13.11, 15.20, 11.50, 12.44, 15.15, and 15.14 eV, respectively, by measurements of photoionization efficiency spectra. Ab initio molecular orbital calculations have been performed to investigate the reaction mechanism of dissociative photoionization of 1,3-butadiene. On the basis of experimental and theoretical results, seven dissociative photoionization channels are proposed: C4H5+ + H, C4H4+ + H2, C4H3+ + H2 + H, C3H3+ + CH3, C2H4+ + C2H2, C2H3+ + C2H2 + H, and C2H2+ + C2H2 + H2. Channel C3H3+ + CH3 is found to be the dominant one, followed by C4H5+ + H and C2H4+ + C2H2. The majority of these channels occur via isomerization prior to dissociation. Transition structures and intermediates for those isomerization processes were also determined.
The photodissociation spectroscopy and dynamics resulting from excitation of the 2A″← 2A″ transition of CH2CFO have been examined using fast beam photofragment translational spectroscopy. The photofragment yield spectrum reveals vibrationally resolved structure between 29 870 and 38 800 cm−1, extending ∼6000 cm−1 higher in energy than previously reported in a laser-induced fluorescence excitation spectrum. At all photon energies investigated, only the CH2F+CO and HCCO+HF fragment channels are observed. Both product channels yield photofragment translational energy distributions that are characteristic of a decay mechanism with a barrier to dissociation. Using the barrier impulsive model, it is shown that fragmentation to CH2F+CO products occurs on the ground state potential energy surface with the isomerization barrier between CH2CFO and CH2FCO governing the observed translational energy distributions. © 2004 American Institute of Physics.
Photofragment translational spectroscopy experiments employing tunable vacuum ultraviolet photoionization yielded absolute photoionization cross sections for vinyl and propargyl radicals at 10 eV of 11.1±2.2 and 8.3±1.6 Mb, respectively. From these values, the photoionization efficiency curves from 7.8–10.8 eV for these radicals were placed on an absolute scale. © 2003 American Institute of Physics.
A combination of velocity map ion imaging, mass spectrometry, and a laser-based vacuum ultraviolet light source was used to perform a new measurement of the absolute photoionization cross section of the propargyl radical. The measurements are in good agreement with the recent determination of Savee et al. [J. Chem. Phys. 2012, 136, 134307], and significantly larger than an earlier determination. The results are discussed and rationalized in terms of some general ideas about absolute photoionization cross sections. The potential utility of these ideas is illustrated by using recent cross section measurements for a number of molecular radicals, including methyl, allyl and 2-propenyl, phenyl, and vinyl.
Theoretical studies of F atom reaction with trans-1,3-butadiene were carried out at the CCSD(T)/6-311G(d,p)/B3LYP/6-311G(d,p) levels. Energies and structures for all reactants, products and transition states were determined. Two reaction pathways involving the formation of the complexes CH2CHCHFCH2 and CH2CHCHCH2F were found in this reaction. Theoretical results suggest that the H atom channel observed in previous crossed beam experiment occurs likely via these two long-lived complex formation pathways. For the complex CH2CHCHFCH2 pathway, another reaction channel (C2H3+C2H3F) is also accessible. Relative importance of the C2H3+C2H3F channel versus the H formation channel via the same reaction pathway has also been estimated, suggesting that it would be di±cult to observe the C2H3+C2H3F channel in a crossed molecular beam experiment. Theoretical analysis also shows that the HF formation proceeds via direct abstraction mechanisms, though it is likely a minor process in this reaction.
Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.
We investigated two-body (binary) and three-body (triple) dissociations of ethanedial, propanal, propenal, n-butane, 1-butene, and 1,3-butadiene on the ground potential-energy surfaces using quantum-chemical and Rice-Ramsperger-Kassel-Marcus calculations; most attention is paid on the triple dissociation mechanisms. The triple dissociation includes elimination of a hydrogen molecule from a combination of two separate terminal hydrogen atoms; meanwhile, the rest part simultaneously decomposes to two stable fragments, e.g., C(2)H(4), C(2)H(2), or CO. Transition structures corresponding to the concerted triple dissociation were identified using the B3LYP/6-311G(d,p) level of theory and total energies were computed using the method CCSD(T)/6-311+G(3df, 2p). The forward barrier height of triple dissociation has a trend of ethanedial < propanal < propenal < n-butane < 1-butene < 1,3-butadiene, pertaining to the reaction enthalpy. Ratios of translational energies of three separate fragments could be estimated from the transition structure of triple dissociation. The synchronous concerted dissociation of propanal, propenal, and 1-butene leading to three different types of molecular fragments by breaking nonequivalent chemical bonds is rare. The triple dissociation of propanal, n-butane, 1-butene, and 1,3-butadiene were investigated for the first time. To outline a whole picture of dissociation mechanisms, some significant two-body dissociation channels were investigated for the calculations of product branching ratios. The triple dissociation plays an important role in the three carbonyl compounds, but plays a minor or negligible role in the three hydrocarbons.
The collisionless photodissociation dynamics of isobutene (i-C(4)H(8)) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H + C(4)H(7) and CH(3) + CH(3)CCH(2). Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH(3) loss channel lies 13 kcal mol(-1) higher in energy, the CH(3):H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations.
The photodissociation dynamics of the tert-butyl radical (t-C(4)H(9)) were investigated using photofragment translational spectroscopy. The tert-butyl radical was produced from flash pyrolysis of azo-tert-butane and dissociated at 248 nm. Two distinct channels of approximately equal importance were identified: dissociation to H + 2-methylpropene, and CH(3) + dimethylcarbene. Neither the translational energy distributions that describe these two channels nor the product branching ratio are consistent with statistical dissociation on the ground state, and instead favor a mechanism taking place on excited state surfaces.
Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical C(6)H(5) at 248 and 193 nm. At 248 nm, the only dissociation products observed were from H atom loss, attributed primarily to H+o-C(6)H(4) (ortho-benzyne). The observed translational energy distribution was consistent with statistical decay on the ground state surface. At 193 nm, dissociation to H+C(6)H(4) and C(4)H(3)+C(2)H(2) was observed. The C(6)H(4) fragment can be either o-C(6)H(4) or l-C(6)H(4) resulting from decyclization of the phenyl ring. The C(4)H(3)+C(2)H(2) products dominate over the two H loss channels. Attempts to reproduce the observed branching ratio by assuming ground state dynamics were unsuccessful. However, these calculations assumed that the C(4)H(3) fragment was n-C(4)H(3), and better agreement would be expected if the lower energy i-C(4)H(3)+C(2)H(2) channel were included.
Using pulsed H-atom Lyman-alpha laser-induced fluorescence spectroscopy along with a photolytic calibration approach, absolute H-atom product quantum yields of phi(H-b13d) = (0.32+/-0.04) and phi(H-b12d) = (0.36+/-0.04) were measured under collision-free conditions for the 193 nm gas-phase laser flash photolysis of buta-1,3- and buta-1,2-diene at room temperature, which demonstrate that nascent H-atom formation is of comparable importance for both parent molecules. Comparison of the available energy fraction, f(T-b13d) = (0.22+/-0.03) and f(T-b12d) = (0.13+/-0.01), released as H+C(4)H(5) product translational energy with results of impulsive and statistical energy partitioning modeling calculations indicates that for both, buta-1,3- and buta-1,2-diene, H-atom formation is preceded by internal conversion to the respective electronic ground state (S(0)) potential energy surfaces. In addition, values of sigma(b-1,3-d-L alpha) = (3.5+/-0.2)x10(-17) cm(2) and sigma(b-1,2-d-L alpha) = (4.4+/-0.2)x10(-17) cm(2) for the previously unknown Lyman-alpha (121.6 nm) radiation photoabsorption cross sections of buta-1,3- and buta-1,2-diene in the gas-phase were determined.
The nu(4) + nu(5) combination band, which appears relatively weak in the IR absorption spectrum, has been identified with exceptionally high intensity in the IR emission spectra from highly vibrationally excited acetylene, which is produced with approximately 71 kcal mol(-1) of vibrational energy from the 193 nm photolysis of vinyl bromide. The 'fundamental' transition of this combination band, from the (0,0,0,1(1),1(-1)) level to the zero point, occurs at 1328 cm(-1). The intensity and frequency of this band as well as the nu(3) and nu(5) bands, IR active but with lower emission intensity, as a function of the acetylene energy can be modeled accurately using the normal mode harmonic oscillator model with frequency anharmonicity corrections. Good fitting results are achieved even though the normal mode quantum numbers are no longer good for levels in the high energy region and the combination band is forbidden in the harmonic oscillator model. The identification of this intense combination band in emission, compared to its weak intensity in the absorption spectrum, highlights the necessity to include in consideration the combination bands for assignment of emission spectra in general and in particular emission from vibrationally hot acetylene which is ample in combustion, atmospheric, and interstellar environments.
A unified synthetic approach to diverse polycyclic scaffolds has been developed using transition-metal-mediated cycloaddition and cyclization reactions of enynes and diynes. The tert-butylsulfinamide group has been identified as a particularly versatile lynchpin in these reactions, with a reactivity profile uniquely suited for efficient, stereoselective substrate synthesis and downstream transformations. This approach provides 10 distinct, functionalized scaffold classes related to common core structures in alkaloid and terpenoid natural products.
The vibronic excitation spectrum of phenylcyclopenta-1,3-diene (PCP3D) has been recorded in a supersonic expansion using resonant-two-photon ionization (R2PI) and laser-induced fluorescence (LIF) techniques. The spectrum is dominated by the S(0)-S(1) origin transition (31,739 cm(-1)), with several low-frequency vibronic bands in the first 400 cm(-1), followed by a sharp cut-off in intensity due to turn-on of a non-radiative process. Single vibronic level fluorescence (SVLF) spectra were recorded for the S(1) origin and several vibronic bands of PCP3D. The excitation and emission spectra show that the molecule is planar with C(s) symmetry in both the ground and excited states. Torsional potentials were simulated from the observed torsional structure in the excitation and emission spectra. The S(0) potential (V(2) = 1237 cm(-1), V(4) = -256 cm(-1)) is associated with a flat-bottomed potential supporting large inter-ring angular changes with little cost in energy (+/-36 degrees at 200 cm(-1)), with a barrier of 1237 cm(-1) at the perpendicular geometry. The S(1) potential is much stiffer about the planar geometry, with a calculated barrier five times larger than in S(0) (V(2) = 6732 cm(-1), V(4) = -477 cm(-1)). Based on the torsional assignments, weak bands in the same frequency region assigned earlier to the structural isomer phenylcyclopenta-1,4-diene [J. J. Newby, J. A. Stearns, C. P. Liu and T. S. Zwier, J. Phys. Chem. A, 2007, 111, 10914-10927] have been re-assigned as hot bands arising from v'' = 1 in the inter-ring torsion, nu(57).
Photodissociation dynamics of C(2)F(5)I near 280 and 304 nm has been investigated on a small and simple time-of-flight photofragment translational spectrometer (PTS). On this new PTS, the photolyzed and ionized fragments, not accelerated by electric field, travel freely for a short flight path (<50 mm) and are detected by microchannel plates. In the spectra of the I(*)((2)P(1/2)) channel at 281.73 and 304.02 nm, vibrational peaks with spacing of approximately 350 cm(-1) are partially resolved, indicating the preferential excitation of CF(2) wag mode (nu(11)=366 cm(-1)) of C(2)F(5) photofragment. The fraction of the available energy disposed into the internal energy is higher than 50% for both I(*) channel and I channel, showing the high excitation of vibration in the C(2)F(5) fragments. The fragment recoil anisotropy parameter beta(I(*)), determined to be 1.70 at 281.73 nm and 1.64 at 304.02 nm, reveals that I(*) atoms are produced predominantly from the parallel (3)Q(0) <-- N transition. The anisotropy parameter beta(I), determined to be 1.25 at 279.71 nm and 0.88 at 304.67 nm, implies that I atoms are produced from two excited states, i.e., direct dissociation via the perpendicular (3)Q(1) <-- N transition, and indirect dissociation via the parallel (3)Q(0) <-- N transition then curve crossing to the (1)Q(1) potential energy surface. Analysis on the recent studies with vibrational state resolution in the photodissociation of alkyl iodides in the A band reveals that the "symmetric bending" mode on alpha-carbon of alkyl iodides is the preferential vibrational excitation mode, which can be explained by the classic impulsive model.
Following the initial report of the detection of fundamental transitions of all nine vibrational modes of the vinyl radical [Letendre , L. ; Liu , D.-K. ; Pibel , C. D. ; Halpern , J. B. ; Dai , H.-L. J. Chem. Phys. 2000 , 112 , 9209] by time-resolved IR emission spectroscopy, we have re-examined the assignments of the vibrational modes through isotope substitution. Precursor molecules vinyl chloride-d3, vinyl bromide-d3, and 1,3-butadiene-d6 are used for generating vibrationally excited vinyl-d3 through 193 nm photolysis. The nondeuterated versions of these molecules along with vinyl iodide and methyl vinyl ketone are used as precursors for the production of vinyl-h3. IR emission following the 193 nm photolysis laser pulse is recorded with nanosecond time and approximately 8 cm(-1) frequency resolution. A room-temperature acetylene gas cell is used as a filter to remove the fundamental transitions of acetylene, a photolysis product, in order to reduce the complexity of the emission spectra. Two-dimensional cross-spectra correlation analysis is used to identify the emission bands from the same emitting species and improve the S/N of the emission spectra. Isotope substitution allows the identification of several low-frequency vibrational modes. For C2H3, the assigned modes are the nu4 (CC stretch) at 1595, nu5 (CH2 symmetric bend) at 1401, nu6 (CH2 asymmetric + alpha-CH bend) at 1074, nu8 (CH2 + alpha-CH symmetric out-of-plane (oop) bend) at 944, and nu9 (CH2 + alpha-CH asymmetric oop bend) at 897 cm(-1). For C2D3, the modes are the nu5 (CD2 symmetric bend) at 1060, nu6 (CD2 asymmetric + alpha-CD bend) at 820, and nu8 (CD2 + alpha-CD symmetric oop bend) at 728 cm(-1).
Vacuum ultraviolet radiation was generated from an undulator at the Advanced Light Source Synchrotron facility and used for photoionization detection of reaction products in a new universal crossed molecular beams machine. A description of the machine and its performance is presented. Initial experiments on the photodissociation of methylamine (CH{sub 3 }NHâ), ozone (Oâ), oxalyl chloride [ (OCCl)â] as well as the reactive scattering of Cl with Câ Hâ show many of the advantages of photoionization in comparison to electron impact ionization, which has been exclusively used in such instruments in the past. {open_quotes}Momentum matching{close_quotes} of reaction products is much more easily accomplished than in electron impact studies due to suppression of dissociative ionization. The tunability of the vacuum ultraviolet radiation can be used to suppress background from residual gases especially when it is desired to detect free radical reaction products. Even when the tunability cannot be used to suppress background, the fact that little heat is generated by the ionizing beam allows background to be substantially suppressed by cryogenic pumping. The energy resolution of the apparatus is comparable to instruments that have previously been designed with electron impact ionization which have more than twice as long a flight path. This new instrument provides outstanding performance for fundamental studies of chemical dynamics. {copyright} {ital 1997 American Institute of Physics.}
The ultraviolet photoelectron spectrum of HXC=C− (X=C2H3) is reported along with ab initio calculations. The adiabatic electron affinity of the X 1A′ state is measured to be 0.914(15) eV for C4H4 and 0.909(15) eV for C4D4. The term energy of the C4H4 ã3A′ state is 1.923(15) eV and the b 3A″ term energy is 2.035(30) eV. Geometries and frequencies of several stationary points on the C4H4 and C4D4 neutral and anion surfaces at the configuration interaction with singles and doubles level of theory are reported, as well as an intrinsic reaction coordinate calculation at the restricted Hartree Fock level on the C4H4 singlet surface. Calculations and experiment are combined to estimate the lifetime of singlet vinylvinylidene for rearrangement to vinylacetylene to be 20–200 fs, corresponding to lifetime broadening of 35–3 meV. © 1995 American Institute of Physics.
The photodissociation dynamics of propyne and allene are investigated in two molecular beam/photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to photoionize the photoproducts. The primary dissociation channels for both reactants are C3H3+H and C3H2+H2. Measurement of the photoionization efficiency curves on the VUV instrument shows that the C3H3 product from propyne is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactants is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in both cases. © 1999 American Institute of Physics.
Photofragment translational spectroscopy has been used to investigate the dissociation dynamics of 1,2-butadiene at 193 nm. Ionization of scattered photoproducts was accomplished using tunable VUV synchrotron radiation at the Advanced Light Source. Two product channels are observed: CH3+C3H3 and C4H5+H. The C3H3 product can be identified as the propargyl radical through measurement of its photoionization efficiency curve, whereas the C4H5 product cannot be identified definitively. The translational energy P(ET) distributions suggest that both channels result from internal conversion to the ground electronic state followed by dissociation. The P(ET) distribution for the C4H5 product is sharply truncated below 7 kcal/mol, indicating spontaneous decomposition of the slowest C4H5 product. © 2001 American Institute of Physics.
Advances in the study of photodissociation dynamics over the past 30 years are reviewed. An overview of experimental techniques that have been developed to extract photofragment energy and angular distributions is presented, followed by a discussion on several current topics of interest in the field of photodissociation. 181 refs., 10 figs.
The photoionization spectrum of vinyl radical is reported, from its observed threshold to 1160 A˚. Two methods of preparation have been employed; (a) the abstraction reaction of F atoms with C2H4, and (b) the pyrolysis of divinyl mercury at 1200 K. In both experiments, relatively sharp autoionization structure is observed, and interpreted as a Rydberg series converging to the excited 3A‘ state of vinyl cation. The analysis leads to an adiabatic ionization energy of ∼10.7 eV for this state, with a structure similar to that of vinyl radical but with an increased C–C distance. The observed ionization threshold for the ground state of vinyl cation is 8.59±0.03 eV with the F atom reaction, and 8.43±0.03 eV with the pyrolysis method. The lower value in the latter experiment is interpreted as a hot band. The relatively low value of the photoionization cross section near threshold implies a large geometry change between vinyl radical and ground state vinyl cation. A progression in the in-plane C–H bending vibration is indicated in the photoionization spectrum; it is quite possible that the vibrational 0–0 transition lies one quantum lower than our detected limit. With this bracketed adiabatic ionization potential and the appearance potential of C2H+3 (C2H4), a C–H bond energy in ethylene of 107–110 kcal/mol (0 K) is deduced.
In the present paper, we report the direct absorption spectra of the 1 1B+u←1 1A−g transitions of gas phase butadiene, deuterated and methylated butadienes, and the cis and trans isomers of hexatriene cooled to low rotational and vibrational temperatures in supersonic molecular jets. These jet absorption spectra allow the more accurate determinations of Franck–Condon factors, upper state vibrational intervals and vibronic band homogeneous widths. We discuss the experimental constraints that the measurements reported here and in the previous paper of this series impose on theoretical models of the equilibrium structures and relaxation dynamics of the 1 1B+u excited states of the small linear polyenes.
We have measured the near ultraviolet absorption spectrum of 2,3-dideuterobutadiene to provide a complete set of experimental Bu←X vibrational intervals and bandwidths for all symmetrically deuterated butadienes. These vibrational intervals and bandwidth ratios are compared with the ground state vibrational frequencies and frequency ratios of the molecules. The prominent vibrational frequency interval observed in transitions to the Bu state of butadiene is demonstrated to arise predominantly from a kinetic coupling of the C=C stretching and CH wagging vibrations. The experimental bandwidth ratios are shown to correlate with single quanta of the ground state au CH2 twist frequency interval ratios. From the latter, a plausable decay path for the Bu excited state of butadiene is deduced. The implications of these conclusions on prior and present attempts to determine the butadiene Bu equilibrium geometry and to understand polyene spectroscopy, photochemistry, and photophysics are discussed.
The ultrafast dynamics of the 11Bu state of 1,3-butadiene was investigated in real time in a femtosecond pump–probe experiment utilizing the RE2PI scheme. A lifetime of 35 ± 10 fs was found for the 11Bu state of the jet-cooled molecule.
Ab initio calculations have been carried out on the lowest energy isomers of C4H5 to predict spectroscopic properties and relative stabilities. Geometry optimizations were carried out on stationary point conformations of seven configurational isomers at UHF, B3LYP, MP2, CISD, QCISD, MCSCF(7,7), and MCSCF(9,9) levels of theory. Single-point energies were evaluated at the MP4, CCSD(T), MCSCF(11,11), and multireference CISD levels. Disparities as large as 70 kJ mol-1 are found between relative energies predicted by single- and multireference methods for the same isomer. Comparison to experimental values suggests that the multireference methods inadequately model the relative correlation energy. Zero-point corrected relative energies (in kJ mol-1) obtained at the QCISD level with a 6-311G(d,p) basis set are the following: 2-butyn-1-yl (0); 1-butyn-3-yl (10); 1,2-butadien-4-yl (13); cyclobuten-3-yl (17); 1,3-butadien-2-yl (50); 1,3-butadien-1-yl (59); and 1-butyn-4-yl (64). Relative energies calculated by multireference methods are higher, and decrease slowly as the active space size increases. Relative energies and hyperfine constants obtained at the QCISD level are in agreement with available experimental data and empirical estimates. Some of these isomers are candidates for relocalization, a phenomenon that results in predicted multiple minima and unusually flat vibrational potential energy surfaces for the isoelectronic C3H3O isomers. Of the present series of molecules, only 2-butyn-1-yl exhibits an especially flat bending potential along the appropriate isomerization coordinate. Predicted vibrational transition frequencies and intensities, dipole moment components, Fermi contact hyperfine constants, and conformational potential energy curves are presented.
The photolysis of 1,3-butadiene was investigated using xenon (1470 Å) and krypton (1236 Å) resonance radiation and was compared with the 2200-2600-Å photolysis. Collisional deactivation of the excited 1,3-butadiene molecule observed in the far-ultraviolet photolysis is absent in the vacuum-ultraviolet region. The distribution of major products formed in the 1470- and 1236-Å photolysis, acetylene, ethylene, and vinylacetylene, showed neither appreciable energy nor pressure dependence. The cleavage of the central carbon-carbon bond to produce two vinyl radicals was demonstrated by means of H2S as a free-radical interceptor. It is suggested that vinylacetylene is formed through hydrogen atom elimination, as opposed to a molecular hydrogen elimination. In the krypton photolysis ionization is expected, but no products could be identified as originating exclusively from an ion-molecule path. Speculation as to the formation of ethane, ethyl radicals, and propylene via an ion-molecule reaction path is discussed.
An undulator beamline, consisting of VUV high resolution and high flux branch lines, devoted to chemical dynamics experiments has been designed at the Advanced Light Source. The radiation source is an undulator having a 10-cm period, and the fundamental in the energy range from 6 to 30 eV is utilized by the experiments. The higher harmonics of the undulator due to the operation at high K values is suppressed by a novel gas filter. In the first branch, high-flux 2% bandwidth radiation is directed toward an end station for photodissociation. A photon flux calculation predicts 1016 photon/s at this end station. In the second branch, highly monochromatized radiation is sent to an end station for photoelectron spectroscopy and photoionization studies. For this purpose a vertical dispersion 6.65-m off-plane Eagle mounting was chosen for the monochromator in anticipation of achieving a resolving power of ~ 5 × 104 with a 1200 grooves/mm grating and 1 × 105 to 2 × 105 with a 4800 grooves/mm grating.
Simple equations for calculating bond dissociation energies were deduced empirically from the great number of experimentally determined bond dissociation energies now available in the literature. Thus, for the bond R1-R2 where R1 is CH3, CX3, C6H5, RC≡C-, R2C=CR-, RCO and CN, and R2 is R1, H, X, CN, OH, OR, NR2, NO, NO2 and SR, the bond dissociation energy (D) is given by D = 71∈1∈2 where ∈1 is a constant characteristic of the group R1. The ∈ of a group R = A1C2 bonds on the righthand side A3A2 is related to the three substituents on the carbon atom attached to the bond in question by ∈g = 0.43 + 0.162 ∑∈i. The calculated bond dissociation energies agree in most cases within 1-2 kcal. with the observed ones.
Photoisomerization dynamics of s-trans butadiene is investigated by a semiclassical surface hopping trajectory method. The Heisenberg model Hamiltonian is developed to describe two covalent states involved in this process, 11Ag and 21Ag states. This model Hamiltonian well reproduces a global structure of the potential energy surfaces of these states and the nonadiabatic coupling of an ab-initio method. It was found that the nonadiabatic decay from the 21Ag to the 11Ag state takes place at the three partially twisted CC bond conformations. The CCC bending motions largely enhance the nonadiabatic transitions. The internal vibrational relaxation associated with the nonadiabatic transition is also analyzed. © 1997 American Institute of Physics.
The singlet–triplet splitting in methylene has been determined from the measurements of fragment velocities from ketene photodissociation at 351 and 308 nm in a molecular beam. The splitting is found to be 8.5±0.8 kcal/mol. This agrees with many experimental results, but not with the value of 19.5 kcal/mol derived from recent photodetachment experiments on CH−2.
At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular beam experiments. A photon flux of 1016 photon/sec has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionization studies. At this second end station a peak flux of 3 X 1011 was observed for 25,000 resolving power. This monochromator has achieved a resolving power of 70,000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a VUV monochromator.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
We report the zero kinetic energy photoelectron spectrum of the propargyl radical, C3H3. From the spectrum an ionization energy of 69 953 +/- 10 cm(-1) (8.673 eV) is deduced. Vibrational frequencies are obtained for the totally symmetric normal modes of the propargyl cation, as well as some combination and overtone bands. Both the frequencies and the relative intensities agree well with the predictions from recent ab initio calculations. (C) 2000 American Institute of Physics. [S0021-9606(00)02006-7].
An approach for detecting the vibrational spectrum of transient species is demonstrated on the vinyl radical. Photodissociation of carefully chosen precursors at selected photolysis wavelengths produce highly vibrationally excited radicals. Infrared (IR) emission from these radicals is then measured by time-resolved Fourier transform spectroscopy with nanosecond time resolution. All nine vibrational bands of the vinyl radical, generated from four different precursors, are obtained and reported here for the first time. © 2000 American Institute of Physics.
Resonance Raman spectra of buta‐1,3‐diene‐d0 and buta‐1,3‐diene‐1,1,4,4‐d4 have been obtained with ultraviolet excitation from 239.5 to 199.9 nm. Activity of the first overtone of mode 24, the bu symmetry CCC chain deformation mode, is observed with excitation energy below the origin of the 1 1Bu state. This vibronic activity of a nontotally symmetric mode is shown to be evidence of resonance with the 2 1Ag state of butadiene. A quantitative analysis of the ratio of intensities of 2ν24 to ν9, the ag symmetry CCC chain deformation mode, demonstrates that enhancement of 2ν24 cannot be due to resonance with the 1 1Bu state. The resonance enhancement behavior of this overtone band also shows that it is of vibronic origin rather than Franck–Condon allowed. The intensity pattern seen for the modes of bu symmetry is fully consistent with the results of a quantitative calculation of vibronic activity for the eight bu symmetry modes. The 2 1Ag electronic state is estimated to be ca. 0.25 eV below the 1 1Bu electronic state. Overtones of out‐of‐plane C–H bending and CH2 twisting modes are seen with excitation radiation near the peak of the transition to the 2 1Ag state, indicating that the 2 1Ag state of butadiene has appreciably lower resistance to deformation along out‐of‐plane coordinates than does the ground electronic state. This is consistent with the expectations of semiempirical calculations.
The photolysis of 1,3-butadiene in the vapor phase was studied in the presence of inert gases such as krypton, diethyl ether, and cyclohexane. In every instance, an increase in the pressure of the added gas led to a decrease in all of the volatile products. The behavior of 1,2-butadiene and 2-butyne were exceptional in that they showed a slight increase in yield when a small quantity of an inert gas was added. But at higher pressures, even in these cases, the yields decreased. A consideration of the radiative lifetime of the first-singlet excited state of 1,3-butadiene suggests that the observed pressure dependence cannot be attributed to a quenching reaction from that state. It seems likely that all of the volatile products in the vapor phase photolysis arise from a vibrationally excited ground state molecule of 1,3-butadiene that is formed by the internal conversion of the electronically excited singlet state. Photolysis of 1,3-butadiene in solution, which is equivalent to photolysis at very high pressures, yielded none of the volatile products that were observed in the vapor phase but only cyclobutene, bicyclo [1.1.0] butane, dimers, and a polymer. The cyclobutene and bicyclobutane are believed to be products of isomerization from the electronically excited state of 1,3-butadiene.
The formation of a polymer in the gas-phase photolysis was found to have an induction period and to depend on the square of the number of photons that were absorbed. In all probability, the polymer is formed in the photolysis of a volatile product(s) of the primary photodecomposition.
Infrared multiphoton dissociation of CF2Cl2 has been reinvestigated by the crossed laser‐molecular beam technique using a high repetition rate CO2 TEA laser. Both the atomic and molecular chlorine elimination channels were observed: (1) CF2Cl2→CF2Cl+Cl and (2) CF2Cl2→CF2+Cl2. No evidence was found for secondary dissociation of CF2Cl at laser energy fluences up to 8 J/cm2. Center‐of‐mass product translational energy distributions were obtained for both dissociation channels. In agreement with previous work, the products of reaction (1) are found to have a statistical translational energy distribution. The products of reaction (2) are formed with a mean translational energy of 8 kcal/mol, and the distribution peaks rather sharply about this value, indicating a sizeable exit barrier to molecular elimination. The product branching ratio was directly determined. Reaction (2) accounts for roughly 10% of the total dissociation yield in the fluence range 0.3–8 J/cm2. These results provide an additional test of the statistical theory of unimolecular reactions.
Threshold collision-induced dissociation in a guided ion beam mass spectrometer is used to determine thermochemistry and structure for the Fe+-alkene and metallacyclobutane structures of FeC3H6+. A flow tube source (which ensures thermalization) is used to produce FeC3H6+ as the adduct of Fe+ with cyclopropane or propene, or by reaction of Fe+ with propane or cyclobutanone. Differences in reactivity and thermochemistry of ions produced in different ways along with results of the bimolecular reactions of Fe+ with cyclopropane and propene are used to infer structural differences. We determine 0 K bond dissociation energies of 39.6 +/- 1.5 kcal/mol for Fe+.propene, 31.8 +/- 1.0 kcal/mol for the metallacycle to dissociate to Fe+ + cyclopropane, and 82.6 +/- 1.5 kcal/mol for Fe+-CH2. Arguments are also presented for elimination of ethylidene from Fe+.propene, and the threshold for this process provides the first experimental measurement of the DELTA(f)H-degrees-0 of CH3CH (ethylidene), 73 +/- 7 kcal/mol.
The mechanisms of the three primary processes in the vapor phase photolysis of 1,3-butadiene have been investigated by the use of deuterium labeling on the end carbon atoms. None of the processes proceeds by the obvious pathway exclusively. Thus ethylene and acetylene are formed not only by a 1,3 shift but also via an intermediate cyclobutene and a third path which gives C2H2D2 and C2D2. Two mechanisms seem to be applicable to the other two primary processes which give 1,2-butadiene and H2 + C4H4 respectively.
Negative ion photoelectron spectroscopy and gas-phase proton transfer kinetics were employed to determine the CH bond dissociation energies of acetylene, ethylene, and vinyl radical: Dâ(HCC-H) = 131.3 {plus minus} 0.7 kcal molâ»Â¹, Dâ(CHâCH-H) = 109.7 {plus minus} 0.8 kcal molâ»Â¹, and Dâ(CHâC-H) = 81.0 {plus minus} 3.5 kcal molâ»Â¹. The strengths of each of the other CH and CC bonds in acetylene and ethylene and their fragments were derived. The energy required to isomerize acetylene to vinylidene was also determined: HC{triple bond}CH â HâC{double bond}C: ÎH{sub isom,0} = 47.4 {plus minus} 4.0 kcal molâ»Â¹. As part of this study, proton transfer kinetics in a flowing afterglow/selected-ion flow tube apparatus were used to refine the acidities of ethylene, acetylene, and vinyl.
Electron affinities and Delta H-acid are combined in a thermochemical cycle to arrive at bond dissociation energies for allene, methylacetylene, and the propargyl radical: D-0(CH2=C=CH-H) = 88.7 +/- 3 kcal mol(-1), D-0(H-CH2C=CH) = 90.3 +/- 3 kcal mol(-1), D-0(CH3C=C-H) = 130.2 +/- 3 kcal mol(-1), and D-0(CH2=C=C-H) = 100 +/- 5 kcal mol(-1). Electron affinity measurements were determined using negative ion photoelectron spectroscopy and yielded the following for the propargyl, 1-propynyl, and propadienylidene radicals: EA(CH2=C=CH) = 0.918 +/- 0.008 eV, EA(CH3C=C) = 2.718 +/- 0.008 eV, and EA(CH2=C=C) = 1.794 +/- 0.008 eV. Gas-phase acidity measurements were made using proton transfer kinetics in a flowing afterglow/selected-ion flow tube and yielded the following for allene, methylacetylene, and the propargyl radical: Delta G(acid)(CH2=C=CH-H) = 372.8 +/- 3 kcal mol(-1), Delta G(acid)(H-CH2C=CH) = 374.7 +/- 3 kcal mol(-1), Delta G(acid)(CH3C=C-H) = 373.4 +/- 2 kcal mol(-1), and Delta G(acid)(CH2=C=CH) = 364 +/- 5 kcal mol(-1). Delta G(acid) was converted to Delta H-acid by employing Delta S-acid: Delta H-acid(CH2=C=CH-H) = 381.1 +/- 3 kcal mol(-1), Delta H-acid(H-CH2C=CH)= 382.7 +/- 3 kcal mol(-1), Delta H-acid(CH3C=C-H) = 381.1 +/- 3 kcal mol(-1), and Delta H-acid(CH2=C=CH) = 372 +/- 5 kcal mol(-1). Evidence is provided for the isomerization of the allenyl anion (CH2=C=CH-) to the 1-propynyl anion (CH3C=C-) in the proton transfer reactions of CH2=C=CH- with CH3OH and CH3CH2OH. This complexity limits the precision of experimental measurements. This study explores the intricacies of determining gas phase acidity values by proton transfer reactions for systems in which isomerization can occur.
An empirical corresponding-states relation for liquid viscosity for a wide variety of substances is given. This relationship, which correlates the experimental viscosity with the reduced temperature parameter Tm/T, allows liquids to be categorized according to their chemical and molecular nature. The status of other empirical viscosity-temperature correlations is also discussed.
A single-pulse shock tube has been used to study the kinetics of ethylene pyrolysis in the range 1300-1800°K., where the chief products are acetylene, 1,3-butadiene and hydrogen. The reaction to form acetylene is essentially first order, and that to form butadiene is second order, with respect to ethylene partial pressure. The reactions seem to be molecular rather than free-radical in mechanism. The hydrogenation of acetylene to ethylene also was studied, and by relating the forward and reverse reactions by the equilibrium constant, it was shown that the shock tube temperature calculations were accurate within 30-50°.
A systematic study of the 213.8-nm (zinc line) photochemistry of 1,3-butadiene has been made either in the absence or in the presence of various additives - such as radical scavengers (O2, NO, DI) and collisional quenchers - in the gas phase (pressure between 1 and 500 Torr). The major fate of the photoexcited 1,3-butadiene molecule is isomerization to the 1,2-butadiene structure which may then decompose to methyl and C3H3 radicals (Φ = 0.64 ± 0.04 at 1 Torr of 1,3-butadiene). Minor processes include decomposition to the acetylene + ethylene couple (Φ = 0.22 ± 0.02) or to vinylacetylene (Φ = 0.038 ± 0.003) and molecular hydrogen. These two minor processes occur from different excited states. Some 2-butyne (Φ < 0.015) is formed by a unimolecular isomerization process. The photolysis of 1,3-butadiene-1,1,4,4-d4 indicates that at least three different intermediates are involved in the formation of molecular ethylene and acetylene. The C3H3 radicals are not easily intercepted by DI: k(C3H3 + 1,3-butadiene)/k(C3H3 + DI) = 0.09 ± 0.03. Also at 21°C and for [DI]/[1,3-butadiene] = 10, the highest ratio used, Φ(allene + propyne)/Φ(CH3D) = 0.72 and a fraction of the C3H3 radicals are still not accounted for (reaction with 1,3-butadiene and/or recombination?). The relative energies obtained by ab initio RHF-SCF geometry optimizations for the doublet electronic state of the C3H3 radical structures are E(propargyl) < E(propyn-1-yl) < E(cyclopropen-1-yl) < E(allenyl). General valence bond geometry optimizations and a multiconfigurational self-consistent-field surface scan also show that the propargyl species (2B1 state) is the lowest energy one. There are probably at least two distinct C3H3 radical structures (different states) present in the far-UV photolysis of 1,3-butadiene.
Franck-Condon factors are calculated for alternative forms of the potential energy surface of the 11Bu+ excited state of butadiene. A comparison of these results with the experimental absorption spectrum indicates that the 11Bu+ state of butadiene may have a nonplanar minimum, but that the twist around a terminal bond is no larger than 30°; for the 21Ag- excited state larger distortions from planarity may occur. In treating the transition from a planar ground state to nonplanar excited states, we have shown that a large number of modes and combination terms have significant Franck-Condon factors. The complex nature of the excited-state surfaces and the coupling between the 11Bu+ and 21Ag- states are discussed and suggested as an explanation of some of the unusual photophysical properties of butadiene.
The microwave spectrum of the charge-transfer complex between dimethylamine and sulfur dioxide was studied with a pulsed molecular beam Fourier transform microwave spectrometer. The rotational constants (in MHz) of (CH3)2NH.SO2 are A = 4445.495 (3), B = 2063.031 (1), and C = 1752.470 (1). In addition to the normal isotopic form, the rotational spectra of the (CH3)2NH.34SO2, (CH3)2(15)NH.SO2, (CH3)2ND.SO2, and two (CH3)2NH.SO18O isotopic species were assigned. Stark effect measurements gave electric dipole components of mu-a = 4.025 (1), mu-c = 1.747 (2), and mu-total = 4.388 (1) D. The structure of this complex lacks any symmetry plane. The nitrogen lone pair points toward the S atom, nearly perpendicular to the plane of the sulfur dioxide, and one methyl group staggers the oxygen atoms. The nitrogen-to-sulfur distance of 2.34 (3) angstrom is about 0.08 angstrom longer than in the trimethylamine-SO2 complex which correlates with the relative strength of the complexes. From the dipole moment and the nitrogen nuclear quadrupole coupling constants, an upper limit is estimated for electron transfer from the nitrogen to the sulfur atom of 0.25 electron. Ab initio calculations also conclude that a methyl group staggers the two oxygens of SO2.
A scheme is presented for the calculation of electron impact total ionization ratios for organic molecules. The scheme is based on the additivity of atomic ionization cross sections. The coefficients for the calculation are determined by a linear regression using 179 total ionization cross section measurements taken from the literature. The average error in the prediction of relative total ionization cross section for the 179 molecules was 4.69 % by this approach. Other approaches employing diamagnetic susceptibility and molecular volume are shown to be inferior to the atomic additiilty scheme.
1,3-Butadiene (1,3-C4H6) was heated behind reflected shock waves over the temperature range of 1200–1700 K and the total density range of 1.3 × 10−5 −2.9 × 10−5 mol/cm3. Reaction products were analyzed by gas-chromatography. The concentration change of 1,3-butadiene was followed by UV kinetic absorption spectroscopy at 230 nm and by quadrupole mass spectrometry. The major products were C2H2, C2H4, C4H4, and CH4. The yield of CH4 for a 0.5% 1,3-C4H6 in Ar mixture was more than 10% of the initial 1.3-C4H6 concentration above 1500 K. In order to interpret the formation of CH4 successfully, it was necessary to include the isomerization of 1,3-C4H6 to 1,2-butadiene (1,2-C4H6) and to include subsequent decomposition of the 1,2-C4H6 to C3H3 and CH3. The present data and other shock tube data reported over a wide pressure range were qualitatively modeled with a 89 reaction mechanism, which included the isomerizations of 1,3-C4H6 to 1,2-C4H6 and 2-butyne (2-C4H6). © 1996 John Wiley & Sons, Inc.
The high temperature pyrolysi of 1,3-butadiene has been investigated in the shock tube with two time-resolved diagnostic techniques: laser schlieren measurements of density gradient with 1, 2, 4, and 5% C4H6 in Ar or Kr, 0.26 < P2 < 0.66 atm, over 1550–2200 K, and time-of-flight mass spectra for 3% C4H6–Ne, P5 ∼ 0.4 atm, 1400–2000 K. When combined with a recent single-pulse shock tube product analysis covering 1050–2050 K, these measurements permit a complete modeling of major species in C4H6 pyrolysis. Extrapolated density gradients and product analyses show initiation is dominated by C4H6 → 2C2H3., significant falloff and Arrhenius curvature being seen in the derived rates. A restricted rotor, Gorin model RRKM fit to these rates with reasonable parameters generates
The derived barrier, ΔH = 99 ± 4 kcal/mol, translates to ΔH,298 = 63.4 ± 2 kcal/mol for the heat of formation of vinyl radical. A mechanism for the formation of all products detected in the above experiments is given, together with a successful but semiquantitative kinetic model for major products. The measurements require the rate of vinyl radical dissociation, C2H3 + M → C2H2 + H + M, to be extremely low, k < 109 cm3/mol s for 1600 K, so that the dominant chain carrier in C4H6 pyrolysis is vinyl radical.
The potential energy surfaces of the , , and electronic states of trans-butadiene have been investigated with the complete-active-space self-consistent-field (CASSCF) method as well as second-order perturbation theory based on the CASSCF reference (CASPT2). Symmetry-adapted valence internal coordinates are employed to describe large-amplitude deformations from the ground-state equilibrium geometry. Both in-plane and out-of-plane deformations have systematically been explored. Four coordinates are identified as particularly effective in inducing a crossing of the and surfaces: two totally symmetric coordinates (symmetric C–C double-bond stretching and symmetric C–C–C bending) as well as the conrotatory and disrotatory torsions of the terminal CH2 groups. A single coordinate of bu symmetry is found to be active as – coupling mode. On the basis of these ab initio results, a simple five-mode vibronic coupling model of the – conical intersection is proposed.
The potential-energy functions of the and 2 excited valence states of trans-butadiene have been characterised by the CASPT2 method. Based on these ab initio data, a vibronic-coupling model describing the conical intersection of the and states has been constructed. UV resonance-Raman and absorption spectra have been calculated, employing the time-dependent approach. The time-dependent wave-packet calculations reproduce the expected ultrafast (≈30 fs) radiationless decay of the optically bright state into the dark state.
By exciting 1,3-butadiene in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization, we found an internal-conversion time of about 110 fs, within which one can distinguish five consecutive phases and time constants. The first two times (10 and 44 fs) are assigned to two phases on the initially excited 1Bu surface and departure from it to the `dark' 2Ag state. The latter is then left in a remarkably short time (18 fs). Obviously the wave packet is very early (already in the 1Bu state) accelerated towards the 2Ag/1Ag conical intersection (CI). There follow two more short processes (both 18 fs) on the ground-state surface and then single-bond isomerization, taking 270 fs at the given excess energy (6.2 eV).
The A(2 1 A g) state of polyenes has been shown by Kohler and co-workers to be of central importance for the understanding of polyene photophysics and photochemistry. The twin-state model is used to provide a physical explanation for the well-known frequency exaltation of the a g symmetric stretch mode frequency upon excitation of the molecule from the ground X(1 1 A g) state to the A(2 1 A g) state and for the increased stabilization of the planar form. The smaller members of the polyene series, ethylene and butadiene, are nonfluorescent, while higher members are. It is shown that the direct (singlet) photochemistry of all polyenes can be largely accounted for by assuming that these two lowest lying A g states are connected by a conical intersection. The nature of the products and the stereochemical characteristics of the photoreactions can be rationalized using the phase-change theorem of Longuet-Higgins (Longuet-Higgins, H. C. Proc. R. Soc. London A 1975, 344, 147). A general procedure for locating the conical intersections and their associated products is suggested.
Rate constants for the unimolecular dissociation of 1,3-butadiene have been measured with the pulsed laser flash absorption technique, following butadiene disappearance at 222 nm. The results are in excellent agreement with previous laser-schlieren measurements interpreted with a ΔH°298 = 100 kcal/mol heat of dissociation. A new RRKM calculation agreeing with both sets of rate constants gives log k∞(s−1) = 17.03 ± 0.3 – 94(kcal/mol)/RT. These data and product measurements using ARAS, single-pulse product analysis, and time-of-flight mass spectrometry, in shock tubes, all provide independent evidence against any major participation by molecular reactions in the dissociation. The only dissociation channel, or combination of channels, consistent with all the measurements is C-C scission to two vinyl radicals. However, the extremely slow rate of H-atom formation seen in ARAS experiments then requires an unacceptably low rate of vinyl dissociation.
A crossed beams apparatus for study of thermal energy neutral-neutral reactions is described. The detector comprises a high efficiency (∼0.1%) electron bombardment ionizer, quadrupole mass filter, scintillation ion counter, and gated scalers synchronized with the beam modulation. The ionizer is nested within three chambers, each pumped by a separate ion pump and the innermost attached to a liquid nitrogen trap. The design is such that molecules which pass through the ionizer without being ionized fly on into another differentially pumped region before hitting a surface. The entire detector unit, including pumps and trap, is mounted on a rotatable platform (angular range ∼140°) which forms the lid of the scattering chamber. The beam sources also comprise modular units, mounted in differentially pumped side chambers which insert into ports in the scattering chamber. Angular distributions of reaction products have been measured for several reactions of Cl, Br, or D atoms with halogen molecules or hydrogen halides. Product velocity distributions have also been measured by time of flight for some of these reactions. In most cases, the partial pressure of interfering background species in the ionization region was ⪝ 10-15 Torr, and satisfactory data could be obtained with reactive scattering signals of a few counts per second.
A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (≪5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported. © 1995 American Institute of Physics.
The potential energy surfaces for the electrocyclic reactions of butadiene at the ground and lowest excited states are calculated by CASSCF molecular orbital methods. The reaction paths for the formation of bicyclobutane via decay from the lowest excited singlet state of butadiene through a conical intersection are clarified. The difference between the reaction pathways producing cyclobutene and bicyclobutane is also explained from the concept of conservation of the dynamical momentum. The dynamical momentum on the excited-state surface from cis-butadiene favors the reaction pathway to cyclobutene rather than that to bicyclobutane.
A properly reversible, strong-collision RRKM calculation of allene half arrow right over half arrow left propyne isomerization rates is presented. The calculation uses literature properties for all but the barrier and average energy transfer, The results agree with the few mutually consistent shock-tube experiments using an optimum E0=64 kcal/mol and an assumed, routine average energy transfer - [DELTAE]all = 160 cm-1. The barrier is in complete accord with the best current theoretical estimate.
We have formulated a chemical kinetic model to predict the growth of higher hydrocarbons in a lightly sooting C2H2/O2/Ar flame. The predictions of the model compare favorably with the experimental results of Bastin et al. (Twenty-Second Combustion Symposium). Analysis of the mechanism shows that reactions of 1CH2 play a central role in forming the C3 and C4 hydrocarbons that ultimately lead to ring formation. Several possibilities are considered for the cyclization reaction. The most likely candidates involve reaction complexes in which the hydrogen atoms are not optimally placed. This point is discussed in some detail. We argue that the “first ring” is most likely formed by reaction of two propargyl radicals, C3H3 + C3H3 ⇐ C6H5 + H or C3H3 + C3H3 ⇐ C6H6.
The rovibrational distributions of H2 photoeliminated from 1,3-butadiene upon excitation at 212.8 nm have been measured via 1 + 1 resonance-enhanced two-photon ionization. The measured rotational distributions are in reasonable agreement with a prior distribution, while the experimental vibrational distribution is substantially hotter than the prior. These results indicate that the photoelimination occurs via a concerted mechanism through a symmetric transition state in which the HH distance is greater than the equilibrium H2 value.
The quantum yield of disappearance of benzene vapor at 1849 A is 0.9 ± 0.3. The major irradiation product appears to be a valence isomer of benzene, tentatively identified as "benzvalene." Addition of diluent N2 reduces the rate of formation of the product but, up to 50 mm total pressure, increases its maximum concentration. Small amounts of fragmentation products, i.e., methane, ethane, ethylene, and acetylene, are also observed, as well as considerable amounts of polymeric or carbonaceous deposit on the cell walls. These products may be formed in the secondary photolysis of "benzvalene.".