H2 Elimination Channel Research Articles

Observation of H displacement and H2 elimination channels in the reaction of O(3P) with 1-butene from crossed beams and theoretical studies

We report preliminary combined experimental/theoretical results on O(3P)+1-butene reaction dynamics with focus on atomic hydrogen displacement and molecular hydrogen elimination channels. Dynamics and relative yield of the ethylvinoxy+H and ethylketene+H2 product channels are characterized in crossed beam experiments. Stationary points and energetics of triplet/singlet C4H8O potential energy surfaces (PESs) are calculated at CCSD(T)/CBS and CASPT2 level. O(3P) attack occurs on both unsaturated C-atoms with preference for the less substituted one leading, among other products, to C2H5CHCHO+H via an exit barrier on the triplet PES, and to C2H5CHCO+H2 via a very high exit barrier on the singlet PES following intersystem crossing.

Reaction of Np atom with H₂O in the gas phase: reaction mechanisms and ab initio molecular dynamics study.

The gas-phase reaction of an Np atom with H2O was investigated using density functional theory and ab initio molecular dynamics. The reaction mechanisms and the corresponding potential energy profiles for different possible spin states were analyzed. Three reaction channels were found in the mechanism study: the isomerization channel, the H2 elimination channel, and the H atom elimination channel. The latter two were observed in the dynamics simulation. It was found that the branching ratio of the title reaction depends on the initial kinetic energy along the transition vector. Product energy distributions for the reaction were evaluated by performing direct classical trajectory calculations on the lowest sextet potential energy surface. The results indicate that most of the available energy appears as the translational energy of the products. The overall results indicate that the H2 elimination channel with low kinetic energy is thermodynamically favored but competes with the H atom elimination channel with higher kinetic energy.

Investigation of the reactions of U, U+and U2+with ammonia: mechanisms and topological analysis

Reactions of uranium atom and ions (U, U+ and U2+) with NH3 have been studied in the gas phase using density functional calculations. The detailed description of reaction mechanisms and diverse topological analysis provide insights into the chemistry of uranium species. The reaction pathways leading to different products are closely examined. In the case of U and U+ reacting with NH3, the isomerization channel (the products are H2UNH and H2UNH+, respectively) and H2 elimination channel (decomposed to UNH + H2 and UNH+ + H2) were investigated. In the U2+ + NH3 reaction, UNH2+ was found to be the only product generated by the dehydrogenation of the HUNH22+ intermediate. The obtained results are compared to recent experimental studies. Bonding evolution along with the reactions was investigated using diverse topological analyses, including electron localization function, atoms in molecules, and natural bond orbital. A comparison with other results reported in the literature for similar systems, such as the reactions of uranium species with NH3, H2O and CH4, is made to underline the similarities and differences of the reaction mechanisms.

Ab Initio Molecular Dynamics Study of the Reaction of U+ and U2+ with H2O in the Gas Phase: Direct Classical Trajectory Calculations

The gas phase reactions of U(+) and U(2+) with H2O were investigated using an ab initio molecular dynamics method. All of the information along the minimum energy path were calculated with density functional theory (DFT) and coupled cluster methods. For U(+) with H2O, the molecular dynamics simulations yield a branching ratio of 86% for the H2 elimination channel to 14% for the H atomic elimination channel in agreement with the quadruple ion trap mass spectrometry (QIT/MS) experimental ratio of 91% to 9%. In the case of U(2+) + H2O, there is a crossing of the potential energy surfaces (PES) after the first transition state. Crossing seams between the PES and possible spin inversion processes were studied by means of the intrinsic reaction coordinate (IRC) approach. For U(2+) with H2O, all trajectories are corresponds to H atom elimination channel, this is consistent with the Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) experimental results. The chemical bonding evolution along the reaction pathways was discussed by using topological methodologies of the electron localization function (ELF).

Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.

Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

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(1D), N(2D), O(1D) – with H2(D2), 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(3P) and C(3P), 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. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113 2.1. Soft electron-ionisation detection 114 2.2. Product angular and velocity distributions 116 2.3. Crossed molecular beam experiments with variable beam crossing angle 118 2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120 3.1. Reaction C(1D) + H2 122 3.2. Reaction N(2D) + H2 126 3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133 4.1. Reaction O(3P) + C2H2 134 4.1.1. Product angular and TOF distributions 135 4.1.2. Determination of branching ratios 138 4.1.3. Determination of product ionisation energies 138 4.2. Reaction C(3P) + C2H2 138 4.2.1. H and H2 elimination channels, and branching ratios 140 4.2.2. Ionisation energy of c/l-C3H radicals 145 4.3. Reaction O(3P) + C2H4 145 4.3.1. Observation of all product channels and branching ratios 146 4.4. Reaction C(3P) + C2H4 151 4.4.1. H-elimination channels 151 4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158

An ab initio Study on the Molecular Elimination Reactions of Methacrylonitrile

Ab initio quantum chemical molecular orbital calculations have been performed for the unimolecular decomposition of methacrylonitrile (), especially for HCN and molecular elimination channels. Structures and energies of the reactants, products, and relevant species along the individual reaction pathways were determined by MP2 gradient optimization and MP4 single point energy calculations. Direct four-center elimination of HCN and three-center elimination of H2 channels were identified. In addition, H or CN migration followed by HCN or H2 elimination channels via the methylcyanoethylidene intermediate was also identified. Unlike the case of crotonitrile previously studied, in which the dominant decomposition process was the direct three-center elimination of HCN, the most important reaction pathway should be the direct threecenter elimination of in the case of methacrylonitrile.뤀댐�⨀螑㌄쀅�⨀䀠ﶖ⨀聢�⨀Ă䀠煥獣愠獵扨敡瀀ｿｿｿｿ䂂�⨀䂂�⨀?늀Ѐက섂돀遠�⨀堘�⨀肴�⨀ဈ�⨀餂돐側�⨀墵�⨀／ࠀ䀈�⨀夂덐蠀㠀駚돀遠�⨀ꁠﲖ⨀�⨀䢕�⨀돐梕�⨀��⨀炕�⨀뤏덐肕�⨀렏頏ꢕ�⨀਀ꢕ�⨀㢖�⨀਀㢖�⨀좖�⨀਀좖�⨀뢗�⨀਀뢗�⨀�⨀਀က�⨀줁?⨀؀က�⨀਀�⨀

Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ mol−1 formation of HCCCH2 (propargyl)+H is observed to be the dominant channel with a nearly forward–backward symmetric angular distribution in the center-of-mass (cm) frame; about 35% of the total available energy is channeled into translation indicating that the propargyl radical is highly internally excited; formation of less stable C3H3 isomer(s) is minor (2%). As ET increases, formation of appreciable, increasingly larger fractions of less stable propyn-l-yl and/or cyclopropenyl isomers is also observed. These findings are consistent with the integral cross-section measurements. While formation of propargyl is thought to proceed via an osculating complex mechanism following addition of C(3PJ) to the double bond of ethylene, the dynamics of formation of the less stable isomers is going through a long-lived complex, as witnessed by an isotropic cm angular distribution. The H2 elimination channel leading to C3H2 formation has not been found to occur, which suggests that inter-system-crossing to the ground singlet C3H4 potential energy surface manifold has low probability and/or the H2-elimination process on the triplet surface is characterized by a very large exit potential barrier.

Reactions of 2-Propanol with Cu+ in the Gas Phase: A Density Functional Theory Study

The gas phase reactions between Cu+ and 2-propanol have been investigated using density functional theory at the B3LYP/6-311+G(d) and B3LYP/6-311++G(d,p) levels of theories. Seven reaction pathways have been studied. Of all reaction paths, the mechanism involving an electrophile-induced one step syn elimination of H2O has the lowest energy barrier. Dehydration also occurs via the intermediate corresponding to C−O insertion of Cu+. On the H2 elimination channel, reaction begins with C−H insertion of Cu+, followed by hydrogen migrations to form H2−Cu+−acetone or the H2−Cu+−2-propenol intermediates. Our results show that the reaction with copper cation insertion into the H−OCH(CH3)2 bond is unlikely, for its activation energy is 24.5 kcal/mol higher than those of the reactants (Cu+ and 2-propanol). The mechanism involving C−C insertion of Cu+ is also energetically unfavorable. The predicted potential energy surface of reaction and relative energies of the fragmentation products are consistent with available ...

Dynamics of the C+C2H2 reaction from differential and integral cross-section measurements in crossed-beam experiments

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(3PJ, 1D2)+C2H2(X 1Σg+), in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJ mol−1. The reaction C(3PJ)+C2H2(X 1Σg+) has been found to lead to C3H+H and C3+H2 products in comparable amounts. Both H and H2 elimination pathways are found to proceed through the formation of a C3H2 long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H2 elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C3H2 potential-energy surfaces. The reaction C(1D2)+C2H2(X 1Σg+) has been found to lead to formation of C3H+H, with a C3H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(3PJ)+C2H2(X 1Σg+)→C3H+H(2S1/2) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies ET=0.38−25.5 kJ mol−1, the H(2S1/2) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (ET)−0.80±0.03 below ET=10 kJ mol−1. These findings provide direct evidence that the C(3PJ)+C2H2 reaction can occur under the physical conditions prevailing in dense interstellar clouds and, in particular, that it may be the source of both C3H and C3 species in these extreme environments.

Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion.

The dynamics of some elementary reactions of N(2D), C(3P,1D) and CN(X2 sigma +) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection. The novel capability of producing intense, continuous beams of the radical reagents by a radio-frequency discharge beam source was exploited. From angular and velocity distribution measurements obtained in the laboratory frame, primary reaction products have been identified and their angular and translational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(2D) + CH4, which is found to lead to H + CH2NH (methylenimine) and H + CH3N (methylnitrene); no H2 elimination is observed. In the reaction N(2D) + H2O the N/H exchange channel has been found to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H2 is negligible. Formation of H + H2CCCH (propargyl) is the dominant pathway, at low collision energy (Ec), of the C(3P) + C2H4 reaction, while at high Ec formation of the less stable C3H3 isomers (cyclopropenyl and/or propyn-1-yl) also occurs; the H2 elimination channel is negligible. The H elimination channel has also been found to be the dominant pathway in the C(3P,1D) + CH3CCH reaction leading to C4H3 isomers and, again, no H2 elimination has been observed to occur. In contrast, both H and H2 elimination, leading in comparable ratio to C3H + H and C3(X1 sigma g+) + H2(X1 sigma g+), respectively, have been observed in the reaction C(3P) + C2H2(X1 sigma g+). The occurrence of the spin-forbidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between triplet and singlet manifolds of the C3H2 potential energy surfaces. The reaction CN(X2 sigma +) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been discussed in the light of recent theoretical calculations on the relevant potential energy surfaces. Previous, lower resolution studies on C and CN reactions carried out using pulsed beams are noted. Finally, throughout the paper the relevance of these results to combustion chemistry is considered.

Reaction Pathways of Singlet Silylene and Singlet Germylene with Water, Methanol, Ethanol, Dimethyl Ether, and Trifluoromethanol: An ab Initio Molecular Orbital Study

Ab initio molecular orbital calculations have been performed to explore the reaction potential energy surfaces of singlet silylene and germylene with water, methanol, ethanol, dimethyl ether, and trifluoromethanol. We have identified two new reaction channels on each reaction surface, except for reactions involving dimethyl ether. The previously unreported reaction channels involve H2 elimination following the initial formation of an association complex. For reactions involving singlet silylene and water, a simple activated complex theory (ACT) analysis predicts that these newly identified reaction channels are equally likely to be accessed as the previously identified 1,2 hydrogen atom shift channels. For reactions involving singlet germylene and water, a similar ACT analysis predicts that the H2-elimination channels will occur in preference to the 1,2 hydrogen shift. Indeed, the room-temperature rate constants for H2 elimination from the germanium complex are predicted to be approximately 5 orders of ma...

Reaction Rate Constants for Singlet Silylene and Singlet Germylene with Water, Methanol, Ethanol, Dimethyl Ether and Trifluoromethanol: Competition between H-Atom Migration and H2 Elimination

Stationary points on the reaction potential energy surfaces of singlet silylene and singlet germylene with water, methanol, ethanol, dimethyl ether and trifluoromethanol have been used to predict reaction rate constants for temperatures between 100 and 1500 Kelvin. We have previously identified two new reaction channels on each reaction surface, except for reactions involving dimethyl ether [J. Phys. Chem. A, 2001, 105, 1185]. The previously unreported reaction channels involve H2 elimination following the initial formation of an association complex. A simple Activated-Complex Theory (ACT) analysis predicts that in the case of singlet silylene reacting with water, the newly identified reaction channels are equally likely to be accessed as previously identified 1,2 H-atom migration channels. The H2-elimination channels are slightly disfavored upon reaction of singlet silylene with methanol and ethanol, but become the preferred reaction channels with trifluoromethanol as the reaction partner. For reactions involving singlet germylene with water and with methanol, the ACT analyses predict that the H2-elimination channels will occur in preference to 1,2 H-atom migration. Indeed, the room temperature rate constants for H2 elimination from the germanium complexes are predicted to be approximately five orders of magnitude greater than for the H-atom migration channels.

Dynamics of Atomic and Molecular Hydrogen Elimination from Small Alkanes Following 157-nm Excitation

Photodissociation of a series of C3−C9 alkanes following 157 nm excitation has been investigated using the photofragment translational spectroscopy (PTS) technique. Three series of alkanes, normal alkanes, branched alkanes, and cyclic alkanes, were studied and the atomic hydrogen (H) and molecular hydrogen (H2) elimination from their photodissociation were experimentally investigated. Experimental results show that the dynamics of the H and H2 elimination processes from larger n-alkanes is quite similar to that of propane. Dissociation of the branched alkanes is, however, significantly different from that of the normal alkanes. C−C bond fission is more significant in the branched alkanes. There are also notable differences among the branched alkanes for the H and H2 elimination processes. Photodissociation dynamics of small cyclic alkanes are significantly different from that of the acyclic alkanes. This is likely due to their significantly different electronic structures caused by the large ring strain in small cyclic alkanes. The H and H2 elimination channels for larger cyclic alkanes are quite similar to the normal alkanes. Relative branching ratios of the H and H2 elimination channels for all the alkane molecules are determined and compared.

Site specific dissociation dynamics of propane at 157 nm excitation

Photodissociation of propane at 157 nm excitation has been investigated using the photofragment translational spectroscopic (PTS) technique. Three dissociation channels have been experimentally observed: the atomic hydrogen (H) elimination, the molecular hydrogen (H2) elimination, and the methyl radical (CH3) elimination. Product translational energy distributions have been measured and relative branching ratios have also been estimated, which indicate that all these three processes are significant. The site effects on the H and H2 elimination have been quantified by investigating the photodissociation of the four propane isotopomers: CH3CD2CH3, CD3CH2CD3, CH2DCH2CH2D, and CD3CD2CD3. Experimental results indicate that the yield of the H elimination from the two terminal CH3 groups (terminal-H elimination) is larger than that from the internal CH2 group (internal-H elimination). H2 elimination from the internal carbon atom of propane (2,2-H2 elimination) is the dominant process while H2 elimination channels from the vicinal carbon atoms (1,2-H2 elimination) are less significant. Minor processes of H2 elimination from terminal carbon atoms (1,1-H2 and 1,3-H2 elimination) were also observed. The translational energy distributions also show that the dynamics of H and H2 elimination from different sites are significantly different. Relative branching ratios of H and H2 elimination processes from different sites have also been determined.

Dynamics of H2 and C2H4 Elimination in the Y + C2H6 Reaction

The reaction between ground-state atomic yttrium (a2DJ) and ethane (C2H6) has been studied in crossed molecular beams with 157 nm photoionization detection of products. Studies have been carried out at several collision energies, ranging from 〈Ecoll〉 = 16.1 kcal/mol to 〈Ecoll〉 = 29.8 kcal/mol. At 〈Ecoll〉 ≥ 18.1 kcal/mol, reactive scattering is observed, with both YH2 + C2H4 and YC2H4 + H2 products being formed. Detected products of the YH2 + C2H4 channel are found to exhibit very little translational energy, suggesting that there is little or no potential energy barrier above the final product energy to ethylene elimination from an (H)2Y(C2H4) intermediate. Translational energy distributions for the YC2H4 + H2 channel, resulting from elimination of H2 from a common intermediate, demonstrate that a slightly larger fraction of energy available to these products is channeled into translation. The YH2 + C2H4 channel is found to dominate the H2 elimination channel by greater than an order of magnitude, which suggests that a small potential energy barrier to the less endoergic H2 elimination channel exists. In lower collision energy studies, only nonreactive scattering of Y atoms from ethane is found to occur. This observed collision energy threshold behavior for the onset of YH2 and YC2H4 product formation allows determination of the barrier to C−H bond insertion of Y atoms into ethane to be 19.9 ± 3.0 kcal/mol.

Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H2 and NbC2+H2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%–50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5±2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y–H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D0(Y–CCH)=110.2±2.0 kcal/mol.

New low background crossed molecular beam apparatus: Low background detection of H2

A low background and almost hydrocarbon free (∼1×10−14 Torr) molecular beam apparatus with an improved universal detector, based on electron bombardment ionization, has been constructed for crossed molecular beam research. Extremely high vacuum (∼1×10−12 Torr) for the detector’s ionization region is achieved using multiple ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head (∼10 K) is used to pump the detector’s ionization region. Using this arrangement, the H2 background in the detector can be reduced by about two orders of magnitude in comparison with previously built similar instruments. Therefore, the signal-to-noise for detecting H2 product detection sensitivity is substantially enhanced, making experimental studies of H2 elimination channels in photodissociation processes much easier. Backgrounds at m/e=28 (CO+), 16 (CH4+,O+), 15 (CH3+), 14 (CH2+), and 13 (CH+) in the ionization detection region are also significantly reduced. In this article, we will describe the new apparatus in detail and present some preliminary results of the unimolecular decomposition studies of CH3OH at 157 nm excitation to demonstrate the capabilities of low background detection H2 using this new instrument.

Dynamics of ethylene photodissociation from rovibrational and translational energy distributions of H2 products

The dynamics of H2 elimination from ethylene was studied via a pump and probe technique utilizing an ultrahigh resolution vacuum ultraviolet laser system. H2 product internal and translational energy distributions were obtained for the photodissociation at 193 nm. The distribution of energy in H2 product from the dissociation of (1,1)-dideuteroethylene is also presented. Two separate H2 elimination channels are inferred: a 1,1 elimination producing the vinylidene radical and a 1,2 elimination producing the acetylene molecule. Differences between the vibrational, rotational, and translational energy distributions for these two channels are discussed and correlations between product internal and translational energy are presented. A comparison with ab initio calculations of the transition state configurations for these processes is made. We suggest that the H2 elimination process may be nonstatistical in nature. The D atom elimination from C2D4 was examined and kinetic energy distribution for this product measured.

Photodissociation of ethylene at 193 nm

The photodissociation of ethylene at 193 nm was studied by measuring the product translational energy distributions for the H+C2H3 and H2+C2H2 channels. In agreement with previous workers, it was determined that atomic and molecular elimination occur in relatively equal amounts. Using 1,1 D2CCH2 and 1,2 cis HDCCDH, it was shown that both acetylene and vinylidene are formed and that the acetylene/vinylidene ratio is approximately 2/3 in the molecular elimination. This H2 elimination channel has a translational energy distribution peaked at around 20 kcal/mol, indicating that it is a concerted process with a substantial exit barrier. It was found that the H atom elimination channel is best described as a simple bond rupture occurring after internal conversion of the electronically excited molecule to the vibrationally excited ground state ethylene. Some of the primary C2H3 product has sufficient internal energy to spontaneously decompose to H+HC≡CH. At higher laser intensity a large fraction of the C2H3, however, absorbs another photon and fragments to H+H2C=C: (1A1 and 3B2).