Ab initio and RRKM study of photodissociation of azulene cation

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.     

Photodissociation of 1,3,5-triazine: an ab initio and RRKM study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of 1,3,5-triazine at different wavelengths of the absorbed photon. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level, and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that, after being excited by 275, 248, or 193 nm photons, the triazine molecule isomerizes to an opened-ring structure on the first singlet excited-state potential energy surface (PES), which is followed by relaxation into the ground electronic state via internal conversion. On the contrary, excitation by 285 and 295 nm photons cannot initiate the ring-opening reaction on the excited-state PES, and the molecule relaxes into the energized ring isomer in the ground electronic state. The dissociation reaction starting from the ring isomer is calculated to have branching ratios of various reaction channels significantly different from those for the reaction initiating from the opened-ring structure. The existence of two distinct mechanisms of 1,3,5-triazine photodissociation can explain the inconsistency in the translational energy distributions of HCN moieties at different wavelengths observed experimentally.     

Photodissociation of benzene under collision-free conditions: an ab initio/Rice-Ramsperger-Kassel-Marcus study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of benzene at various wavelengths upon absorption of one or two UV photons followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G2M level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for C(6)H(5)+H, C(6)H(4)+H(2), C(4)H(4)+C(2)H(2), C(4)H(2)+C(2)H(4), C(3)H(3)+C(3)H(3), C(5)H(3)+CH(3), and C(4)H(3)+C(2)H(3) have been calculated subsequently using both numerical integration of kinetic master equations and the steady-state approach. The results show that upon absorption of a 248 nm photon dissociation is too slow to be observable in molecular beam experiments. In photodissociation at 193 nm, the dominant dissociation channel is H atom elimination (99.6%) and the minor reaction channel is H(2) elimination, with the branching ratio of only 0.4%. The calculated lifetime of benzene at 193 nm is about 11 micros, in excellent agreement with the experimental value of 10 micros. At 157 nm, the H loss remains the dominant channel but its branching ratio decreases to 97.5%, while that for H(2) elimination increases to 2.1%. The other channels leading to C(3)H(3)+C(3)H(3), C(5)H(3)+CH(3), C(4)H(4)+C(2)H(2), and C(4)H(3)+C(2)H(3) play insignificant role but might be observed. For photodissociation upon absorption of two UV photons occurring through the neutral "hot" benzene mechanism excluding dissociative ionization, we predict that the C(6)H(5)+H channel should be less dominant, while the contribution of C(6)H(4)+H(2) and the C(3)H(3)+C(3)H(3), CH(3)+C(5)H(3), and C(4)H(3)+C(2)H(3) radical channels should significantly increase.     

Photodissociation dynamics of C6HxF6-x (x = 1-4) at 193 nm

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C(6)H(3)F(2) and C(6)H(2)F(3) from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made.     

Photodissociation and photoionization of 2,5-dihydroxybenzoic acid at 193 and 355 nm

Photodissociation and photoionization of 2,5-dihydroxybenzoic acid (25DHBA), at 193 and 355 nm were investigated separately in a molecular beam using multimass ion imaging techniques. Two channels competed after excitation by one 193 nm photon. One channel is dissociation from the repulsive excited state along O-H bond distance, resulting in H atom elimination from meta-OH functional group. The other channel is internal conversion to the ground state, followed by H(2)O elimination. Some of the fragments further proceeded to secondary dissociation. On the other hand, absorption of one 355 nm photon gave rise to H(2)O elimination channel on the ground state. Absorption of more than one 355 nm photon resulted in the three-body dissociation which also occurs on the ground state. Dissociation on the excited state does not play a role at 355 nm. The large concentration ratio (2×10(5)), between neutral fragments and cations produced from 355 nm multiphoton excitation indicates that internal conversion followed by dissociation, is the major channel for 355 nm multiphoton excitation. Multiphoton ionization is a minor channel. Multiphoton ionization of 25DHBA clusters only produces 25DHBA cations. Neither anion nor protonated 25DHBA cation were observed. It is very different from the ions produced from solid matrix-assisted laser desorption/ionization (MALDI), experiments. This suggests that protonated 25DHBA and negatively charged 25DHBA generated in MALDI experiments does not simply result from the ionization following proton transfer reactions or charge transfer reactions of the clusters in the gas phase.     

Photodissociation dynamics of phenol

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 相似文献   

Unimolecular dissociation of the CH3OCO radical: an intermediate in the CH3O + CO reaction

This work investigates the unimolecular dissociation of the methoxycarbonyl, CH(3)OCO, radical. Photolysis of methyl chloroformate at 193 nm produces nascent CH(3)OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the CH(3)O + CO and CH(3) + CO(2) product channels. Both electronic ground- and excited-state radicals undergo competitive dissociation to both product channels. The experimental product branching to CH(3) + CO(2) from the ground-state radical, about 70%, is orders of magnitude larger than Rice-Ramsperger-Kassel-Marcus (RRKM)-predicted branching, suggesting that previously calculated barriers to the CH(3)OCO --> CH(3) + CO(2) reaction are dramatically in error. Our electronic structure calculations reveal that the cis conformer of the transition state leading to the CH(3) + CO(2) product channel has a much lower barrier than the trans transition state. RRKM calculations using this cis transition state give product branching in agreement with the experimental branching. The data also suggest that our experiments produce a low-lying excited state of the CH(3)OCO radical and give an upper limit to its adiabatic excitation energy of 55 kcal/mol.     

Photodissociation Dynamics of Benzene at 193 nm

Photodissociation of benzene at 193 nm has been investigated using the photofragment translational spectroscopy (PTS) technique. H atom elimination channel for benzene at 193 nm is from a one‐photon dissociation process, while H₂ and CH₃ elimination channels come from a two‐photon excitation process.     

Photodissociation dynamics of ClN3 at 193 nm

Photofragment translational spectroscopy was used to identify the primary and secondary reaction pathways in 193 nm photodissociation of chlorine azide (ClN(3)) under collision-free conditions. Both the molecular elimination (NCl+N(2)) and the radical bond rupture channel (Cl+N(3)) were investigated and compared with earlier results at 248 nm. The radical channel strongly dominates, just as at 248 nm. At 193 nm, the ClN(3) (C (1)A(")) state is excited, rather than the B (1)A(') state that is accessed at 248 nm, resulting in different photofragment angular distributions. The chlorine translational energy distribution probing the dynamics of the radical bond rupture channel shows three distinct peaks, with the two fastest peaks occurring at the same translational energies as the two peaks seen at 248 nm that were previously assigned to linear and "high energy" N(3). Hence, nearly all the additional photon energy relative to 248 nm appears as N(3) internal excitation rather than as translational energy, resulting in considerably more spontaneous dissociation of N(3) to N(2)+N.     

On the conformational memory in the photodissociation of formic acid

The photodissociation of formic acid at 248 and 193 nm was investigated by classical trajectory and RRKM calculations using an interpolated potential energy surface, iteratively constructed using the B3LYP/aug-cc-pVDZ level of calculation. Several sampling schemes in the ground electronic state were employed to explore the possibility of conformational memory in formic acid. The CO/CO2 branching ratios obtained from trajectories initiated at the cis and at the trans conformers are almost identical to each other and in very good accordance with the RRKM results. In addition, when a specific initial excitation that simulates more rigorously the internal conversion process is used, the calculated branching ratio does not vary with respect to those obtained from cis and trans initializations. This result is at odds with the idea of conformational memory in the ground state proposed recently for the interpretation of the experimental results. It was also found that the calculated CO vibrational distributions after dissociation of the parent molecule at 248 nm are in agreement with the experimental available data.     

Photodissociation dynamics of nitrobenzene and o-nitrotoluene

Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO(2) elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO(2) elimination channels were determined to be NONO(2)=0.32+/-0.12 (193 nm), 0.26+/-0.12 (248 nm), and 0.4+/-0.12(266 nm) for nitrobenzene and 0.42+/-0.12(193 nm) and 0.3+/-0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.     

Photodissociation dynamics of indole in a molecular beam

Photodissociation of indole at 193 and 248 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. H atom elimination was found to be the only dissociation channel at both wavelengths. The photofragment translational energy distribution obtained at 193 nm contains a fast and a slow component. Fifty-four percent of indole following the 193 nm photoexcitation dissociate from electronically excited state, resulting in the fast component. The rest of 46% indole dissociate through the ground electronic state, giving rise to the slow component. A dissociation rate of 6 x 10(5) s(-1), corresponding to the dissociation from the ground electronic state, was determined. Similar two-component translational energy distribution was observed at 248 nm. However, more than 80% of indole dissociate from electronically excited state after the absorption of 248 nm photons. A comparison with the potential energy surfaces from the ab initio calculation has been made.     

Photodissociation dynamics of benzyl alcohol at 193 nm

Photodissociation dynamics of benzyl alcohol, C(6)H(5)CH(2)OH and C(6)H(5)CD(2)OH, in a molecular beam was investigated at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed, including OH elimination and H(2)O elimination from the ground electronic state, H atom elimination (from OH functional group), and CH(2)OH elimination from the triplet state. The dissociation rate on the ground state was found to be 7.7 × 10(6) s(-1). Comparison to the potential energy surfaces from ab initio calculations, dissociation rate, and branching ratio from Rice-Ramsperger-Kassel-Marcus calculations were made.     

Photodissociation of isobutene at 193 nm

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.     

Ab initio and RRKM study of the HCN/HNC elimination channels from vinyl cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).     

Photodissociation dynamics of vinyl fluoride (CH2CHF) at 157 and 193 nm: Distributions of kinetic energy and branching ratios

Using photofragment translational spectroscopy and tunable vacuum-ultraviolet ionization, we measured the time-of-flight spectra of fragments upon photodissociation of vinyl fluoride (CH2CHF) at 157 and 193 nm. Four primary dissociation pathways--elimination of atomic F, atomic H, molecular HF, and molecular H2--are identified at 157 nm. Dissociation to C2H3 + F is first observed in the present work. Decomposition of internally hot C2H3 and C2H2F occurs spontaneously. The barrier heights of CH2CH --> CHCH + H and cis-CHCHF --> CHCH + F are evaluated to be 40+/-2 and 44+/-2 kcal mol(-1), respectively. The photoionization yield spectra indicate that the C2H3 and C2H2F radicals have ionization energies of 8.4+/-0.1 and 8.8+/-0.1 eV, respectively. Universal detection of photoproducts allowed us to determine the total branching ratios, distributions of kinetic energy, average kinetic energies, and fractions of translational energy release for all dissociation pathways of vinyl fluoride. In contrast, on optical excitation at 193 nm the C2H2 + HF channel dominates whereas the C2H3 + F channel is inactive. This reaction C2H3F --> C2H2 + HF occurs on the ground surface of potential energy after excitation at both wavelengths of 193 and 157 nm, indicating that internal conversion from the photoexcited state to the electronic ground state of vinyl fluoride is efficient. We computed the electronic energies of products and the ionization energies of fluorovinyl radicals.     

Photodissociation dynamics of formyl fluoride (HFCO) at 193 nm: branching ratios and distributions of kinetic energy

Following photodissociation of formyl fluoride (HFCO) at 193 nm, we detected products with fragmentation translational spectroscopy utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Among three primary dissociation channels observed in this work, the F-elimination channel HFCO-->HCO+F dominates, with a branching ratio approximately 0.66 and an average release of kinetic energy approximately 55 kJ mol(-1); about 17% of HCO further decomposes to H+CO. The H-elimination channel HFCO-->FCO+H has a branching ratio approximately 0.28 and an average release of kinetic energy approximately 99 kJ mol(-1); about 21% of FCO further decomposes to F+CO. The F-elimination channel likely proceeds via the S1 surface whereas the H-elimination channel proceeds via the T1 surface; both channels exhibit moderate barriers for dissociation. The molecular HF-elimination channel HFCO-->HF+CO, correlating with the ground electronic surface, has a branching ratio of only approximately 0.06; the average translational release of 93 kJ mol(-1), approximately 15% of available energy, implies that the fragments are highly internally excited. Detailed mechanisms of photodissociation are discussed.     

Photoproduct channels from BrCD2CD2OH at 193 nm and the HDO + vinyl products from the CD2CD2OH radical intermediate

We present the results of our product branching studies of the OH + C(2)D(4) reaction, beginning at the CD(2)CD(2)OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD(2)CD(2)OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD(2)CD(2)OH), and of the secondary dissociation of vibrationally excited CD(2)CD(2)OH radicals (OH, C(2)D(4)/CD(2)O, C(2)D(3), CD(2)H, and CD(2)CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD(2)CD(2)O and DBr + CD(2)CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C(2)D(3)) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C(2)D(3) product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.     

Insights into mechanistic photodissociation of chloroacetone from a combination of electronic structure calculation and molecular dynamics simulation

The stationary and intersection structures on the S(0) and S(1) potential energy surfaces of CH(3)COCH(2)Cl have been determined by the CAS(10,8)/cc-pVDZ optimizations and their relative energies are refined by the CASPT2//CAS(10,8)/cc-pVDZ single-point calculations. Non-adiabatic molecular dynamics simulations were performed on the basis of the state-averaged CAS(10,8)/cc-pVDZ calculated energies, energy gradients, and Hessian matrix for the S(0) and S(1) states. It is found that the features of the S(1) potential energy surface and non-adiabatic effect control the selectivity of the two α-C-C bond fissions, which provides a reasonable explanation why one α-C-C bond was observed as a primary channel and the other is ruled out even if CH(3)COCH(2)Cl is excited at 193 nm. The β-C-Cl fission is determined to be a dominant channel once the CH(3)COCH(2)Cl molecule is excited to the S(1) state and the β-C-Cl:α-C-C branching ratio is estimated by the RRKM rate theory to be 15:1 at 193 nm, which is overestimated in comparison with the value of ~11:1 inferred experimentally. The present calculation reveals that the α-C-C fission might take place in the ground electronic state as a result of the S(1) → S(0) internal conversion upon photolysis at 308 nm. However, the measured kinetic energy distributions of the α-C-C fission products suggest that the fission does not involve internal conversion to the ground state. To solve this issue, we need to perform non-adiabatic quantum dynamics simulation on accurate S(0), S(1), and S(2) potential energy surfaces, which is still a challenging task currently.     

Photodissociation pathways of 1,1-dichloroacetone

We present a comprehensive investigation of the dissociation dynamics following photoexcitation of 1,1-dichloroacetone (CH(3)COCHCl(2)) at 193 nm. Two major dissociation channels are observed: cleavage of a C-Cl bond to form CH(3)C(O)CHCl + Cl and elimination of HCl. The branching between these reaction channels is roughly 9:1. The recoil kinetic energy distributions for both C-Cl fission and HCl elimination are bimodal. The former suggests that some of the radicals are formed in an excited electronic state. A portion of the CH(3)C(O)CHCl photoproducts undergo secondary dissociation to give CH(3) + C(O)CHCl. Photoelimination of Cl(2) is not a significant product channel. A primary C-C bond fission channel to give CH(3)CO + CHCl(2) may be present, but this signal may also be due to a secondary dissociation. Data from photofragment translational spectroscopy with electron impact and photoionization detection, velocity map ion imaging, and UV-visible absorption spectroscopy are presented, along with G3//B3LYP calculations of the bond dissociation energetics.