Theoretical Studies of the Photolytic Decomposition of Vinyl Bromide at 193 nm

Abstract

The decomposition dynamics of vinyl bromide upon single-photon excitation at 193 nm have been investigated by using classical trajectory methods on adiabatic excited-state potentials that have been obtained by using empirical and a b initio configuration interaction (CI) methods. The excited-state potential surfaces are represented by a global analytic hypersurface previously developed for the vinyl bromide ground state with the C-Br bonding Morse-type potential replaced with one of the repulsive C-Br interactions obtained in the empirical or ab initio calculations Energetic considerations suggest that the dissociation dynamics of vinyl bromide upon photolysis at 193 nm involves excitation to three or four repulsive C-Br states which include the A'A"(na*), 63A"(na*) and C3A'(no*) potentials. The effects of a vertical excitation from the ground state to the A'A"(na*) and E3A'(na*) states have been determined by the computation of 300 or more trajectories in each case. The results show that the only products for these excitations are vinyl radicals and either Br(2P3n) or Br (2Pln) atoms. No HBr is observed. This result is consistent with the hypothesis advanced in our previous study of vinyl bromide dissociation on the ground-state surface where we suggested that the HBr formed in previously reported beam experiments [Zsr. J. Chem. 1989,29, 3831 is produced subsequent to internal conversion to the ground state. Combination of the trajectory results with the measured Br/HBr ratio of 1.28 indicates that the internal conversion probability lies in the range 0.44-0.64. The calculated translational energy distributions for C2H3 and either Br(2P3n) or Br(2Pln) atoms are peaked at energiessignificantly in excess of that observed in the beam experiments. This is interpreted to mean that the ab initio excited-state potentials are too repulsive. Comparison with the experimental data suggests that, in the region around the C-Br equilibrium distance, the ab initio energies are too large by about 16 kcdmol. The computed full width at half-maximum for all distributions is much smaller than the experimental result, suggesting that decomposition is occurring from more than one excited electronic surface. It is shown that a good fit to the measured translational energy distribution can be obtained from a linear combination of the distributions computed by using three empirical potentials whose energy at the equilibrium C-Br separation has been reduced by about 16 kcaYmol from that predicted by the ab initio calculations. The values of the expansion coefficients indicate that about 60% of the bromine atoms are formed in the 2P3n ground state. This is close to the statistical result based upon a simple count of available spin states.

193 nm photolysis of vinyl bromide: Nascent product distribution of the C2H3Br→C2H2 (vinylidene)+HBr channel

“…13,22 Direct dissociation from a barrierless repulsive surface could occur, or the initial excitation could be followed by internal conversion to the ground state leaving the VBr with significant excess vibrational energy. The VBr is initially excited to a number of closely spaced purely repulsive potential energy surfaces.…”

“…12,13 This reaction was used for the production of vibrationally excited vinyl radicals in our study of the vinyl vibrational modes. 12,13 This reaction was used for the production of vibrationally excited vinyl radicals in our study of the vinyl vibrational modes.…”

“…12,13 This reaction was used for the production of vibrationally excited vinyl radicals in our study of the vinyl vibrational modes. 13 There are two possible mechanisms for the elimination of HBr from the VBr precursor. 13 There are two possible mechanisms for the elimination of HBr from the VBr precursor.…”

Direct comparison of 3-centre and 4-centre HBr elimination pathways in methyl-substituted vinyl bromides

“…9 Fig. 1 contains a schematic diagram, based on previous calculations, 10,11 of the lowest lying electronic states of VBr responsible for the UV absorption and atomic and molecular fragmentation processes. In the optically accessed state with p CC p CC * character, the ethylenic component relaxes from planarity in the Franck-Condon region along the torsional angle coordinate around the CQC bond.…”

“…14 This p CC s CBr * state is purely repulsive along the C-Br stretching coordinate, promoting C-Br bond cleavage. 11 The twisted ethylenic motion in the p CC p CC * state can also avoid the crossing to the p CC s CBr * state, instead further relaxing towards a conical intersection with the ground state at a twisted-pyramidal geometry of one methylene group. 12 The internal conversion probability at this conical intersection in VBr is estimated to lie in the range 0.44-0.64.…”