THEORETICAL-STUDIES OF THE PHOTOLYTIC DECOMPOSITION OF VINYL BROMIDE AT 193 NM

The decomposition dynamics of vinyl bromide upon single-photon excitat ion at 193 nm have been investigated by using classical trajectory met hods on adiabatic excited-state potentials that have been obtained by using empirical and ab initio configuration interaction (CI) methods. The excited-state potential surfaces are represented by a global analy tic hypersurface previously developed for the vinyl bromide ground sta te with the C-Br bonding Morse-type potential replaced with one of the repulsive C-Br interactions obtained in the empirical or ab initio ca lculations Energetic considerations suggest that the dissociation dyna mics of vinyl bromide upon photolysis at 193 nm involves excitation to three or four repulsive C-Br states which include the ($) over tilde A(1)A ''(pi sigma), ($) over tilde b(3)A ''(pi sigma*) and ($) over t ilde c(3)A'(n sigma) potentials. The effects of a vertical excitation from the gr state to the ($) over tilde A(1)A ''(pi sigma) and ($) o ver tilde c(3)A'(n sigma) states have been determined by the computat ion of 300 or more trajectories in each case. The results show that th e only products for these excitations are vinyl radicals and either Br (P-2(3/2)) or Br(P-2(1/2)) atoms. No HBr is observed. This result is c onsistent with the hypothesis advanced in our previous study of vinyl bromide dissociation on the ground-state surface where we suggested th at the HBr formed in previously reported beam experiments [Isr. J. Che m. 1989, 29, 383] 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 probabili ty lies in the range 0.44-0.64. The calculated translational energy di stributions for C2H3 and either Br(P-2(3/2)) or Br(P-2(1/2)) atoms are peaked at energies significantly in excess of that observed in the be am 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 kcal/mol. The compu ted full width at half-maximum for all distributions is much smaller t han the experimental result, suggesting that decomposition is occurrin g from more than one excited electronic surface, It is shown that a go od fit to the measured translational energy distribution can be obtain ed from a linear combination of the distributions computed by using th ree empirical potentials whose energy at the equilibrium C-Br separati on has been reduced by about 16 kcal/mol from that predicted by the ab initio calculations. The values of the expansion coefficients indicat e that about 60% of the bromine atoms are formed in the P-2(3/2) groun d state. This is close to the statistical result based upon a simple c ount of available spin states.