STATISTICAL AND NONSTATISTICAL DYNAMICS IN THE UNIMOLECULAR DECOMPOSITION OF VINYL BROMIDE

Rate constants have been computed for three unimolecular decomposition reactions of vinyl bromide for several energies in the range 5.23-7.6 7 eV, using statistical variational efficient microcanonical sampling- transition-state theory (EMS-TST) on a global vinyl bromide potential energy surface. The EMS-TST results are compared with those obtained f rom a previously reported classical trajectory study on the same poten tial energy surface [J. Phys. Chem. 1995, 99, 2959] in order to assess the extent to which vinyl bromide unimolecular decomposition is gover ned by statistical dynamics. For the three-center HBr elimination reac tion, it is found that k(EMS-TST) is greater than k(trajectory) by a f actor of 1.5-3.5 over the energy range considered. For the C-Br bond s cission, the EMS-TST and trajectory results at lower energies are equa l within the statistical error in the trajectory calculations, while a t higher energies k(EMS-TST) exceeds k(trajectory) by a factor of 1.4- 2.9. The EMS-TST calculations also reproduce a surprising result from the trajectory study, that the rate constant for three-center HBr elim ination is an order of magnitude greater than that for C-Br bond sciss ion throughout the energy range, even though the barrier height for th e latter reaction is 0.34 eV lower. These results imply that three-cen ter HBr elimination and C-Br bond scission are governed by statistical dynamics. For the three-center H-2 elimination reaction, however, k(t rajectory) is greater than k(EMS-TST) by a factor of 2-4 at lower ener gies and a factor of 5-7 at higher energies. This result necessarily i mplies that the dynamics of the three-center H-2 elimination are nonst atistical. The nonstatistical behavior for this reaction is attributed to a breakdown in the coupling among vibrational modes as the H-2 fra gment departs, which leaves energy in excess of the statistically pred icted amount in the dissociation coordinate. A study of intramolecular vibrational relaxation (IVR) rates and pathways in vinyl bromide [J. Phys. Chem. 1996, 100, 8085] supports this conclusion. The IVR analysi s also shows that such a breakdown in mode-to-mode coupling does not e xist for the three-center HBr elimination and that nearly global rando mization of the internal energy rapidly occurs as the system moves thr ough the transition-state region for HBr elimination. Thus, the nature of IVR on the vinyl bromide potential surface used in this work is co nsistent with the present EMS-TST results showing that three-center HB r elimination is well-described by statistical reaction rate theory, w hile three-center H-2 elimination is not.