Ronald D. Kay and Lionel M. Raff
J. Phys. Chem. A 101(6), 1007 (1997)
Abstract
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Rate constants have been computed for three unimolecular
decomposition reactions of vinyl bromide for several energies in
the range 5.23 - 7.67 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 from a previously reported classical
trajectory study on the same potential energy surface [J. Phys.
Chem. 1995, 99, 2959] in order to assess the extent to which vinyl
bromide unimolecular decomposition is governed by statistical
dynamics. For the three-center HBr elimination reaction, it is
found that k(EMS-TST) is greater than k(trajectory) by a factor of
1.5 - 3.5 over the energy range considered. For the C-Br bond
scission, the EMS-TST and trajectory results at lower energies are
equal within the statistical error in the trajectory calculations,
while at 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 elimination is an order of magnitude greater than that
for C-Br bond scission throughout the energy range, even though
the barrier height for the latter reaction is 0.34 eV lower.
These results imply that three-center HBr elimination and C-Br bond
scission are governed by statistical dynamics. For the three-
center H2 elimination reaction, however, k(trajectory) is greater
than k(EMS-TST) by a factor of 2 - 4 at lower energies and a factor
of 5 - 7 at higher energies. This result necessarily implies that
the dynamics of the three-center H2 elimination are nonstatistical.
The nonstatistical behavior for this reaction is attributed to a
breakdown in the coupling among vibrational modes as the H2 fragment
departs, which leaves energy in excess of the statistically predicted
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
analysis also shows that such a breakdown in mode-to-mode coupling
does not exist for the three-center HBr elimination and that nearly
global randomization of the internal energy rapidly occurs as the
system moves through the transition-state region for HBr elimination.
Thus, the nature of IVR on the vinyl bromide potential surface used
in this work is consistent with the present EMS-TST results showing
that three-center HBr elimination is well-described by statistical
reaction rate theory, while three-center H2 elimination is not.