THEORETICAL-STUDIES OF THE THERMAL GAS-PHASE DECOMPOSITION OF VINYL BROMIDE ON THE GROUND-STATE POTENTIAL-ENERGY SURFACE

The reaction dynamics of the thermal, gas-phase decomposition of vinyl bromide has been investigated using classical trajectory methods on a global, analytic potential-energy surface fitted to the results of nb initio electronic structure calculations and experimental thermochemical, spectroscopic, and structural data. The saddle-point geometries and energies for several decomposition channels are determined using 6-31G(d,p) basis sets for carbon and hydrogen and Huzinaga's (4333/433/4) basis set augmented with split outer s and p orbitals and an f orbital for bromine. Electron correlation is incorporated using Moller-Plesset fourth-order perturbation theory with all single, double, triple, and quadruple excitations included, The calculated transition-state energies without zero-point energy corrections relative to vinyl bromide are four-center HBr elimination (3.530 eV), three-center HBr elimination (3.196 eV), four-center H-2 elimination (4.159 eV), and three-center H-2 elimination (4.618 eV). The global potential is written as a sum of the different reaction channel potentials connected by parametrized switching functions. The average absolute difference between Delta E values for the various decomposition channels obtained from the global surface and experimental measurement is 0.076 eV. Predicted equilibrium geometries for reactants and products are in good to excellent accord with experiment. The average absolute difference between the fundamental harmonic vibrational frequencies predicted by the global surface and those obtained from Raman and IR spectra varies from 10.2 cm(-1) for H2C=CHBr to 81.3 cm(-1) for H2C=CH. The potential barriers for six decomposition channels agree with the ab initio calculations to within an average difference of 0.124 eV. The dissociation dynamics of vinyl bromide on the ground-state surface is investigated at several excitation energies in the range 4.0-6.34 eV. The results show the following: (1) The decomposition dynamics follows a first-order rate law. (2) At thermal energies, the only brominated decomposition product is HBr. The results indicate that a previously reported activation energy for this process is too small. (3) As the excitation energy increases, other decomposition channels become important. At E = 6.44 eV, the reaction channels are, in order of importance, H-2 elimination (38.1%), HBr formation (44.5%), Br atom elimination (4.6%), and C-H bond fission (2.6%). (4) The percentage of the total excitation energy partitioned into product relative translational motion and HBr internal energy upon HBr elimination is nearly independent of the total excitation energy. (5) Comparison of the calculated and measured relative translational energy distributions for product Br atoms upon C-Br bond fission and time-of-flight spectra for C2H2 upon HBr elimination indicates that in previously reported photolysis experiments Br atom dissociation is occurring on an excited electronic surface but HBr elimination is taking place on the ground-state surface subsequent to internal conversion. (6) Both HBr and H-2 eliminations occur almost exclusively via a three-center mechanism. For both three- and four-center dissociation reactions, both C-X bonds rupture nearly simultaneously. (7) The computed branching ratios suggest that the dynamics on the ground-state surface is nonstatistical. This result is in accord with previously reported generalizations concerning factors that promote nonstatistical behavior and with the calculated rate for intramolecular vibrational energy transfer to and from the C-Br stretching mode.