Effects of multidimensional potential-energy surface topology on the unimolecular dissociation of vinyl bromide

Scope and method of study. The purpose of this study was to examine the effects of multidimensional potential-energy surface topology on the unimolecular dissociation and energy transfer dynamics of vinyl bromide in the gas phase using classical trajectory methods. The global potential-energy surface is parameterized and written as a sum of the different reaction channel potentials connected by switching functions. In this investigation, only the major reaction channels; namely 3-center HBr and H2, H and Br atom dissociation channels are included. The parameters of the channel potential-energy surface which represents the system either in reactant, product or in transition state for vinyl bromide undergoing dissociation, are fitted to the results of ab initio calculations using 6-31 G(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 f orbital with an exponent of 0.5 for bromine, at the level of fourth order Moller Plesset perturbation theory. The topological features of the potential-energy surface are changed one at a time to study the sensitivity of the dissociation and energy transfer dynamics.;Findings and conclusions. The rate of dissociation, branching ratios, reaction mechanism and vibrational distribution of HBr are calculated at different excitation energy in the range 4.5--6.44 eV. Comparisons of the results obtained from the present investigation with those obtained from the previous studies on an empirical surface indicate that variation of the potential energy with respect to stretching, bending and torsion should be modeled accurately. The excellent agreement between experimental and calculated vibrational energy distribution indicates that HBr dissociation is taking place on the ground electronic state subsequent to the internal conversion. The total relaxation rate can be characterized by first order rate law. Energy-transfer pathways and relaxation rate coefficients are found to be very sensitive to small variation in the surface curvature at equilibrium and along the reaction coordinates. While intimate details of intramolecular energy transfer are very sensitive to the fine details of the potential-surface curvatures, the gross behavior of energy transfer dynamics is not.