Classical dynamical investigations of reaction mechanism in three-body hydrogen-halogen systems

A theoretical investigation of the reaction mechanisms in eight different three‐body hydrogen‐halogen reactions has been accomplished by means of Monte Carlo averages over classical trajectories computed on reasonable semiempirical potential‐energy surfaces. Two of the surfaces employed contain a single adjustable parameter characteristic of a halogen core charge while six others result from unadjusted computations. The analysis shows that dynamic effects resulting from momentum transfer require the reactions ${\mathrm{M}}_2+\mathrm{X}=\mathrm{MX}+\mathrm{M}\mathrm{\hspace{0.17em}(}\mathrm{M=H,D,\hspace{0.17em}or\hspace{0.17em}T;\; X=Br\hspace{0.17em}or\hspace{0.17em}I})$ to proceed through excited vibrational states of M₂. Reaction of the ground vibrational state is dynamically forbidden in each case. Reaction rate coefficients, associated activation energies, pre‐exponential factors, and kinetic isotope effects calculated from rotationally averaged reaction cross sections are in reasonable agreement with experiment. The computations further suggest an origin for the isotope effect that is significantly different from that derived from statistical mechanical theory. A comparison of the relative magnitudes of the experimental activation energy and vibrational level spacing can be used to qualitatively predict the importance of dynamic effects in homogeneous gas‐phase reactions of diatomic species.