Variational transition state theory and vibrationally adiabatic transmission coefficients for kinetic isotope effects in the Cl-H-H reaction system

Variational transition state theory and semiclassical adiabatic ground-state transmission coefficients are applied to calculate the kinetic isotope effects for D and T substitution in the reaction Cl+H₂ at 245-445 K. The calculated isotope effects differ significantly from those calculated previously using conventional transition state theory and semiempirical potential energy surfaces. We use variational transition state theory and conventional transition state theory with the Wigner tunneling correction to adjust three new semiempirical surfaces to the experimental data. No one set of calculations is completely successful. The potential energy surfaces that are most successful at predicting the HD/DH intramolecular kinetic isotope effect have the earliest saddle points (the saddle points are collinear with R‡_Cl-H=2.64-2.78a₀, R‡_H-H= 1.88-1.72a₀). For each surface studied except one, the canonical variational transition states are located past the saddle point for some of the isotopic reactions and earlier than the saddle point for others. The exceptional surface is the one of Valencich and co-workers; that surface has an early saddle point, but the variational transition states are always earlier than the saddle point.