Kinematics and dynamical effects in the total and differential excitation functions. A variational transition state theory analysis

Abstract

A variational transition state theory, previously developed to account for the kinetic energy dependence of the reaction cross section, has been extended to include the effect of the curvature of the reaction coordinate as well as to calculate the energy disposal of elementary reactions. A systematic study of the influence of kinematics and dynamical factors in the kinetic energy dependence of both the energy disposal and reaction cross section has been made. The theory was applied to ideal systems with different mass combinations as well as to the M + RX → MX + R reaction family (M = Na, K, Rb; R = CH 3; X = Cl, Br, I). One of the most important findings of the present study is that for a given potential energy surface there is a correlation between the energy disposal as well as the excitation function, with the actual location of the maximum in the curvature of the reaction coordinate. Those systems for which the location of the maximum of the curvature along the reaction coordinate lies close to that of maximum gradient in the potential energy showed a clear non-adiabatic behaviour, i.e. there is a significant flux between the (translational) energy along the reaction coordinate and the (vibrational) energy perpendicular to it. Therefore they showed the higher fraction in vibrational energy of the products as well as the higher collision energy dependence of the reaction cross section (i.e. higher slope of σ R versus E T.