Properties of canonical variational transition state theory for association reactions without potential energy barriers

A typical association reaction without a potential energy barrier, H + CH₃ →» CH₄, is used as a model in canonical variational transition state theory (CVTST) calculations for the 200-1000 K temperature range. The calculations demonstrate how the attractiveness of the long-range radial potential and the attenuation of anisotropy in the long-range radial potential for an association reaction affect (1) the "looseness" or "tightness" of the transition state and (2) the size and temperature dependence of the association rate constant. As potential energy surfaces with a Morse function radial potential are varied, there is an abrupt "transition" between outer (free-rotor type) and inner (vibrator type) transition states, with the two transition states coexisting over a very narrow range of temperatures. These two types of transition states are expected to coexist over a wider range of temperatures for potential surfaces with long-range radial potentials proportional to r^-4. The outer and inner transition states are expected to dominate at low and high temperatures, respectively. A previously proposed semiempirical model is extended to interpret trends in the CVTST calculations.