The role of kinematic mass in simple collision models of activated bimolecular reactions

A new treatment of steric effects in activated bimolecular reactions is presented. It is based on a development of the angle-dependent line-of-centres (ADLOC) model and treats the case where the critical surface dividing reagents and products is non-spherical. It is argued that it is the energy associated with motion perpendicular to the equipotential contours at the critical dividing surface, not that directed along the line-of-centres, which must exceed the potential energy at the barrier. The approach takes explicit account of the need to conserve total angular momentum and brings out the importance of a kinematic mass. Opacities, excitation functions, and the effects of reagent rotation are calculated for the reactions O + HCI → OH + Cl and O + HBr → OH + Br and are compared with the predictions of the simplest version of the ADLOC model and with the results of earlier classical trajectory calculations. The present model reproduces the trajectory results quite well without the need to take into account the effects of long-range interactions between the reagents as they approach the critical dividing surface.