Transition state theory for multichannel addition reactions: Multifaceted dividing surfaces

A variational multifaceted dividing surface generalization of the variable reaction coordinate (VRC) approach is described. This approach involves the incorporation and optimization of multiple pivot points for each fragment. Illustrative applications to a variety of barrierless reactions with multiple addition channels are presented. For the addition of H atoms to propargyl radical a high level ab initio potential is employed and comparisons are made with trajectory simulations and with prior implementations of VRC-TST. The multifaceted VRC-TST results agree with the trajectory results to within 5-10% as do prior approximate multifaceted VRC-TST results, obtained via the neglect of the flux through certain connecting surfaces. In contrast, results based on the sums of properly variational single faced results differ significantly, being similar to15-20% greater. Notably, the optimal multifaceted transition state dividing surfaces are again in qualitative accord with contours of the radical molecular orbital. Applications to the CH3 + CH3 and C2H3 + O-2 reactions further illustrate the dependence of the results on the use of multiple pivot points, while also illustrating the implementation of directly determined density functional interaction energies. Interestingly, these a priori results are in reasonable agreement with experiment for both these reactions.