Nuclear-motion corrections to Born-Oppenheimer barrier heights for chemical reactions

We use diatomics-in-molecules theory to estimate the nuclear-motion contributions to atom-diatom potential energy surfaces for reactive systems, with special emphasis on the saddle point region. We examine the reactions of H, O, F, and Cl with H₂ and H with Cl₂. Nuclear-motion corrections at the saddle point are in the range 0.007-0.07 kcal/mol for cases with one or two hydrogens and classical barrier heights in the range 0.7-3 kcal/mol and are about 0.2 kcal/mol for cases with two hydrogens and classical barrier heights in the range 8-12 kcal/mol. For F+H₂ with parameters such that the predicted nuclear-motion correction is 0.03 kcal/mol at the saddle point, the correction is as large as 0.2 kcal/mol elsewhere on the surface. Isotopic substitution of D for H changes the classical barrier height by 0.003-0.1 kcal/mol for the cases studied.