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.