Potential energy surfaces for simple chemical reactions:. Application of valence-bond techniques to the Li + HF → LiF + H reaction

Abstract

Ab initio multi-structure valence-bond calculations have been performed to determine the potential energy surface governing the reaction Li + HF → LiF + H. Results for both linear and non-linear nuclear geometries are presented. The system is a prototype for many heavier alkali metal plus hydrogen halide reactions which have been studied using the crossed molecular beams technique. The ab initio valence-bond results are improved by applying corrections, within the framework of the orthogonalized Moffitt (OM) method, for the atomic errors present. The orbital basis set used was of double zeta quality, and was augmented by some extra orbitals. Preliminary calculations on the neutral and ionic diatomic species were performed to ensure the adequancy of the valence-bond structure basis sets used and care was taken to ensure that the basis sets provided an adequate description of F^–, HF^– and LiF^–. The endoergicity of the reaction, ignoring the zero-point vibrational energies, was predicted by the ab initio and OM methods to be 5.8 and 2.5 kcal mol^–1 respectively, as compared with the experimental value of 2.6 kcal mol^–1. Besides the ground state potential energy surface, several surfaces for excited electronic states have been calculated and are presented. The relationship of the ground state potential energy surface to the reactive cross section, and its variation with energy, is discussed. The ground and excited state potential energy surfaces are compared with previously proposed models and a mechanism for the production of alkali metal ions in hyperthermal alkali metal atom–hydrogen halide collisions is proposed.