Potential energy surface for the Li+HF→LiF+H reaction

The three dimensional potential energy hypersurface for Li+HF→LiF+H has been studied at the self-consistent field (SCF) and configuration interaction (CI) levels of electronic structure theory with a medium-sized basis set that included polarization functions. The ’’corner’’ of the reaction channel was first mapped by calculation of a lattice of points, then further calculations were carried out to characterize selected points along the minimum energy pathway more precisely. The classical reaction endothermicity was 2.9 kcal/mole, but with zero-point corrections, the reaction was found to be exothermic by 1.7 kcal/mole. As the Li atom approaches the diatomic, it first forms a bent complex with 4.5 kcal/mole of stabilization energy before reaching the transition state. The latter, also bent with an angle of 74° was located in the exit channel and is predicted to be 10 kcal/mole above the reactants. Force constants, vibrational frequencies, and zero-point energies of the complex and the transition state were calculated. After applying zero-point corrections to the transition state, the threshold energy for reaction was reduced to 6.4 kcal/mole, which will probably be further reduced to ∼4 kcal by higher order correlation effects. Our results were compared with previous theoretical efforts and with qualitative theories concerned with the transition state angle and its exit bias. A qualitative discussion of the dynamics over the surface emphasizing interrelationships between the translational, vibrational energy and the LiFH angle ϑ is also presented.