ANALYTICAL POTENTIAL-ENERGY SURFACE FOR METHANE IN TERMS OF INTERATOMIC FORCES

Ab initio bending potentials for methane are compared to potentials for the mutual interaction of non-bonded hydrogen atoms. Bonding and non-bonding contributions have been incorporated in a simple, analytical expression for the dependence of potential energy on molecular geometry. The expression, with four fixed and ten fitted parameters, agrees with 112 ab initio points within a root-mean-square deviation of 4 kJ mol-1. Variational transition-state theory rate constants and harmonic vibrational frequencies have been calculated from the fitted potential surface. For the functional form used, non-bonded repulsions between hydrogen atoms contribute 29% to the bending force constant at the potential minimum, reduce the bond dissociation energy by 120 kJ mol-1 and cause the formation of a potential energy ridge when H approaches the edge of CH3.