The relation of chemical potentials and reactivity studied by a state path sum

Abstract

Exact quantum transition probabilities for collinear F + H₂ → FH + H on two different potential energy surfaces are analyzed in terms of regional effects of the potentials. The potentials considered are (a) a weakly curved rotated harmonic model potential and (b) a strongly curved rotated Morse potential fitted to the ab initio SCFCI surface of Bender et al. In both potentials a, b the energy along the reaction path has the same parameters, and three successive regions 1–3 are analysed. The main results are: (1) at very low total energies the reactants are reflected from the activation barrier (region 1 of potentials a, b). (2) in the downhill region 2 of potentials a and b the molecules are weakly and strongly excited, respectively. Region 2 of the realistic potential b also causes H₂ back reflection, depending on the total energy (although there is no barrier in region 2!); (3) In the final shallow sloped, weakly curved region 3 of potential a, part of the excited molecules are again de-excited before reaching the product's configuration. On the contrary, region 3 of potential b has a non-curved reaction path, and the reaction is adiabatic therein, i.e. the van der Waals minimum is irrelevant for vibrational excitation. Practical aspects of the state path sum method are discussed in detail.