COMPARISON OF AB-INITIO AND EMPIRICAL POTENTIALS FOR H-ATOM ASSOCIATION WITH DIAMOND SURFACES

Canonical variational transition-state theory (CVTST) is used to compa re H + CH3 and H + diamond {111} association rate constants calculated from the Brenner empirical potential function and molecular anharmoni c potentials written with switching (MAPS) functions, Previous work [J . Am. Chem. Sec. 1987, 109, 2916; J. Chem. Phys. 1994, 101, 2476] has shown that the MAPS functions, derived from ab initio calculations, gi ve rate constants in agreement with experiment. For the 300-2000 K tem perature range, the Brenner potential function gives CVTST H + CH3 and H + diamond {111} association rate constants which are 159-30 and 49- 7 times smaller, respectively, than the values from the MAPS functions . An analysis of the Brenner potential function shows that it inaccura tely represents the intermediate and long-range H---C association pote ntial, which controls the structure of the variational transition stat e and the CVTST rate constant. The MAPS functions give H + CH3 and H diamond {111} variational transition states with similar properties. Angular momentum and external rotation have no effect on the Il + diam ond {111} association rate constant, which makes it approximately an o rder-of-magnitude smaller than that for H + CH3 association.