COMPARISON OF MOLECULAR-DYNAMICS AND VARIATIONAL TRANSITION-STATE-THEORY CALCULATIONS OF THE RATE-CONSTANT FOR H-ATOM ASSOCIATION WITH THE DIAMOND (111) SURFACE

The quasiclassical trajectory method was used to study the dynamics an d kinetics of H-atom association with a C-atom radical site on the dia mond {111} surface. The calculations employed an analytic potential-en ergy surface derived previously [P. de Sainte Claire, P. Barbarat, and W. L. Hase, J. Chem. Phys. 101, 2476 (1994)] from high-level ab initi o calculations. The trajectory rate constant calculated here of 1.7+/- 0.3 X 10(13) cm(3) mol(-1)s(-1) for temperatures of 1000 and 2000 K is in excellent agreement with the one calculated previously on the same analytic potential-energy surface using canonical variational transit ion-state theory. For H atoms impinging perpendicularly and directly o nto the C-atom radical site, the association probability is sensitive to the frequencies of the diamond lattice and the treatment of the lat tice's zero-point energy. However, trajectories with this orientation make a negligible contribution to the thermal rate constant, which is found to be nearly insensitive to the lattice frequencies and Zero-poi nt energy. Trajectories, for which H atoms move toward the surface wit h a small angle relative to the surface plane and pass above the C-ato m radical site before associating, make an important contribution to t he association rate constant.