AB-INITIO POTENTIAL AND VARIATIONAL TRANSITION-STATE THEORY RATE-CONSTANT FOR H-ATOM ASSOCIATION WITH THE DIAMOND(111) SURFACE

High-level ab initio calculations were performed to determine accurate analytic potential energy functions for interactions a gas-phase H-at om has with H-atoms attached to the diamond (111) surface and with C-a tom radical sites on this surface. The nonbonded potential between the gas-phase H-atom and H-atoms attached to the surface was determined f rom coupled-cluster at; initio calculations, including single, double, and perturbatively applied triple excitations [CCSD(T)], with the 6-3 11++G(2df,p) basis set. The resulting nonbonded potential is nearly id entical to that found previously from both theory and experiment for i nteractions between II-atoms on different hydrocarbon molecules. In th e ab initio calculations, a C-atom radical, site on the diamond surfac e is represented by a constrained tert-butyl radical. Radial and small -displacement angular potentials for a H-atom interacting with this ra dical were determined from unrestricted quadratic configuration intera ction calculations, with single, double and perturbatively applied tri ple excitations [UQCISD(T)], with the 6-31G* basis set. UQCISD(T) cal culations were performed on the H+CH3-->CH4 reaction system with both the 6-31G* and 6-311++G(3df,3pd) basis sets to calibrate the accuracy of the 6-31G* basis set results for the H-atom plus constrained tert -butyl radical. The above information was used to construct an analyti c potential energy function for H-atom association with a radical site on the (111) surface of diamond, which was then employed in a canonic al variational transition state theory (CVTST) calculation of the asso ciation rate constant. The resulting rate constant is 1.8-2.1x10(13) c m(3) mol(-1) s(-1) for the 1000-2000 K cm temperature range. It is ins ensitive to the gas-phase H-atom/surface H-atom nonbonded potential an d the potential for the diamond lattice. The H+diamond (111) CVTST rat e constant is used to estimate a rate constant of 4X10(13) cm(3) mol(- 1) s(-1) for H+tert-butyl association at 298 K. The UQCISD(T)/6-31G c alculations give a H-C(CH3)(3) bond dissociation energy which is only 1 kcal/mol lower than the experimental value.