EMPIRICAL POTENTIAL FOR METHYL-RADICAL ASSOCIATION WITH DIAMOND SURFACES

An analytical potential-energy function for . CH3 association with car bon-atom radical sites on diamond surfaces is derived from experimenta l energies, structures, and vibrational frequencies, high-level ab ini tio calculations, and properties of potential-energy functions for H . + . CH3, . CH3 + . CH3, and H . + . C(CH3)(3) association. This . CH3 + diamond potential-energy function is a molecular anharmonic potenti al written with switching (MAPS) functions and is identified by MAPS/M eDIAM. It is a sum of lattice (V-lattice), nonbonded (V-nonbonded), an d radial site (V-CH3,V-site) potential terms. There are many general p roperties of potential-energy functions for alkyl radical association reactions which are transferable and can be used to help construct V-C H3,V-site. Additional properties of V-CH3,V-site are determined by ab initio calculations using the restricted and unrestricted quadratic co nfiguration-interaction method, with single, double, and perturbative triple excitations and the 6-31G* basis set. The MAPS/MeDIAM potentia l function is used to study . CH3 association with a carbon-atom radic al site on the diamond {111} terrace. The energy for . CH3 adsorption with this site is calculated to be 17 kcal/mol lower than the ab initi o CH3-C(CH3)(3) bond dissociation energy. A canonical variational tran sition-state theory calculation, based on the MAPS/MeDIAM potential, g ives a value of 0.06 10(13) cm(3) mol(-1) s(-1) for the . CH3 + diamon d {111} terrace site association rate constant at 1500 K. A linear fre e-energy relationship is shown for the kinetics of . H and . CH3 assoc iation with a diamond {111} terrace site.