An ab initio electronic structure study of methyl adsorption and reaction on cluster models for the diamond surface

Electronic structure calculations were carried out for a series of hydrogen-terminated carbon clusters designed to model the 100- and 111 -diamond surfaces, C_d(100) and C_d(111). The subjects of the calculations were: (1) methyl radical (CH₃) adsorption on an activated diamond surface; and (2) hydrogen abstraction from adsorbed methyl via reaction with gas-phase atomic hydrogen. The largest clusters were treated at the MP2/6-31G^*//HF6-31G ^* level of theory. The results of higher level calculations on smaller clusters were used to estimate corrections to the MP2/6-31G ^*//HF/6-31G^* energies. It is concluded that methyl adsorption is 6.8 kcal mol^-1 more exothermic on C_d(100) than on C_d(111). Also, the barrier for hydrogen abstraction from methyl adsorbed on C_d(100) is 2.4 kcal mol^-1 lower than that for abstraction from methyl adsorbed on the C_d(111) surface. Finally, the abstraction reaction energy is 0.8 kcal mol^-1 lower for methyl adsorbed on C_d(100) compared to methyl adsorbed on C_d(111).