Hydrogen Abstraction from a Diamond Surface. Ab Initio Quantum Chemical Study with Constrained Isobutane as a Model

Abstraction of terminal hydrogens on a diamond I1 1 I } surface by atomic hydrogen has been offered as the possible rate-determining elementary step in the mechanism of low-pressure diamond growth by chemical vapor deposition. We use ab initio multiconfiguration self-consistent-field methods to estimate the activation energy for this abstraction reaction. We do this by first computing features of the potential energy surface for hydrogen abstraction from gas-phase isobutane and then computing features of the potential energy surface for this same system imposing constraints that mimic those found in a diamond lattice. Our results indicate that, although 5.4 kcal/mol of the CH bond energy in isobutane is attributable to structural relaxation of the radical, most of this radical relaxation energy (4.5 kcal/mol of it) is realized even with geometric constraints similar to those in a diamond lattice. We therefore predict bonds to a diamond surface to be only about 1 kcal/mol stronger than corresponding bonds to a gas-phase tertiary-carbon atom. The effect of the geometrical constraints on the activation energy for the hydrogen abstraction reaction is even smaller: all but 0.2 kcal/mol of the gas-phase radical relaxation energy at the transition state is realized even with the imposition of lattice-type constraints. Our results therefore support the use in kinetic modeling or molecular dynamics simulations of activation energies taken from analogous gas-phase hydrocarbon reactions with little or no adjustment.