THEORETICAL-STUDIES OF HYDROGEN-ABSTRACTION REACTIONS FROM DIAMOND AND DIAMOND-LIKE SURFACES

Reaction probabilities, cross sections, rate coefficients, frequency factors, and activation energies for hydrogen-atom abstraction from a hydrogen-covered C (111) surface have been computed using quantum wave packet and classical trajectory methods on the empirical hydrocarbon #1 potential hypersurface developed by Brenner. Upper bounds for the abstraction rates, activation energies, and frequency factors have been obtained for six different chemisorbed moieties on a C(111) diamond surface using a classical variational transition-state method. For the hydrogen-covered surface, the results of the wave packet/trajectory calculations give k(T) = 1.67 x 10(14) exp(-0.46 eV/k(b)T) cm3/mol s, which is about a factor of 2.9 less than the gas-phase abstraction rate from tertiary carbon atoms at 1200 K. The variational calculations show that the activation energies for hydrogen-atom abstraction vary from 0.0 to 1.063 eV. Some sp2-bonded hydrogen atoms can be removed in a barrierless process if adjacent to a carbon radical. In contrast, abstractions that produce a methylene carbon are associated with much larger activation energies in the range 0.49-0.82 eV. Abstraction from nonradical chemisorbed ethylene structures of the type that might be formed by the chemisorption of acetylene at two lattice sites is a particularly slow process with a 1.063 eV activation energy. Hydrogen abstraction from sp3 carbon atoms have activation energies approximately 0.4 eV. The results suggest that phenomenological growth models which assume either an equilibrium distribution between surface hydrogen/H-2 or a common abstraction rate for surface hydrogen atoms are unlikely to be accurate.