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

Reaction probabilities, cross sections, rate coefficients, frequency f actors, and activation energies for hydrogen-atom abstraction from a h ydrogen-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 ab straction rates, activation energies, and frequency factors have been obtained for six different chemisorbed moieties on a C(111) diamond su rface using a classical variational transition-state method. For the h ydrogen-covered surface, the results of the wave packet/trajectory cal culations give k(T) = 1.67 x 10(14) exp(-0.46 eV/k(b)T) cm3/mol s, whi ch is about a factor of 2.9 less than the gas-phase abstraction rate f rom 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 ba rrierless process if adjacent to a carbon radical. In contrast, abstra ctions that produce a methylene carbon are associated with much larger activation energies in the range 0.49-0.82 eV. Abstraction from nonra dical chemisorbed ethylene structures of the type that might be formed by the chemisorption of acetylene at two lattice sites is a particula rly slow process with a 1.063 eV activation energy. Hydrogen abstracti on 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.