Effects of radical site location and surface doping on the radical chain-reaction on H-Si(100)-(2 × 1): A density functional theory study

The radical chain-reactions of allyl mercaptan, ethylene oxide, propylene oxide, and 1,3-butadiene molecules on the H-Si(100)-(2 × 1) are systematically investigated on the basis of a hybrid (ONIOM) model. The formation of γC-, δC-, or δS-site radical intermediate reduces the kinetic selectivity of H-abstraction reactions. The H-abstraction activation energies have the order of across dimer row < interdimer (in the same dimer row) < intradimer H-abstraction, contrasting to the previously reported tendency of βC-site radical intermediate. The steric factor greatly affects the direction-selectivity of radical chain-reaction. The discrepancy between chain-reaction of allyl mercaptan [J. Am. Chem. Soc. 2007, 729, 12304] and trimethylene sulfide molecules [J. Phys. Chem. C 2007, 111, 11965] is rationalized by the doping effect of silicon substrate. It suggests the doping of silicon substrate can alter the direction of the surface chain-reaction. Furthermore, we also theoretically predict the self-directed growth behaviors of ethylene oxide, propylene oxide, and 1,3-butadiene molecules on the H-Si(100)-(2 × 1). The predicted growth behavior of 1,3-butadiene molecules is in good agreement with recent experimental observations. © 2008 American Chemical Society.