Competing mechanisms in the optically activated functionalization of the hydrogen-terminated Si(111) surface

Recent experiments have shown that organic monolayers on silicon surfaces can be formed through the optically activated surface reaction of H-terminated Si surfaces with terminally unsaturated organic molecules (Eves et al. J. Am. Chem. Soc. 2004, 126, 14318; Sun et al. J. Am. Chem. Soc. 2005, 127, 2514). Possible mechanisms for the formation of this monolayer involve the abstraction of a H atom either at the same attachment site of the molecule (Path A) or from a neighboring site (Path B). Using periodic Density Functional Theory calculations together with an efficient method for finding reaction pathways, we examine both optically activated reaction mechanisms for an alkene and an aldehyde reacting with H-Si(111). Our results show that while Path A is energetically more favorable its significant barrier is likely to limit its viability. Path B on the other hand encounters a much lower H atom abstraction barrier and appears to be more viable.