A QM/MM model for O(³P) reaction with an alkyl thiolate self-assembled monolayer

A QM/MM potential energy surface model is developed for the reaction of O(³P) with a self-assembled monolayer (SAM) of alkyl thiolate chains on Au{111}. The QM part of the model consists of O(³P) and CH₃-(CH₂)_n- surface moieties of a group of the SAM's central alkyl thiolate chains. A model for this QM part is developed by refitting the PM3 semiempirical electronic structure theory to specific reaction parameters (SRPs). In this model, the CH₃-CH₂- group represents the surface moieties and experimental and high-level ab initio data for the O(³P) + C₂H₆ → OH + C₂H₅ reaction are used to fit the SRPs. The resulting QM/MM potential energy surface is used in an initial trajectory simulation of O(³P) reaction with the n-hexyl thiolate SAM/Au{111} surface as a function of incident energy and azimuthal angle. The trajectories are restricted to the triplet potential energy surface. The results of this preliminary and incomplete trajectory study are compared with a related recent experimental study of O(³P) with liquid squalane (D.J. Garton, T.K. Minton, M. Alagia, N. Balucani, P. Casavecchio, G.G. Volpi, Faraday Discuss. 108 (1997) 387). A complete trajectory study of the O(³P) + n-hexyl thiolate SAM reaction dynamics, based on this QM/MM model, will be reported later.