Sulfur−Gold Orbital Interactions which Determine the Structure of Alkanethiolate/Au(111) Self-Assembled Monolayer Systems

Abstract

With the aim of understanding the nature of the S−Au(111) bonding in organosulfur/Au(111) self-assembled monolayer (SAM) systems, orbital interactions in the adsorption of methanethiolate (-SCH3) in various binding sites of a three-layer slab model and an Au42 cluster model of Au(111) surface are investigated. The methods of choice are crystal orbital overlap population (COOP) and crystal orbital Hamilton population (COHP) analyses for a periodic slab model and fragment molecular orbital (FMO) analyses for the cluster model. The origin of the S−Au(111) bond and the binding site preference are discussed from the viewpoint of orbital interaction. The site preference is in the order of three-fold hollow (fcc and hcp) > bridge > on-top. The second layer Au atoms have little influence on the S−Au(111) bonding, and adsorptions to the fcc and hcp sites are almost identical with respect to energy and S−Au bonding nature. Although σ-type S−Au orbital interactions dominate the S−Au(111) bonding in the on-top model, π-type S−Au orbital interactions play an important role in the bridge, fcc, and hcp models. FMO results explain the vertical S−C bonds in the hollow models and the tilted S−C bonds in the on-top and bridge models.