Structure and Binding of Alkanethiolates on Gold and Silver Surfaces: Implications for Self-Assembled Monolayers

We performed ab initio geometry optimization of HS and CH₃S on cluster models of Au(111), Au(100), Ag(111), and Ag(100) surfaces, at the RECP Hartree-Fock + electron correlation (MBPT2) level. From these we determined classical force field parameters, thus opening the possibility for realistic molecular dynamics (MD) simulations of self-assembled alkanethiolate monolayers on gold and silver surfaces. We find that there are two chemisorption modes, very close in energy, for thiolates on Au(111) surfaces. In the first, the surface-S-C bond angle is ~ 180° (sp hybridization), while in the second it is ~104° (sp³ hybridization). This suggests a possible mechanism for the annealing of alkanethiolate monolayers. We present herein the first molecular mechanics energy minimization using these new force field parameters. Self-assembled monolayers of alkanethiolates on gold and silver are different. The ab initio calculations presented here provide the fundamental understanding of the chemisorption of alkanethiolates on gold and silver surfaces, the surface-adsorbates bonds, and the structure of the monolayers thus formed.