Packing and Molecular Orientation of Alkanethiol Monolayers on Gold Surfaces

Preliminary calculations based on a simple model give a good description of the molecular orientation and packing of alkanethiol monolayers on gold surfaces. These calculations suggest that alkanethiol molecules on gold have a total alkyl chain axis tilt of approximately 38° in a plane that bisects the methylene H-C-H angles, followed by a rotation about the alkyl chain axis of ≈46°. The alkyl chain tilt is a function of the sulfur-sulfur spacing in a hexagonal crystal lattice layer and maximizes the attractive interactions between neighboring molecules. These results are in agreement with the molecular orientations obtained from grazing angle FTIR experiments on dodecanethiol monolayers on a (111) gold surface.