Chirality and Rotation of Asymmetric Surface-Bound Thioethers

A combination of theory and experiment is used to interrogate the surface bonding geometry and rotational dynamics of a simple asymmetric thioether, butyl methyl sulfide. Calculations predict that binding occurs to a Au surface through one of the lone pairs on the sulfur atom and that the alkyl tails lie almost parallel to the surface. The unbonded lone pair remains unperturbed, and the geometry around the S atom is essentially tetrahedral, and hence two surface-bound enantiomers of the prochiral molecule are formed with an R S inversion barrier that is predicted to be large. The rapidly rotating molecules can be clearly distinguished upon atomic-scale imaging by their mirror image pinwheel appearance. The binding of asymmetric dialkyl sulfides leads to the formation of surface-bound enantiomers, which has important consequences for studies in which thioether linkers are used in conjunction with chiral moieties and indicates that diastereomeric effects may be at play in these systems.