Atop adsorption site of sulphur head groups in gold-thiolate self-assembled monolayers

Using normal incidence X-ray standing wave (NIXSW) analysis, we show that the sulphur head group occupies the atop site for methyl-, butyl- and octyl-thiolates on Au(1 1 1), with a Au–S distance of 2.50 ± 0.05 Å, and not the hollow or bridging sites favoured by theoretical calculations. Based on the results of recent theoretical calculations and NIXSW experiments for decyl-thiolate (C10H21S–), it is argued that thiolates may reconstruct the Au surface in such a way as to create a structure in which the increasing intermolecular interactions with increasing chain length may lead to head group displacements away from atop sites.