Understanding the Chemisorption of 2-Methyl-2-propanethiol on Au(111)

The chemisorption of 2-methyl-2-propanethiol on the Au(111) surface is studied using density functional
theory, interpreting the observed lack of pitting of the surface and the appearance of the unusual (2√7 ×
√7)R19.1°−2 adsorbate lattice. Bader charge analysis indicates that the adsorbate forms as a thiyl species,
with the S−Au bond displaying primarily Au d to S p dative covalent character. While modulation of this
contribution is shown to provide the driving force for possible chemisorption to supersurface gold adatoms
rather than to the flat surface, the advent of a minor bonding process involving S p to Au s dative covalent
bonding that is inactive for linear alkanethiol chemisorption is found to control the properties of chemisorbed
branched alkanethiols. This contribution is shown to enhance the binding to the flat surface, inhibiting adatom
chemisorption and thus preventing pitting of the surface, as well as stabilizing the adsorbate structure in
which the sulfur sits above an fcc site. This stabilization makes the fcc and more-usual distorted bridge sites
nearly equal in energy and leads to the occupancy of both sites in a complex (2√7 × √7)R19.1°−2 adsorbate
lattice.