Theoretical Characterization of Cyclic Thiolated Gold Clusters

The density functional theory is used to explore structural, energetic, vibrational, and optical properties of thiolated gold clusters (MeSAu)x with x = 2-12. Clusters up to (MeSAu)4 adopt Au-S ring conformations, and crownlike structures are formed for larger sizes. The clusters are essentially polymeric and show convergence in structural and energetic properties at (MeSAu)4. The nature of the Au-S bond is polar covalent with a degree of cyclic electron delocalization. Vibrational analysis reveals characteristic Au-S stretch vibrations at approximately 300 cm(-1). Effects of ligand substitution are studied in the case of the tetramer by comparing the results for methylthiolate with hexylthiolate, benzenethiolate, and glutathionate (GS). The choice of ligand has clear effects on electronic properties. For example, the optical gap is approximately 1.5 eV lower for (GSAu)4 than for (MeSAu)4.