Computational studies of copper, silver, and gold alkanethiolates and alkaneselenates

Ab initio and density functional calculations have been performed for metal alkanethiolates and metal alkaneselenates containing one or three metal atoms of copper, silver, or gold. Alkanethiolates and alkaneselenates with alkane chains of one, three, and six carbons in length were considered. Equilibrium geometries, charge distributions, and HOMO-LUMO energy gaps were obtained. The objective of the study was to investigate invariant geometrical and electronic properties with respect to metal atom substitution, sulfur/selenium substitution, or alkane chain length. The study was carried out in order to help explain recent surface-enhanced electronic Raman scattering (SEERS) spectra of self-assembled monolayers of alkanethiolates on roughened gold surfaces. The observed SEERS transitions have been shown to be nearly independent of substitution of silver for gold, substitution of selenium for sulfur, and alkane chain lengths of 9-18 carbons. From the calculations, little change in the sulfur-carbon or selenium-carbon bond distances was observed as the metal atom was varied from copper to silver to gold. On the other hand, the metal-sulfur and metal-selenium distances varied by over 0.2 Å as a function of metal atom. Significant differences also were observed for the charges of the metal atom and the sulfur or selenium atom. However, the charge of the carbon atom attached directly to sulfur or selenium was found to be invariant to metal or S/Se substitution for alkane chains greater than one carbon in length. Finally, the HUMO-LUMO gap was determined to be rather insensitive to metal atom and sulfur/selenium substitution.