Vibrational Normal Modes of Small Thiolate-Protected Gold Clusters

This report addresses a density functional theory study of the vibrational normal modes of small-sized thiolate-protected gold clusters. Calculated far-infrared (far-IR) and low-frequency Raman spectra of established thiolate-protected gold clusters (4, 12, 18, 19, 20, 24, and 25 Au atoms) show characteristic peaks in the range of 20–350 cm–1 that can be attributed to the breathing mode of the gold core, Au–S–Au bending modes and Au–S stretching modes. It was found that as cluster size increases the Au13 breathing mode of Au25 cluster emerges. This study reveals both systematic and specific characteristics that may provide a basis for identification ("fingerprinting") by far-IR and low-frequency Raman spectroscopy of the many diverse gold–thiolate cluster compounds.