X‑ray Crystal Structure and Theoretical Analysis of Au_{25–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₁₈^– Alloy

The atomic arrangement of Au and Ag atoms in Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [−SR–Au–SR–Au–SR−] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au_18.3Ag_6.7(SCH₂CH₂Ph)₁₈. This composition is in reasonable agreement with the composition Au_18.8Ag_6.2(SCH₂CH₂Ph)₁₈ measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au₁@Au_5.3Ag_6.7@6×[−SR–Au–SR–Au–SR−]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster