Divide and protect:: Capping gold nanoclusters with molecular gold-thiolate rings

Density functional theory calculations are used to explore phosphine- and thiolate-protected gold nanoclusters, namely, Au-39(PH3)(14)Cl-6 and Au-38(SCH3)(24). For Au-38(SCH3)(24), a novel structural motif is predicted, consisting of ringlike (AuSCH3)(4) units protecting a central Au-14 core. The calculated optical spectrum of this species features a large optical gap ( about 1.5 eV) and a prominently peaked structure, correlating with experimental findings of "molecular-like spectra" of thiolate-protected 1.1 nm gold nanoparticles. Ligand-ligand interactions and steric effects in the ligand shell are suggested as possible driving forces toward an ordered gold core structure. A novel mechanism for ligand-exchange reactions on gold clusters is proposed.