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)(PH(3))(14)Cl(6) and Au(38)(SCH(3))(24). For Au(38)(SCH(3))(24), a novel structural motif is predicted, consisting of ringlike (AuSCH(3))(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.