The total structure determination of thiol-protected Au clusters has long been a major issue in cluster research. Herein, we report an unusual single crystal structure of a 25-gold-atom cluster (1.27 nm diameter, surface-to-surface distance) protected by eighteen phenylethanethiol ligands. The Au25 cluster features a centered icosahedral Au13 core capped by twelve gold atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly symmetric structure is distinctly different from recent predictions of density functional theory, and it also violates the empirical golden rule "cluster of clusters", which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand-gold core interactions. The Au25(SR)18 clusters exhibit multiple molecular-like absorption bands, and we find the results are in good correspondence with time-dependent density functional theory calculations for the observed structure.

This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).

Structural, electronic, and optical properties of the thiolate-protected Au(38)(SR)(24) cluster are studied by density-functional theory computations (R = CH(3) and R = C(6)H(13)) and by powder X-ray crystallography (R = C(12)H(25)). A low-energy structure which can be written as Au(23)@(Au(SR)(2))(3)(Au(2)(SR)(3))(6) having a bi-icosahedral core and a chiral arrangement of the protecting gold-thiolate Au(x)(SR)(y) units yields an excellent match between the computed (for R = C(6)H(13)) and measured (for R = C(12)H(25)) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au(23) core by using a particle-in-a-cylinder model. Although the alkane thiolate ligands are achiral, the chiral structure of the ligand layer yields strong circular dichroism (CD) in the excitations below 2.2 eV for Au(38)(SCH(3))(24). Our calculated CD spectrum is in quantitative agreement with the previously measured low-energy CD signal of glutathione-protected Au(38)(SG)(24). Our study demonstrates a new mechanism for the strong chiral response of thiolate-protected gold clusters with achiral metal cores and ligands.

We report reversible switching of paramagnetism in a well-defined gold nanoparticle system consisting of atomically monodisperse nanoparticles containing 25 gold atoms protected by 18 thiolates [abbreviated as Au(25)(SR)(18)]. The magnetism in these nanoparticles can be switched on or off by precisely controlling the charge state of the nanoparticle, that is, the magnetic state of the Au(25)(SR)(18) nanoparticles is charge-neutral while the nonmagnetic state is an anionic form of the particle. Electron paramagnetic resonance (EPR) spectroscopy measurements establish that the magnetic state of the Au(25)(SR)(18) nanoparticles possess one unpaired spin per particle. EPR studies also imply an unusual electronic structure of the Au(25)(SR)(18) nanoparticle. Density functional theory calculations coupled with the experiments successfully explain the origin of the observed magnetism in a Au(25)(SR)(18) nanoparticle as arising from one unpaired spin having distinct P-like character and delocalized among the icosahedral Au(13) core of the particle in the highest occupied molecular orbital. The results suggest that the Au(25)(SR)(18) nanoparticles are best considered as ligand-protected superatoms.