Thiolate-Protected Au24(SC2H4Ph)20 Nanoclusters: Superatoms or Not?

We report a new gold thiolate cluster with molecular purity. Electrospray ionization (ESI) mass spectrometry in conjunction with thermogravimetric analysis (TGA), elemental analysis (EA), and H-1 NMR, unambiguously determined the composition of the as-prepared Au nanocluster to be Au-24(SC2H4Ph)(20). The optical absorption spectrum of this cluster shows a highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) transition at 765 nm, indicating quantum confinement of electrons in the particle. The HOMO-LUMO gap (similar to 1.5 eV) of Au-24(SR)(20) is much smaller than that of Au-20(SR)(16) (similar to 2.1 eV) but slightly larger, than that of Au-25(SR)(18) (similar to 1.3 eV). The number of valence electrons in Au-24(SC2H4Ph)(20) is 4e, which is not predicted by the superatom model.