Properties of a ubiquitous 29 kDa Au:SR cluster compound

The broad, intense peak found near 29 kDa in the laser-desorption mass-spectral abundances of various aurothiol (Au:SR) cluster compounds has been used to optimize the preferential formation of the species in that mass range. Recrystallization gives enriched fractions, on the 10 mg scale, that in several cases appear free of species outside that mass range. Elemental analysis and X-ray photoelectron spectra (XPS) confirm the absence of elements other than Au, S, C, and H, while infrared and NMR (¹H,¹³C) spectra are consistent with intact thio groups. The Au 4f_7/2 XPS peak is only slightly shifted (∼0.2 eV) and broadened from that of bulk Au(0) metal, and intense optical absorption extends far into the infrared region (0.5 eV), consistent with a metallic Au core. Recrystallized samples of the R = C4, C6, and C12 materials readily form highly diffractive crystalline films, powders, and single crystals on the scale of 10 μm, consistent with perfect ordering in > 100 nm grains, and a negligible amorphous content. Uniformity is quantified through several independent measures: (1) the mass spectrometrically determined core mass of 29.2 ± 2 kDa is invariant to that of the thiol used, indicating an equivalent Au core diameter of 1.68 ± 0.05 nm, assuming bulk density. (2) The powder X-ray diffraction intensities are sensitively fit to a Au core of 1.64 ± 0.03 nm equivalent diameter: (3) The powder patterns index unambiguously to bcc packing, with nearest-neighbor distances of 2.68 ± 0.02 nm (R = C6) and 3.15 ± 0.02 nm (R = C12). An fcc packing structure with 2.56 ± 0.04 nm distance (11.9 nm³ volume) is found for R = C4. A formulation consistent with this mass spectral, diffraction, and average compositional information is Au_144-146(SR)_50-60, with the structure of the inorganic core being influenced by the type of adsorbate used to produce the entire inorganic/organic assembly.