Raman spectroscopy of benzenethiolates on nanometer-scale gold clusters

Near-infrared (1064-nm) irradiation of neat solid samples of benzenethiolate monolayer-protected gold clusters (MPCs) yields strong, well-resolved Raman spectra of the thiolate groups, comparable to those obtained for the same groups adsorbed at roughened gold electrodes. These clusters are formulated as TOAZ[AuN(SPh)M]Z-, N > M and Z = 3-6, with core diameters of 1.7 and 1.5 nm, and were characterized previously by X-ray scattering, mass spectrometry, infrared spectroscopy, optical spectroscopy, nuclear magnetic resonance, and elemental analysis [Price, R. C.; Whetten, R. L. J. Am. Chem. Soc. 2005]. Numerous previous attempts to obtain spectra on various MPCs yielded only diffuse luminescence bands, as did benzenethiolate MPC samples of TOA2[Au44(SPh)28]2-, with 1.1-nm core diameters. The clusters are free of excess tetraoctylammonium bromide (TOABr) from the synthetic procedure, containing only the necessary TOA+ to maintain charge balance. In situ thermometry, using the anti-Stokes/Stokes intensity ratios, indicated the sample temperature remained below the onset of thermal decomposition. The Raman spectra of the clusters bear a strong resemblance to those obtained for nonmetallic (Au(I)SPh)x polymer samples that are not in resonant absorption at the laser wavelength. The smaller of the two cores, nominally TOA6[Au110(SPh)62], shows clearly a band at 505 cm(-1) assigned to a S-S stretch, suggestive of a moiety resembling diphenyl disulfide on the cluster surface. These results are interpreted with reference to recent reports suggesting a substantial "reconstruction" of the outermost gold layer upon thiolate adsorption (SAM formation).     

On the structure of thiolate-protected Au25

Density functional theory is used to explore the structure of Au25(RS)18. The preferred structure consists of an icosahedral Au13 core protected by 6 RS-Au-RS-Au-RS units. The enhanced stability of the structure as an anion is found to originate from closure of an eight-electron shell for delocalized Au(6s) electrons. The evaluated XRD pattern and optical spectra are in good agreement with experimental data.     

Metastability of gold-carbonyl cluster complexes, Au N(CO) M -

Smaller gold-cluster anions, typified by Au ₇ ^- , adsorb multiple CO molecules in a high-pressure, room-temperature flow-reactor, tending toward previously unknown saturation compositions, Au₇(CO) ₄ ^- . The weakness of the gold-carbonyl adsorption bond is evidenced indirectly by the high CO partial pressure required and more directly by the high probability of fragmentation in the field-free flight region of the reflectron-type time-of-flight mass spectrometer. The analysis of this metastability reveals that the actual distribution f_N,M of products Au₇(CO) _M ^- in the reactor may be highly non-statistical, e.g. with only even-M species present. Received 17 April 2001     

Ionization energies and stabilities of Na n,n<25: shell structure from measurements on cold clusters

The stability patterns found in alkali-atomic clusters and their explanation in terms of electronic and structural factors have been controversial for some time. Generation of very cold Na _n clusters in a novel source and use of a special photoion normalization method resolve the remaining questions by allowing precise determination of photoionization thresholds. This is demonstrated here for several sizes in the 7<n<26 range, where in two crucial cases the interpretations of earlier ionization threshold measurements on oven-beam clusters [M. Kappes, M. Schär, P. Radi, and E. Schumacher, J. Chem. Phys.84, 1863 (1986)] disagreed with the explanation of the observed stability pattern. Combining the new values with the charged-cluster fragmentation energies of Bréchignac et al. [C. Bréchignac, P. Cahuzac, J. Leygnier, and J. Weiner, J. Chem. Phys.90, 1492 (1989)] yields neutral cluster fragmentation energies that successfully account for the famous “magic-number” ledges [W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou, and M.L. Cohen, Phys. Rev. Lett.52, 2141 (1984)]. Our measurements offer decisive support for the applicability of the spherical/spheroidal electronic shell-model to smaller Na _n clusters, even in their low-temperature form.     

Chaotic Expansion of Elements of the Universal Enveloping Algebra of a Lie Algebra Associated with a Quantum Stochastic Calculus

The functional Ito formula, in the form df() = f( + d ) –f(),is formulated and proved in the context of a Lie algebra L associatedwith a quantum (non-commutative) stochastic calculus. Here fis an element of the universal enveloping algebra U of L, andf() + d() – f() is given a meaning using the coproductstructure of U even though the individual terms of this expressionhave no meaning. The Ito formula is equivalent to a chaoticexpansion formula for f() which is found explicitly. 1991 MathematicsSubject Classification: primary 81S25; secondary 60H05; tertiary18B25.     

All-aromatic, nanometer-scale, gold-cluster thiolate complexes

We report a method to generate and isolate 22 and 29 kDa core mass gold:benzenethiolate monolayer-protected cluster (MPC) molecules and a subsequent reaction to yield [(octyl)4N]2[Au44(SC6H5)28] (8.7 kDa core) clusters from these materials. Characterization methods for these 1.0-1.7 nm molecules include vibrational, electronic, 1H NMR, and mass spectrometry, as well as powder X-ray diffraction. The clusters are anionic and charge balanced by tetraoctylammonium. They possess size-dependent optical absorbance bands consistent with size-quantized gold cores and are susceptible to electrochemical degradation in polar organic solvents. To our knowledge this is the first report of all arylthiolate gold MPC molecules, and they represent ideal materials for crystallization and total structural characterization.     

Ultraviolet absorption bands of [Cs n+1 I n ]+ (n<14) clusters

Starting from the large photofragmentation cross sections recently reported for mass-selected alkali-halide clusters [1], we have obtained, for the first time, optical absorption spectra forCs _n+1 I _n ⁺ clusters in the ultraviolet charge-transfer bands. In this short report we focus on the spectra ofn=4, 6 and 13 which start to show absorption between the lowest charge-transfer band of the diatomic (ca. 3.8 eV) and that of the bulk (ca. 5.9 eV). The spectral features are analyzed for these sizes and correlated to their structures qualitatively as a preliminary model.