Search PubMed⌕ Search

Biomedical subjects

Robert L Whetten

Publications and source records attributed to Robert L Whetten.

6 recordsLinked to original sources
PubMed2006

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).

Journal Article↗
PubMed2005

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.

Gold↗
PubMed2005

Efficient low-temperature oxidation of carbon-cluster anions by SO2.

Carbon-cluster anions, CN-, are very reactive toward SO2 (sticking probability of 0.012 +/- 0.005 for C27- at 25 degrees C), in contrast to their inertness toward other common atmospheric gases and pollutants. In flow reactor experiments at ambient temperature and near atmospheric pressure, primary adsorption of SO2 by the carbon cluster anions, N = 4-60, yields CNSO2- or CN-1S-. The inferred elimination of neutral CO2 is also detected as meta-stable decay in collision-induced dissociation. At higher temperatures, the reaction of SO2 with nascent carbon clusters yields CN-1SO- as well as undetected CO. The size-dependent initial reactivity reflects the previously established structural transitions (i.e., from chain to cyclic to cage structures). Such carbon clusters are formed in sooting flames and may act as nuclei for the formation of primary soot particles and serve as models for the local structural features of active soot particle sites for black-carbon soot. The facile generation of reactive carbon-sulfide and -sulfinate units may therefore have implications for understanding the health and environmental effects attributed to the coincidence of soot and SO2.

Anions↗
PubMed2003

Oxygen adsorption on hydrated gold cluster anions: experiment and theory.

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.

Journal Article↗
PubMed2002

Coadsorption of CO and O(2) on selected gold clusters: evidence for efficient room-temperature CO(2) generation.

Spurred by the recent demonstrations of the size- and support-dependent reactivity of supported gold clusters, here we present results on the coadsorption of CO and O(2) on selected anionic gold clusters, Au(N)(-), in the gas phase. O(2) adsorbs in a binary (0,1) fashion as a one-electron acceptor on the Au(N)()(-) clusters, with even-N clusters showing varying reactivity toward O(2) adsorption, while odd-N clusters show no evidence of reactivity. CO shows a highly size-dependent reactivity for Au(N)(-) sizes from N = 4 to 19, but no adsorption on the gold dimer or trimer. When the gold clusters are exposed to both reactants, either simultaneously or sequentially, interesting effects have been observed. While the same rules pertaining to individual O(2) or CO adsorption continue to apply, the preadsorption of one reactant on a cluster may lead to the increased reactivity of the cluster to the other reactant. Thus, the adsorbates are not competing for bonding sites (competitive coadsorption), but, instead, aid in the adsorption of one another (cooperative coadsorption). New peaks also arise in the mass spectrum of Au(6)(-) under CO and O(2) coadsorption conditions, which can be attributed to the loss of a CO(2) molecule (or molecules). By studying the relative amount of reaction, and relating it to the reaction time, it is found that the gas-phase Au(6) anion is capable of oxidizing CO at a rate 100 times that reported for commercial or model gold catalysts.

Journal Article↗