Robert E. Benfield

Exact analytical formulae for mean coordination numbers in clusters

Rigorous analytical formulae for mean nearest-neighbour coordination numbers in clusters as a function of cluster size have been derived for a range of geometries: the tetrahedron, octahedron, cuboctahedron, icosahedron and bcc rhombic dodecahedron. Formulae for outer-neighbour coordination numbers are also reported, including a complete analysis of interatomic distances and mean coordination numbers in icosahedra. The formulae will find application in studies of electronic structure and interpretation of EXAFS data.

An EXAFS Study Of The Cluster Molecule Au55(PPh3)12Cl6

Abstract Gold L3 edge EXAFS has been used to study the coordination environment of gold atoms in the cluster molecule Au55(PPh3)12Cl6. The mean coordination of 7 by other gold atoms is consistent with a 3-shell cuboctahedral structure for the Au55 cluster. The first direct measurement of the Au-Au distance in this cluster shows that the spacing is significantly shorter than that in bulk metallic gold and is consistent with calorimetric work which has shown the Au-Au interactions are stronger in the cluster than in bulk gold. There is no evidence to suggest a significant spread of Au-Au distances in Au55(PPh3)12Cl6, in contrast to lower-nuclearity gold cluster molecules which have peripheral Au-Au distances typically 0.2C longer than those to interstitial gold atoms.

Magnetic nanoparticles through organometallic synthesis: evolution of the magnetic properties from isolated nanoparticles to organised nanostructures.

Co and NiFe nanoparticles (2.7 to 3.3 nm mean diameter) of narrow size distribution have been obtained through the decomposition of organometallic precursors in organic solutions of long alkyl chain ligands, namely oleic acid and hexadecylamine. Materials of various volume fractions were produced. The particles have been structurally characterised by WAXS. Both adopt the bulk structure: HCP in the case of cobalt; a mixture of FCC and BCC for NiFe. Their aptitude to self-assemble either on flat supports or in bulk solid state has been investigated by means of TEM and SAXS. This study suggests the crystallisation of the nanoparticles upon solvent evaporation, especially a local FCC arrangement was observed for the NiFe material. Magnetic measurements (SQUID) confirm this tendency. The blocking temperature depends on the metal volume fraction, i.e. on the anisotropy generated by the dipolar couplings (Ki). We show that, for dense samples, the particles of high intrinsic anisotropy, Ku, (Co) still display an individual behaviour while the soft ones (NiFe) display a collective behaviour.

Chloromethylation of poly(methylphenylsilane)

The chloromethylation of poly(methylsilane) is readily achieved using chloromethyl methyl ether in a tin(IV) chloride-catalysed reaction in chloroform solution at 0 °C. A convenient, less hazardous reaction, in which chloromethyl methyl ether is prepared in situ, is also reported. The accompanying variations of the polymer molecular-weight parameters are recorded for chloromethylations extending to 95% of the substituent phenyl groups, and discussed in terms of chain scissions arising at isolated siloxane linkages formed adventitiously during the isolation of the parent polymer.

Organometallic control at the nanoscale: a new, one-pot method to decorate a magnetic nanoparticle surface with noble metal atoms

A novel cascade of reactions is designed to control in situ the deposition of noble metal islands (e.g. Rh) on top of nanoparticles of a 3d metal (e.g. Fe, Co).

An EXAFS study of some gold and palladium cluster compounds

Gold L3-edge EXAFS measurements at 80 K on Au55(PPh3)12Cl6 confirm that the Au-Au distances in this amorphous metal cluster compound are significantly shorter than in bulk gold. The nearest-neighbour Au-Au distances are all equal within experimental uncertainty. Outer-shell Au-Au distances have also been resolved. The results are consistent with the cuboctahedral structure originally proposed for this cluster, but not the polyicosahedral one recently suggested. Very similar results have been obtained from the sulphonated water-soluble derivative Au55(PPh2C6H4SO3Na)12Cl6. In contrast, EXAFS of Au11{PPh2(p-ClC6H4)}7I3 has clearly resolved the two nearest-neighbour Au-Au distances associated with its icosahedral structure.Palladium K-edge EXAFS has been used to study the cluster Pd561(phen)36O200. The Pd-Pd distance is nearly equal to that in bulk palladium. The results show a cubic close-packed cluster structure for this material, in contrast to the icosahedral structure reported for Pd561(phen)60(OAc)180.

Wurtz synthesis of a high-molecular-weight organostannane—organosilane copolymer

Abstract An organosilane—organostannane copolymer of average molecular mass approaching 10 000 Daltons has been prepared by the reaction of dichloromethylphenylsilane and dichloro-n-butylstannane, in a 4:1 molar ratio, with sodium metal in the absence of a solvent. Its subsequent fractionation using hexane led to a product of M w = 15100 with an almost normal distribution ( M w M n = 2.3 ) in 11% yield. Nuclear magnetic resonance spectroscopy indicates an Si:Sn molar ratio of 6.2:1.

An X-ray-Absorption Study Of Gold Coordination-Compounds - EXAFS Refinements And Double-Electron Excitation Background

X-ray absorption spectra at the Au L edges of Au(PPh3)Cl, Au(PPh3)2Cl and KAuCl4.2H2O have been collected as a function of temperature. Data analysis of the extended X-ray absorption fine structure was carried out with the GNXAS package performing an appropriate treatment of the multiple-scattering effects involving several ligand atoms. The accuracy of the current analysis procedures for the Au L edges is assessed and the sensitivity to coordination numbers and distances is established. Evidence for the presence of a (2p4f) double-electron excitation in the atomic Au background is obtained for the first time. The results presented in this paper are of general importance for improving data analysis at the Au L edges and in particular for improving the EXAFS results in Au cluster compounds.

Structural studies of mesoporous alumina membranes by small angle X-ray scattering

Small-angle X-ray scattering (SAXS) has been used to study the structures of mesoporous alumina membranes. These membranes, produced by anodic deposition, have an arrangement of parallel cylindrical pores centred on a disordered hexagonal lattice. The SAXS intensity profile varies over five orders of magnitude, and is a convolution of a structure factor for the 2D distribution of pore axes with a cylinder form factor for the pores. Rotation of the plane of the membrane changes the pattern from a ring structure for channels parallel to the X-ray beam to a set of vertical spots for channels perpendicular to the beam. The oscillatory pattern changes systematically with the anodic deposition voltage, confirming a linear relationship between voltage and pore separation. Initial results are also reported for alumina membranes containing cobalt nanowires in the pore volume. The SAXS technique complements and extends direct-imaging methods such as electron microscopy, which view only the surface structure for a limited area of the sample. The results indicate that the membranes show greater disorder than normally deduced from other techniques.

STRUCTURAL CHARACTERISATION OF THE GIANT ORGANOMETALLIC PLATINUM CLUSTER PT309(PHEN*)36O30 USING EXAFS

The structure of the high-nuclearity organometallic platinum cluster material Pt-309(phen*)(36)O-30 has been investigated by X-ray absorption spectroscopy. Analysis of Pt L-3-edge EXAFS data, using platinum foil as a reference, shows that the platinum clusters have cubic close packed geometry. A slight contraction of the Pt-Pt bond length from the bulk was observed. XANES analysis shows that the platinum atoms in the clusters have a low mean oxidation state, very close to that in metallic platinum