Predicting the shape and structure of face-centered cubic gold nanocrystals smaller than 3 nm.

Although a number of computational studies have examined the relative stability of icosahedral and decahedral gold clusters from 1 to 3 nm in size, few studies have focussed on the variety of face-centered cubic (fcc) nanoparticles in this size regime. In most cases small fcc gold particles are assumed to adopt the truncated octahedral shape, but in light of the fact that the shape and structure of gold nanoparticles is known to vary, the relative stability of fcc polyhedra may change with size. Presented here are results of first-principles calculations investigating the preferred shape of gold particles less than 3 nm in size. Our results indicate that the equilibrium shape of fcc gold nanoparticles less than 1 nm is the cuboctahedron, but this shape rapidly becomes energetically unstable with respect to the truncated octahedron, octahedron and truncated cube shapes as the size increases.     

Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation

The use of B3LYP/6–31G^* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G^* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G^* are found to do as well as MP2/6–31G^* geometries in the calculation of the enthalpies of formation.     

Polymer contribution to the thermal conductivity and viscosity in a dilute solution (Fraenkel Dumbbell model)

The phase-space kinetic theory for polymeric liquid mixtures is used to obtain an expression for the polymer contribution to the thermal conductivity of a nonflowing, dilute solution of polymers, where the polymer molecules are modeled as Fraenkel dumbbells. This theory takes into account three mechanisms for the energy transport: diffusion of kinetic energy (including the Öttinger-Petrillo term), diffusion of intramolecular energy, and the work done against the intramolecular forces. This paper is an extension of previous developments for the Hookean dumbbell model and the finitely-extensible dumbbell model. A comparison among the dumbbell results suggests that the thermal conductivity increases with chain stiffness. In addition, the zero-shear-rate viscosity and first normal-stress coefficient are also given for the Fraenkel dumbbell model.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday.     

The Convexity Number of a Graph

 For two vertices u and v of a connected graph G, the set I[u,v] consists of all those vertices lying on a u−v shortest path in G, while for a set S of vertices of G, the set I[S] is the union of all sets I[u,v] for u,v∈S. A set S is convex if I[S]=S. The convexity number con(G) of G is the maximum cardinality of a proper convex set of G. The clique number ω(G) is the maximum cardinality of a clique in G. If G is a connected graph of order n that is not complete, then n≥3 and 2≤ω(G)≤con(G)≤n−1. It is shown that for every triple l,k,n of integers with n≥3 and 2≤l≤k≤n−1, there exists a noncomplete connected graph G of order n with ω(G)=l and con(G)=k. Other results on convex numbers are also presented. Received: August 19, 1998 Final version received: May 17, 2000     

Carbon ad-dimer defects in carbon nanotubes

The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications. Barriers for the formation of carbon dimer induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect. The electronic states introduced by the ad-dimers depend on defect structure and tube type and size. Multiple carbon ad-dimers provide a route to structural engineering of patterned tubes that may be of interest for nanoelectronics.     

Synthesis and characterization of mono- and micro6-sulfato hexanuclear zinc complexes of a new symmetric dinucleating ligand

Zinc complexes of a new symmetric dinucleating ligand, N,N'-Bis[2-carboxybenzomethyl]-N,N'-Bis[carboxymethyl]-1,3-diaminopropan-2-ol (H5ccdp) with mixed donating groups, have been studied in the solid state as well as in solution. In methanol, the reaction of stoichiometric and substoichiometric amounts of Zn(ClO4)2 x 6H2O and the ligand H5ccdp, in the presence of K2CO3 or Et3N, afforded a mononuclear zinc complex, [Zn(H2O)6][Zn(H2ccdp)(H2O)2]2 x 12H2O (1). The solid state structure of 1 contains two units of the zinc-ligand anion, [Zn(H2ccdp)(H2O)2]-, and one [Zn(H2O)6]2+ counter cation. The Zn(II) center of the anion is in a distorted octahedral geometry. However, in methanol, the reaction of ZnSO4 x 7H2O and the ligand Hsccdp in the presence of NaOH afforded a unique micro6-sulfato hexanuclear zinc complex, Na6[Zn6(ccdp)3(micro6-SO4)](OH) x 10.5H2O (2). The structure of 2 contains a [ZnII6(micro6-SO4)] core unit which is held together by three heptadentate bridging ligands, ccdp5-. Three of the Zn(II) centers are in highly distorted square pyramidal geometry, the other three Zn(II) centers are in a distorted octahedral geometry.     

Oxidation of organic films by beams of hydroxyl radicals

We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' ' 相似文献