Non-orthogonal tight-binding model for tellurium and selenium

Abstract

The twofold coordinated heavier group-VI elements tellurium and selenium with the trigonal crystal structure have unshared electron pairs (lone pairs) which control the interplay of the intra- and inter-chain interactions and their sensitivity on pressure and temperature. We have developed tight-binding (TB) parameters for the helical structures of tellurium and selenium using the Naval Research Laboratory Tight-Binding (NRL-TB) method. The TB parameters were derived by fitting to the band structures and total energies of density functional theory (DFT) calculations. We have applied the TB parameters to study the phase stabilities of different structures under hydrostatic pressure. We have predicted (without fitting) the volume dependence of the rhombohedral and diamond structures, the bcc to rhombohedral and rhombohedral to simple cubic phase transitions and the elastic constants of the trigonal structures, all in agreement with ab initio and experimental results. While the results for the unrelaxed vacancy formation energies and surface energies are in good agreement with the DFT values, we find large discrepancies for the relaxed values indicating that the present set of TB parameters cannot accurately capture the inter-chain interactions.     

Nucleoside-copper(II) system in water and on 13X-zeolite studied by continuous-wave and pulsed-wave ESR

In order to study the effects of large-surface-area solids on the formation of biomacromolecules, copper(II)-nucleosides complexes were studied in water at high pH and after contact with the cavity walls of 13X-zeolite. The results were mainly collected by electron spin resonance in continuous (cw-ESR) and pulsed (ESE) wave. Particular attention was dedicated to adenosine as a nucleoside model for the formation of compounds which were fully characterized in alkaline water solution and after adsorption on commercial 13X-zeolite. In aqueous solution, adenosine was coordinated to copper(II) through deprotonated hydroxyl groups in the 2′ and 3′ positions of the ribose unit, with the formation of [Cu(adenosine)₂(H₂O)₂]²⁻. When adsorbed on zeolite, both cw-ESR and ESE showed that a fraction of the adsorbed complexes did not change their structure and showed high mobility in the zeolite faujasite cavity. The remaining fraction directly interacted with²⁷Al nuclei of the zeolite framework. Other copper(II)-nucleosides behaved in the same manner. Surprisingly enough, alkaline solutions of nucleosides were able to remove copper (II) from zeolite in the form of [Cu(adenosine)₂(H₂O)₂]²⁻ which freely moved in the supernatant aqueous liquid.     

Isolation,reactions and x-ray structure of tetraethylammonium dihydridotetracarbonylrhenate

The isolation and characterization of the novel hydridic compound [Net₄][H₂Re(CO)₄], obtained from the reaction of Re₂(CO)₁₀ with methanolic KOH, are described. Its reactions with I₂, strong acids, and ethanol are reported. The salt gives triclinic crystals, investigated by X-ray analysis, with cell constants a = 14.493(6), b = 7.564(3), c = 7.930(3) Å, α = 72.97(4)°, β = 81,73(4)°, γ = 96.63(4)°; space group P$\text{1}$. The structure was solved by conventional Patterson and Fourier methods and refined by least-squares up to a final R values of 0.071, for 1586 independent counter data. The anion exhibits a distorted octahedral geometry, with the two hydrido ligands in cis position between themselves. The mean values of the ReC and CO distances are 1.89 and 1.21 Å. The carbonyl groups are significantly bent towards the hydrido ligands.     

Measurement of apparent diffusion coefficients within ultrathin nafion Langmuir-Schaefer films: comparison of a novel scanning electrochemical microscopy approach with cyclic voltammetry

The use of scanning electrochemical microscopy (SECM) to evaluate the apparent diffusion coefficient, Dapp, of redox-active species in ultrathin Nafion films is described. In this technique, an ultramicroelectrode (UME) tip, positioned close to a film on a macroscopic electrode, is used to oxidize (or reduce) a species in bulk solution, causing the tip-generated oxidant (reductant) to diffuse to the film/solution interface. The oxidation (reduction) of film-confined species regenerates the reductant (oxidant) in solution, leading to feedback to the UME. A numerical model is developed that allows Dapp to be determined. For these studies, ultrathin films of Nafion were prepared using the Langmuir-Schaefer (LS) technique and loaded with an electroactive species, either the ferrocene derivative ferrocenyltrimethylammonium cation, FA+, or tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+. The morphology and the thickness of the Nafion LS films (1.5 +/- 0.2 nm per layer deposited) were evaluated using atomic force microscopy (AFM). For comparison with the SECM measurements, cyclic voltammetry (CV) was employed to evaluate the concentration of electroactive species within the Nafion LS films and to determine Dapp. The latter was found to be essentially invariant with film thickness, but the value for Ru(bpy)32+ was 1 order of magnitude larger than for FA+. CV and SECM measurements yield different values of Dapp, and the underlying reasons are discussed. In general, the Dapp values for these films are considerably smaller than for recast Nafion films, which can be attributed to the compactness of Nafion LS films. Nonetheless, the ultrathin nature of the films leads to fast response times, and we thus expect that these modified electrodes could find applications in sensing, electroanalysis, and electrocatalysis.     

A reactive and versatile trirhenium carbonyl cluster obtained by oxidation of [re3 (μ-H)4 (CO)10]−. Synthesis,x-ray characterization and reactivity of re3 (μ-H)3 (CO)10(NCMe)2

Oxidation of [Re₃(μ-H)₄(CO)₁₀]^? with CF₃SO₃H in acetonitrile gives the new complex Re₃(μ-H)₃(CO)₁₀(NCMe)₂. This contains a triangle of metal atoms with the edges bridged by the hydrides (mean ReRe 3.266 Å). The acetonitrile ligands, bound to two metals in a trans-diaxial manner, are easily replaced, giving a variety of derivatives.