A first-principles study of surface and subsurface H on and in Ni(1 1 1): diffusional properties and coverage-dependent behavior

Periodic, self-consistent, density functional theory (GGA-PW91) calculations are performed for both surface and subsurface atomic hydrogen on and in Ni(1 1 1). At a low coverage (θ=0.25 ML), the binding energies (BEs) of a hydrogen atom in surface fcc, subsurface octahedral (first layer), and subsurface octahedral (second layer) sites are −2.89, −2.18, and −2.11 eV, respectively. The activation energy barriers for hydrogen diffusion from the surface to the first subsurface layer and from the first to the second subsurface layer are estimated to be 0.88 and 0.52 eV, respectively. In the entire coverage range studied, hydrogen occupies surface fcc and hcp sites and subsurface octahedral sites. In addition, the magnitude of the BE per hydrogen atom and the magnetization of the nickel slabs both decrease as hydrogen coverage increases. Vibrational frequencies of hydrogen at various surface and subsurface sites are calculated and are in reasonable agreement with experimental data. A phase stability calculation with a 2 × 2 surface unit cell shows that a p(2 × 2)-2H overlayer structure (θ=0.5 ML) and a p(1 × 1)-1H structure (θ=1.0 ML) are stable at low hydrogen pressures, in agreement with numerous experimental results. A very large increase in pressure is required to populate subsurface sites. After such an increase occurs, the first subsurface layer is filled completely.     

Quantum-Chemical Study of the Profiles of Reactions that Form Pyrrole Anion N-Adducts with CS<Subscript>2</Subscript> and CO<Subscript>2</Subscript>

The profiles of reactions leading to pyrrole anion N-adducts with CO₂ and CS₂ have been studied by the ab initio (RHF/6-31+G*, MP2/6-31+G*) and density functional (B3LYP/6-31+G*) methods. Addition of the pyrrole anion to the carbon disulfide molecule is accompanied by the appearance of a minimum corresponding to a pre-reaction complex. The transformation of the complex to the N-pyrrolyldithiocarboxylate anion occurs via a low activation barrier, which is due to repolarization of the C=S bonds. The profile of the reaction leading to the pyrrole anion N-adduct with CO₂ does not contain any intermediate stationary points throughout the whole route from reagents to products.Original Russian Text Copyright © 2004 by V. B. Kobychev, N. M. Vitkovskaya, I. L. Zaitseva, and B. A. Trofimov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 990–993, November–December, 2004.     

等径管道的三维重建

由轮廓线重建物体的现有方法，不能准确地恢复原有管道的形状．本文提出了一种等径管道的三维重建方法，能较为准确地恢复原管道的三维结构．     

A density functional theory computational study of the role of ligand on the stability of CuI and CuII species associated with ATRP and SET‐LRP

Atom transfer radical polymerization (ATRP) and single electron‐transfer living radical polymerization (SET‐LRP) both utilize copper complexes of various oxidation states with N‐ligands to perform their respective activation and deactivation steps. Herein, we utilize DFT (B3YLP) methods to determine the preferred ligand‐binding geometries for Cu/N‐ligand complexes related to ATRP and SET‐LRP. We find that those ligands capable of achieving tetrahedral complexes with Cu^I and trigonal bipyramidal with axial halide complexes with [Cu^IIX]⁺ have higher energies of stabilization. We were able to correlate calculated preferential stabilization of [Cu^IIX]⁺ with those ligands that perform best in SET‐LRP. A crude calculation of energy of disproportionation revealed that the same preferential binding of [Cu^IIX]⁺ results in increased propensity for disproportionation. Finally, by examining the relative energies of the basic steps of ATRP and SET‐LRP, we were able to rationalize the transition from the ATRP mechanism to the SET‐LRP mechanism as we transition from typical nonpolar ATRP solvents to polar SET‐LRP solvents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4950–4964, 2007     

Particle multiplicities in the fission-like reactions of 340 MeV Si on Th

Pre-scission and post-scission multiplicities of neutrons and alpha particles have been simultaneously measured for the fission-like reactions of 340 MeV ²⁸Si on ²³²Th. Dynamical model calculations using HICOL code predict that about 90% of the observed events are of quasi-fission type while the remaining 10% are from compound nucleus fission decay. Moving source fits were carried out to the observed neutron and alpha particle spectra, measured at different angles with respect to the fragment directions. The pre-scission and post-scission neutron multiplicities are deduced to be 8.7±2.0 and 9.4±2.0, respectively. The corresponding multiplicity values for alpha particles are found to be 0.22±0.08 and 0.1±0.03. From the measured post-scission neutron multiplicity, it is inferred that about 65±20 MeV of the initial excitation energy remains at scission. This may be compared to the value of 85±30 MeV estimated from PACE2 statistical model calculations, adjusted to reproduce the measured pre-scission neutron multiplicity. From a comparison of the Statistical Model predictions with the measured pre-scission neutron multiplicity, the fission delay is estimated to be of 5⁺⁷₋₃×10⁻²⁰ s which overlaps with the average duration of fission-like process from the contact to the scission point (2×10⁻²⁰ s) as determined from HICOL-based dynamical calculations. For the delay time deduced as above, the pre-scission alpha particle multiplicity calculated by the PACE2 code is about a factor two larger than the experimental one, demonstrating the difficulties in modelling the alpha particle emission from highly elongated shapes that characterize the fissioning system from the contact point to scission.     

Nitroxyl free radical binding to Si(1 0 0): a combined scanning tunneling microscopy and computational modeling study

The ultra-high vacuum scanning tunneling microscope (UHV-STM) was used to investigate the addition of the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical to the Si(1 0 0) surface. Room temperature studies performed on clean Si(1 0 0)-2 × 1 confirm the proposed binding of the unpaired valence electron associated with the singly occupied molecular orbital (SOMO) of the molecule with a Si dangling bond. A strong bias dependence in the topography of isolated molecules was observed in the range of −2.0 to +2.5 V. Semiempirical and density functional calculations of TEMPO bound to a three-dimer silicon cluster model yield occupied state density isosurfaces below the highest occupied (HOMO) and unoccupied state densities isosurfaces above the lowest unoccupied molecular orbital (LUMO) which trend in qualitative agreement with the bias dependent STM topography. Furthermore, the placement of TEMPO molecules on dangling bonds was controlled with atomic precision on the monohydride Si(1 0 0) surface via electron stimulated desorption of H, demonstrating the compatibility of nitroxyl free radical binding chemistries with nanopatterning techniques such as feedback controlled lithography.     

Cluster and periodic DFT calculations of adsorption and activation of CO2 on the Cu(hkl) surfaces

The adsorption behavior and thermal activation of carbon dioxide on the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces have been investigated by means of density functional theory calculations and cluster models and periodic slabs. According to the cluster models, the optimized results indicate that the basis set of C and O atoms has a distinct effect on the adsorption energy, but an indistinct one on the equilibrium geometry. For the CO₂/Cu(hkl) adsorption systems studied here, the final structure of adsorbed CO₂ is near linear and the preferred modes for the adsorption of CO₂ onto the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces are the side-on adsorption at the cross bridge site with an adsorption energy of 13.06 kJ/mol, the side-on adsorption at the short bridge site (13.54 kJ/mol), and the end-on adsorption on the on-top site with C–O bonds located along the short bridge site (26.01 kJ/mol), respectively. However, the calculated adsorption energies from periodic slabs are lower as compared to the experimental data as well as the cluster model data, indicating that the periodic slab approach of generalized gradient approximation in the density function theory may be not suitable to obtain quantitative information on the interaction of CO₂ with Cu(hkl) surfaces.