Ab Initio Calculations on the H_(2)+D_(2)=2HD Four‐Center Exchange Reaction. I. Elements of the Reaction Surface

We present the results of ab initio calculations on some interesting regions of the reaction surface for the four‐center exchange reaction H_(2)+D_(2)=2HD. These calculations, which use a minimum basis set of Slater orbitals, indicate that for all geometries appropriate to the transition state of the reaction, a barrier height of at least 148 kcal/mole is present. This is far greater than the energy required to produce free radicals and more than three times the experimental energy of activation, 42 kcal/mole. Considering the sources and magnitudes for errors due to correlation and basis set restrictions, we estimate the barrier height for this exchange reaction to be 132 ± 20 kcal/mole exclusive of zero‐point energies. In this paper we discuss the surface as determined by configuration interaction techniques. We find that the most favorable geometries for the exchange reactions are the square, rhombus, and kite configurations. However, all of these states are unstable with respect to H_(2) + 2H. In addition we find no evidence of collision complexes for any of the likely transition state geometries. In the following paper we will examine the G1 wavefunctions for this system in order to obtain an understanding of the factors responsible for the shape of the surface

The electronic structure of pyrazine. Configuration interaction calculations using an extended basis

Extensive ab initio double zeta basis set configuration interaction calculations have been carried out on the nπ^∗ and ππ^∗ states of pyrazine as well as on the low‐lying n and π cations. The calculations corroborate the validity of the valence bond (VB) model for the interaction of lone pair excitations proposed earlier by Wadt and Goddard. Good agreement (errors of ∼0.2 eV) with experiment is obtained (except for the higher‐lying 1ππ∗ states that possess significant ionic character). The calculations indicate that the order of increasing ionization potentials is ^2A_g(n), 2B_(1g)(π), 2B_(1u)(n), and ^2B_(2g)(π). The forbidden 1 ^1B_(2g)(nπ^∗) state is predicted to be 1.30 eV above the allowed 1^ 1B_(3u)(nπ^∗) state. Finally, the calculations indicate that the adiabatic excitation energies to the 1^ 3B_1(nπ^∗) and 1^ 3A_1(ππ^∗) states in pyridine should be nearly degenerate at ∼3.6 eV

Atomic simulations of kinetic friction and its velocity dependence at Al/Al and alpha-Al_2O_3/alpha-Al_2O_3 interfaces

Kinetic friction during dry sliding along atomistic-scale Al(001)/Al(001) and alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces has been investigated using molecular dynamics (MD) with recently developed Reactive Force Fields (ReaxFF). It is of interest to determine if kinetic friction variations predicted with MD follow the macroscopic-scale friction laws known as Coulomb's law (for dry sliding) and Stokes' friction law (for lubricated sliding) over a wide range of sliding velocities. The effects of interfacial commensuration and roughness on kinetic friction have been studied. It is found that kinetic friction during sliding at commensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces exceeds that due to sliding at an incommensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interface. For both interfaces, kinetic friction at lower sliding velocities deviates minimally from Coulombic friction, whereas at higher sliding velocities, kinetic friction follows a viscous behavior with sliding damped by thermal phonons. For atomically smooth Al(001)/Al(001), only viscous friction is observed. Surface roughness tends to increase kinetic friction, and adhesive transfer causes kinetic friction to increase more rapidly at higher sliding velocities

Adhesion and nonwetting-wetting transition in the Al/alpha-Al_2O_3 interface

Using a reactive force field (ReaxFF), we investigated the structural, energetic, and adhesion properties, of both solid and liquid Al/alpha-Al2O3 interfaces. The ReaxFF was developed solely with ab initio calculations on various phases of Al and Al2O3 and Al-O-H clusters. Our computed lattice constants, elastic constants, surface energies, and calculated work of separation for the solid-solid interface agree well with earlier first-principles calculations and experiments. For the liquid-solid system, we also investigated the nonwetting-wetting transition of liquid Al on alpha-Al2O3(0001). Our results revealed that the evaporation of Al atoms and diffusion of O atoms in alpha-Al2O3 lead to the wetting of liquid Al on the oxide surface. The driving force for this process is a decrease in interfacial energy. The nonwetting-wetting transition was found to lie in the 1000–1100 K range, which is in good agreement with sessile drop experiments

Gold-Catalyzed Intramolecular Aminoarylation of Alkenes: C-C Bond Formation through Bimolecular Reductive Elimination

Gold-ilocks and the 3 mol % catalyst: Bimetallic gold bromides allow the room temperature aminoarylation of unactivated terminal olefins with aryl boronic acids using Selectfluor as an oxidant. A catalytic cycle involving gold(I)/gold(III) and a bimolecular reductive elimination for the key CC bond-forming step is proposed. dppm= bis(diphenylphosphanyl)methane

On-line optical measurement and monitoring of yarn density in woven fabrics

This paper describes a vision-based system that monitors the yarn density of woven fabrics on-line. The system is described in terms of its two principal modules, namely, the image acquisition and the image analysis subsystems. The image acquisition subsystem is implemented with standard components on a low-cost personal computer platform. These components consist of a line-scan camera, a DSP-based image acquisition and processing card, and a host personal computer. The image acquisition process is controlled by a software module that runs on the DSP board and accumulates a 2-D image suitable for the density measurement algorithm. The image analysis sub-system, which also runs on the DSP board, implements a novel, yet straightforward, algorithm that utilizes the discrete Fourier transform for monitoring the yarn density of the fabrics from the acquired images. In this algorithm, the Fourier spectrum of the images is covered by contiguous, concentric annular regions that have a prespecified width. The spectrum values within each annular region are summed, normalized, and subsequently used to produce a 1-D signature. Simple statistics of the obtained signatures are the basis for characterizing the fabric in terms of its yarn density. The described system is tested on seven fabrics with common properties but varying yarn densities and has shown to be accurate within 2 yarns per inch in either direction. It is also shown that the obtained accuracy, which is primarily a function of the image resolution, can be greatly improved

Methylrhenium Trioxide Revisited: Mechanisms for Nonredox Oxygen Insertion in an M−CH_3 Bond

Methylrhenium trioxide (MTO) has the rare ability to stoichiometrically generate methanol at room temperature with an external oxidant (H_2O_2) under basic conditions. In order to use this transformation as a model for nonredox oxidative C−O coupling, the mechanisms have been elucidated using density functional theory (DFT). Our studies show several possible reaction pathways to form methanol, with the lowest net barrier (ΔH‡) being 23.3 kcal mol^(-1). The rate-determining step is a direct “Baeyer−Villiger” type concerted oxygen insertion into MTO, forming methoxyrhenium trioxide. The key to the low-energy transition state is the donation of electron density, first, from HOO(−) to the –CH_3 group (making –CH_3 more nucleophilic and HOO− more electrophilic) and, second, from the Re−C bond to both the forming Re−O and breaking O−O bonds, simultaneously (thus forming the Re−O bond as the Re−C bond is broken). In turn, the ability of MTO to undergo these transfers can be traced to the electrophilic nature of the metal center and to the absence of accessible d-orbitals. If accessible d-orbitals are present, they would most likely donate the required electron density instead of the M−CH_3 moiety, and this bond would thus not be broken. It is possible that other metal centers with similar qualities, such as Pt^(IV) or Ir^V, could be competent for the same type of chemistry

Molecular evolution: sex accelerates adaptation

An analysis confirms the long-standing theory that sex increases the rate of adaptive evolution by accelerating the speed at which beneficial mutations sweep through sexual, as opposed to asexual, populations

No N=4 Strings on Wolf Spaces

We generalize the standard $N=2$ supersymmetric Kazama-Suzuki coset construction to the $N=4$ case by requiring the {\it non-linear} (Goddard-Schwimmer) $N=4~$ quasi-superconformal algebra to be realized on cosets. The constraints that we find allow very simple geometrical interpretation and have the Wolf spaces as their natural solutions. Our results obtained by using components-level superconformal field theory methods are fully consistent with standard results about $N=4$ supersymmetric two-dimensional non-linear sigma-models and $N=4$ WZNW models on Wolf spaces. We construct the actions for the latter and express the quaternionic structure, appearing in the $N=4$ coset solution, in terms of the symplectic structure associated with the underlying Freudenthal triple system. Next, we gauge the $N=4~$ QSCA and build a quantum BRST charge for the $N=4$ string propagating on a Wolf space. Surprisingly, the BRST charge nilpotency conditions rule out the non-trivial Wolf spaces as consistent string backgrounds.Comment: 31 pages, LaTeX, special macros are include

Quantising Gravity Using Physical States of a Superstring

A symmetric zero mass tensor of rank two is constructed using the superstring modes of excitation which satisfies the physical state constraints of a superstring. These states have one to one correspondence with quantised operators and are shown to be the absorption and emission quanta of the Minkowski space Lorentz tensors using the Gupta-Bleuler method of quantisation. The principle of equivalence makes the tensor identical to the metric tensor at any arbitrary space-time point. The propagator for the quantised field is deduced. The gravitational interaction is switched on by going over from ordinary derivatives to coderivatives.The Riemann-Christoffel affine connections are calculated and the weak field Ricci tensor $R^{0}_{\mu \nu}$ is shown to vanish. The interaction part $R^{int}_{\mu \nu}$ is found out and the exact $R_{\mu \nu}$ of theory of gravity is expressed in terms of the quantised metric. The quantum mechanical self energy of the gravitational field, in vacuum, is shown to vanish. It is suggested that quantum gravity may be renormalisable by the use of the physical ground states of the superstring theory.Comment: 14 page