摘要:

Car-Parrinello molecular dynamics calculations have been carried out for phosphorus-selenium mixtures of compositions corresponding to overall stoichiometrics PSe and P2Se. Comparison of the calculated structure factors with those obtained in neutron scattering experiements suggests that the real materials are characterized by the presence of substantial numbers of well defined molecular units.

ISSN:0026-8976    

DOI:10.1080/00268970009483339

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH2, OH and CH3) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(= X)XH(M = C., Si; X = 0, S; R = F, Cl, OH, NH2and CH3). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH ≪ R-Si(=O)SH < R-SiSSH(R = H, F, Cl, OH, NH2and CH3). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol−1) has been calculated for ClC(=S)OH.

ISSN:0026-8976    

DOI:10.1080/00268970009483340

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Calculations of the critical properties of binary mixtures of components of equal size are reported using the Guggenheim equation of state. The calculations are used to determine the global phase diagram of binary mixtures. Type VI phase behaviour is predicted successfully indicating that closed-loop liquid-liquid equilibria can be obtained from hard-sphere + van der Waals interactions. Closed-loop liquid-liquid equilibria occur in the region of the global phase diagram characterized by moderately strong unlike interactions and components with very dissimilar critical temperatures. The Guggenheim equation can predict all experimentally known phase behaviour types. In addition, other hypothetical phase behaviour types are also predicted.

ISSN:0026-8976    

DOI:10.1080/00268970009483341

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Solvation properties of solutes in supercritical, medium density solvents have been analysed using hypernetted-chain theory with the emphasis on the solvent-mediated interaction between solute molecules. The solvent and solute molecules are Lennard-Jones particles, and the solute is present at infinite dilution. Also a pair of solute molecules separated by different distances has been considered using reference interaction site model theory. Mainly, solvents at two typical densities (1.09pcand 2.91pc;pcis the critical density) that are in medium and high density regions, respectively, are treated. The temperature is set at 1.04TC(Tcis the critical temperature). When the solute size is larger than the solvent size and the strength of the solute-solvent attractive interaction is greater than that of the solvent-solvent in the medium density region, the solvent structure confined between a pair of solute molecules is largely different from that near a single solute molecule. The confined solvent becomes denser and more stabilized as the distance between the solute molecules decreases, and an attractive interaction is induced between them. The interaction becomes even more attractive as the strength of the solute-solvent attractive interaction increases. The observations are qualitatively different from those in the high density region. Another high density region, which is well below the critical temperature, has been considered, but the behaviour observed is similar to that in the high density region above the critical temperature.

ISSN:0026-8976    

DOI:10.1080/00268970009483342

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Concentration dependent experimental measurements of the ethanol hydroxyl proton chemical shift σHfor binary solutions were carried out. The solvents used were carbon tetrachloride (CCl4), benzene, chloroform, acetonitrile, acetone and dimethylsulphoxide (DMSO). The chemical shift values range from 0.69 ppm (relative to TMS) for dilute ethanol (extrapolated to infinite dilution) in CCl4to 5.34 ppm for neat liquid ethanol.Ab initiocalculations of the ethanol-solvent hydrogen bond energies show a correlation with the values for the chemical shift. The hydrogen bond energies for ethanol-solvent dimers range from 0.63 kcal mol−1for ethanol-CCl4to 9.34 kcal mol−1for ethanol-DMSO. Theoretical calculations show a linear correlation between the deuterium quadrupole coupling parameter XDard the isotropic proton chemical shift σH: XD(kHz) = 291.48 − 14.96 σH, where σHis the proton chemical shift in ppm relative to TMS (R2= 0.99). Using the concentration dependent chemical shift data and this equation, XDia observed to range from 280 kHz for very dilute concentrations in CCl4, where the primary species is ethanol monomer, to 210 kHz for the neat liquid that is comprised primarily of cyclic pentamers.

ISSN:0026-8976    

DOI:10.1080/00268970009483343

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

All derivatives of tetraborane(lO) with one or more boron atoms replaced by gallium have been investigated using quantum chemical methods. Particular attention is given to the relative stability of the ‘butterfly’ arachno- structures and the bis(diboranyl) structures. It is concluded that these two structures have very close energies if the bis(diboranyl) structures contain a central gallium-gallium bond. These gallium substituted bis(diboranyl) structures are thus a fascinating synthetic target.

ISSN:0026-8976    

DOI:10.1080/00268970009483344

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Molecular mechanical force fields are widely used in molecular simulation studies due to their simplicity and efficiency. Although accurate enough for many purposes, force fields are approximations and neglect several effects that are important at an atomic level. Probably the most significant of these, that is not taken into account by the majority of force fields, is the ability of the charge distribution of a molecule to polarize in response to changes in its environment. There are several ways of including polarization effects in force fields but, in this paper, we investigate a relatively new approach, called the chemical potential equalization model, which is based upon the principle of electronegativity equalization and which is a generalization of the class of fluctuating charge models. We detail the principles behind the model and of our implementation of it and then present parametrizations of the model for the molecules, methane and water, and the chloride anion. Gas- and solution-phase calculations to test the parametrizations indicate that the model gives results that are in good agreement with experiment and that compare well to those obtained with non-polarizable force fields and other polarizable models.

ISSN:0026-8976    

DOI:10.1080/00268970009483345

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

For systems of Kihara molecules with circular cores, the values of the reduced critical constants were determined from the fourth-order virial expansion as functions of the core diameter/thickness ratio. From expressions for the reduced functions both for the oblate and prolate shapes, the values of critical constants of four cyclic hydrocarbons and four branched alkanes were evaluated and compared with the experimental data and values obtained from the perturbation theory.

ISSN:0026-8976    

DOI:10.1080/00268970009483346

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor