ISSN:0026-8976    

DOI:10.1080/002689797172796

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The frequency dependent polarizability α(– ωσ; ω1) and the frequency dependent first hyperpolarizability β(– ωσ; ω1, ω2) have been calculated using both self consistent field (SCF) and multiconfigurational SCF (MCSCF) linear and quadratic response functions for six small molecules: BH, CO, HF, H2O, NH3, and H S. Two approximate ways of using the calculated dispersion at the SCF level to describe the frequency dependence at the correlated level, referred to as scaling and addition, have been tested. Calculated frequency dependent MCSCF results are used to investigate the validity of these approximations. Our results indicate that, if the correlation correction to the static average hyperpolarizability is large, then the approximations do not hold. The same conclusion holds if correlation changes the sign of the average first hyperpolarizability at zero frequency. There is also a tendency towards an underestimation of the frequency dependence if scaling or addition is applied. There does not seem to be a simple way to infer anything about the validity of scaling or addition for β‖ from either the lower lying excitations and their intensities or from the frequency dependence of the dipole polarizability.

ISSN:0026-8976    

DOI:10.1080/002689797172804

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Chemically induced dynamic electron polarization (CIDEP) spectra of transient radicals were measured upon photolysis of benzaldehydes (BAs) after 266 nm laser excitation. The substituent effects on CIDEP were studied for a series of substituted derivatives (CH3-, F-, Cl-, Br- and CN-) of BA in 2-propanol at room temperature. For F-, Cl-, and CN-substituted BAs, CIDEP spectra were observed that were due to combinations of, firstly, the triplet mechanism (TM) with emissive (E) phase patterns and, secondly, the S-T,0mechanism (ST0M) of radical pairs with emissiveoabsorptive (E/A) phase patterns. In contrast, for CH3-substituted BAs, CIDEP spectra due only to the ST0M with E/A phase patterns were observed. The contribution of the TM-induced CIDEP shows an increasing trend for BAs with electron-withdrawing substituents (F-, C1-, and CN-) and a decreasing trend for BAs with an electron-donating substituent (CH3-). These substituent effects on the TM-induced CIDEP were found to be parallel to the effects on the reaction rate of hydrogen abstraction of excited BA molecules: both effects are strongly influenced by the (n, π*) and (π, π*) characteristics in the lowest excited triplet states. Furthermore, clear CIDEP signals due to the benzoyl and alpha-hydroxybenzyl radicals, which are produced by self-quenching of triplet BA, were newly obtained upon photolysis of a concentrated acetonitrile solution of BA (5 × 10-2mo1 dm-3) by a 266 nm laser. This result is not consistent with the proposed wavelength effect on the BA photochemistry, but it is consistent with a normal radical reaction mechanism that shows no wavelength effect.

ISSN:0026-8976    

DOI:10.1080/002689797172813

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Molecular dynamics simulations are used to study the wave vector (k) dependent static dielectric properties of a recently proposed model (H. Liu, F. Müller-Plathe and W. F. van Gunsteren, 1995,J. Amer. chem. Soc.,117, 4363) for liquid dimethyl sulphoxide (DMSO). Room temperature results are given for the real-space orientational pair correlationsh110(r) andh112(r), for thek-dependent short- and long-ranged generalized Kirkwoodgfactors,gKS(k) andgKL(k), and for thek-dependent transverse and longitudinal components of the dielectric permittivity tensor, εT(k) and εL(k), over wide ranges ofkvalues. Thesek-dependent quantities T L have been obtained from the Fourier–Hankel transforms of the projectionsh110(r) andh112(r). The functionh110(r) reveals that the relative orientation between neighbouring molecules may result from dipolar forces, and that the local order of the molecular dipoles is consistent with the results from other studies on DMSO. The system's shape-independent Kirkwoodgfactor calculated from thek→ 0 limit ofgKS(k) is 1•6, and using this value in the Kirkwood relation yields ε(0) = 44 for the system's dielectric constant. The asymptotic behaviour ofh112(r) obtained from an auxiliary simulation on a 2048 molecules system yields similar estimates for ε(0). The calculated values for the model's Kirkwoodgfactor and dielectric constant are in very good agreement with experimental data at room temperature. In addition, calculations were made of the contributions to εT(k) and εL(k) from the molecular spatial charge distribution under several different approximations. These contributions are essential for a correct characterization of thek-dependent dielectric properties of the model at finite values ofk.

ISSN:0026-8976    

DOI:10.1080/002689797172822

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Non-equilibrium steady state systems do not obey the equipartition theorem in that the distribution of the random kinetic energy among the three directions is not independent of the mechanism which removes the dissipative heat. This becomes important beyond the linear regime because system properties will be functions of the thermostatting mechanism as well. For homogeneous non-equilibrium molecular dynamics simulations, in most of the cases, the simplest synthetic thermostat is used in which, analogously to equilibrium algorithms, the friction coefficient is identical in thex,yandzdirections. The simplicity is attractive, but computer simulation results are presented that demonstrate that this may not be the most sensible choice physically.

ISSN:0026-8976    

DOI:10.1080/002689797172831

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A comprehensive theoretical treatment is presented of the low-lying electronic states of the molecular ion BBr+correlating with the two lowest dissociation asymptotes B+(1S) Br(2P) and B(2P)+Br+(3P). All-electron CASSCF+CI calculations have been made with averaged atomic natural orbital basis sets. Spectroscopic constants are calculated for the bound states but are in disagreement with an experimental analysis in the literature of a2Πr→ X2Σ+system. It is suggested that a re-examination of the spectroscopy of this species would be worthwhile.

ISSN:0026-8976    

DOI:10.1080/002689797172840

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The crystal structure of the two phases of chlorodifluoromethane (CHClF2) were determined at 70 K and 10 K using Rietveld refinement of neutron powder profiles. The upper phase is tetragonal P42/n (C44h) with eight molecules in the unit cell occupying general positions. The lattice constants area= 10•3711(1) Å andc= 5•5915(2) Å. The lower phase is monoclinic P112/n (C24h) with eight molecules in the unit cell with dimensionsa= 10•1106(2),b= 10•4830(2),c= 5•5868(2) Å and γ = 90•319(2)°. The asymmetric unit has two molecules in general positions. The phase transition is of a displacive type but there are no large changes in the molecular positions or orientations at the transition.

ISSN:0026-8976    

DOI:10.1080/002689797172859

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The condensed Fukui functions have been calculated using the Mulliken population analysis (MPA) and natural population analysis (NPA) schemes for the molecules BH2F, BH2Cl andtransandcisforms of the FC(O)OF and nitrosoethylene molecules and another isomer of nitrosoethylene, oxazete. The changes in the relative reactive sites of these molecules due to the MPA and NPA schemes have been reported. The basis set dependence of the condensed Fukui functions have also been studied. The concept of a larger condensed Fukui function for higher reactivities has been verified for the nitrosoethylene molecule.

ISSN:0026-8976    

DOI:10.1080/002689797172868

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Density profiles have been studied of a hard sphere chain fluid and of dimerizing hard spheres near a crystalline wall represented as a lattice of sticky sites. The associative Henderson–Abraham–Barker equation in the PY approximation is used for the calculation of a structure with the adsorption isotherm assumed to be of the mean-field form. It is shown that the density profiles exhibit a sudden change corresponding to critical behaviour of the adsorption isotherm. The influence of the low-density critical point, caused by the intramolecular correlation in a chain, on the interfacial structure is determined. It is also shown that the associative interparticle interaction forms a sharp edge of the second adlayer. The density of this adlayer is analytically calculated.

ISSN:0026-8976    

DOI:10.1080/002689797172877

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

It is shown that ‘compressible ion’ potentials, which represent the results ofab initioelectronic structure calculations on MgO and CaO and which represent their phase transitions accurately, can be transferred successfully to other alkaline earth oxides by substitution of accepted values for the cation radii. The application of such a potential to ZnO erroneously predicts a ground-state crystal structure of rocksalt symmetry. After extending the interaction model to allow for the presence of induced octupoles on both the zinc and oxide ions the observed wurtzite structure emerges as most stable. The importance of short-range contributions to the induced moments, beyond those predicted by the multipole expansion, is highlighted.

ISSN:0026-8976    

DOI:10.1080/002689797172886

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor