摘要:

A new method is described for calculating NMR spectra of powdered solids spinning at any speed and at any angle relative to the magnetic field. The algorithm provides the intensities and shapes of the centreband and all sidebands through a computation involving the solution of homogeneous linear equations. The method is also applicable to cases where two or more interactions simultaneously affect the spectral response. A simple extension of the algorithm treats cases where an analytical expression for the resonance frequency is not available. The method is demonstrated with simulations and experimental examples of anisotropic chemical-shift spectra obtained by spinning samples at various low speeds and angles. The technique is also applied to a spin-½ nucleus with both an anisotropic chemical shift and a dipolar coupling either to a heteronuclear spin-½ nucleus or to a spin-1 nucleus experiencing a strong quadrupole interaction.

ISSN:0026-8976    

DOI:10.1080/00268979000101761

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The structure factors for saturated liquid CO have been measured at 83·5K and 121·9K by neutron diffraction over a range of momentum transfer 0·5 ⩽>Q⩽ 1>6 Å-1. The data were analysed to derive information about orientational correlations. No evidence was found for such correlations. Moreover, a new analysis of previous data on liquid N2at 77K showed that in this case also the orientational correlations are undetectable.

ISSN:0026-8976    

DOI:10.1080/00268979000101771

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Neutron-diffraction measurements have been performed on orthobaric liquid chlorine at temperaturesT= 202·2 K and 286·6 K. A comparison with previous measurements is made. The intermolecular structure factors and pair correlation functions are reported and discussed in terms of the orientational correlations that strongly influence the structure of liquid chlorine. Comparison is made with molecular-dynamics results obtained by Rodger, Stone and Tildesley employing a new sophisticated potential. Agreement between the present experimental data and the predictions made using this potential is very satisfactory, although some minor changes in the potential are suggested. Finally, we point out the impossibility of explaining the features ofg(r) by using only a limited number of configurations such as the T, end-to-end or staggered parallel ones.

ISSN:0026-8976    

DOI:10.1080/00268979000101781

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A new method to solve the reference HNC equations is developed to treat systems with both asymmetric short-range and long-range interactions. This method is motivated by the work of Patey and co-workers and uses Lado's free-energy minimizing optimization criteria for the reference HNC approximation. The properties of several fluids composed of linear triatomic molecules with various dipole moments or hard-sphere molecules with different-length dipoles are investigated.

ISSN:0026-8976    

DOI:10.1080/00268979000101791

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The effect of molecular flexibility on the pressure second virial coefficientBhas been evaluated for both linear molecules related to CS2and nonlinear symmetric triatomic molecules derived from CS2. Our results show that these effects will not be significant in evaluating pairwise intermolecular potentials for these molecules. Earlier, we found this also to be the case for linear diatomic molecules. This result is different from the case of water, where much larger contributions (up to 10%) were reported by Refsonet al.The dominant contribution toBis caused by molecular bond bending in both linear and nonlinear molecules and is very sensitive to the degree of nonlinearly of the molecule.

ISSN:0026-8976    

DOI:10.1080/00268979000101801

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Different potential models are exploited for deriving the allowed and induced Raman spectra of liquid chlorine atT= 200 K and ρ = 1·414 × 1028m-3. The induced effects are evaluated within the DID (dipole-induced-dipole) scattering mechanism in the point-scatterers approximation. The time correlation functions of allowed and induced polarizabilities, the depolarization ratios, the isotropic and anisotropic spectral shapes and the second spectral moments have been computed for the different potential models and compared with the available experimental data.

ISSN:0026-8976    

DOI:10.1080/00268979000101811

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Reactive scattering of F atoms with C2H5I, C3H7I and (CH3)2CHI molecules has been studied at an initial translational energyE≈ 38 kJ mol-1using a supersonic beam of F atoms seeded in He buffer gas. Centre-of-mass angular distributions of IF scattering show peaking in the forward and backward directions, which is consistent with reaction via persistent F-I-C2H5, F-I-C3H7and F-I-CH(CH3)2complexes with lifetimes greater than two rotational periods. The extended microcanonical theory indicates that the angular distribution of reactive scattering for F + C2H5I arises from a range of transition state configurations generated by internal rotation about the extended C-I bond. The reactive scattering for F + C3H7I arises from a wide range of transition-state configurations, with sharply peaked forward and backward scattering from extended configurations and mildly peaked scattering from more contracted configurations. However, the range of transition-state configurations is much more limited for F + (CH3)2CHI. This distinction confirms the persistence of significant C-I bonding, which restricts the rotation of the IF and C2H5, C3H7or (CH3)2CH fragments in these product transition states.

ISSN:0026-8976    

DOI:10.1080/00268979000101821

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We consider the effect of varying the angular orientation, or configuration, of field and initial relative velocity vectors for crossed-beam molecular scattering in the presence of intense non-resonant radiation. Expressions are presented for integral cross-sections specific to energy transfer in atomic collisons; the analysis can, however, readily be extended to include additional (molecular) degrees of freedom. The configuration dependence of the degeneracy-averaged cross-section appears as an expansion in even-order spherical harmonicsYL0(with the orientation as argument). The expansion takes in spherical harmonics up to order 2p, wherepis the highest-order photon (absorption and/or emission) process involved in the cross-section. We identify cross-sectional components arising from interference between transition-matrix elements; the potential information value of configuration-dependent (degeneracy-averaged) cross-sections, in the light of these interference effects, is considered. We also consider a number of simplifications to the basic cross-section expressions, with the view of facilitating computational analysis.

ISSN:0026-8976    

DOI:10.1080/00268979000101831

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A new algorithm for generalized Langevin-dynamics (GLD) simulations is presented. It is third-order (inclusive) in positions and requires the realization of only one stochastic trajectory. An approximate and very time-saving method is given to generate such trajectories in steps while still closely reproducing the original memory function. The algorithm, in conjunction with a rational evaluation of the friction force, is tested by simulating one krypton atom in dense liquid krypton in which the velocity autocorrelation function is non-trivial and exhibits a pronounced back-scattering minimum. The velocity autocorrelation function is at all times well reproduced and the algorithm is 10–20 times faster than other GLD algorithms.

ISSN:0026-8976    

DOI:10.1080/00268979000101841

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A theoretical study of the restricted primitive model of the electrical double layer using a free-energy density-functional theory is presented. The ion-ion hard-core repulsive contribution to the free energy is incorporated through a non-local excess hard-sphere free-energy density functional. The electrostatic part of the ion-ion direct correlation function of the inhomogeneous electrolyte in the interfacial region is approximated by that of the homogeneous bulk electrolyte. The generalized van der Waals model and the Tarazona model are used to construct the respective excess hard-sphere free-energy density functionals and compared with each other. Each model requires as input a hard-sphere equation of state. The Carnahan-Starling equation of state is chosen. The theory correctly predicts the layering effect of the counterions and the charge-inversion phenomenon. The results for 1:1 and 2:2 electrolytes agree well with Monte Carlo simulations. The Tarazona model gives results in closer agreement with Monte Carlo data than does the generalized van der Waals model. The diffuse-layer potentials predicted by the Tarazona model are not as accurate as those predicted by the generalized hard-rod model, which has been applied to the same problem in a previous study.

ISSN:0026-8976    

DOI:10.1080/00268979000101851

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor