摘要:

The product operator formalism has been developed to evaluate, in closed and analytical form, the time evolution for spin (I= ½) systems in the presence of rf irradiation. This development is based on the fact that the time evolution propagator for the spin systems can be decomposed completely and thoroughly into the order multiplication of a series of elementary propagators. The inherent advantages and characteristic features of the formalism have been retained. As applications of this developed formalism, the Raman magnetic resonance in a weakly coupled two-spin system and the spin-locking process in the AB strongly coupled two-spin system are treated in detail. This formalism also gives a method for the analytic calculation of the spin-locking process in the ABC strongly coupled three-spin system.

ISSN:0026-8976    

DOI:10.1080/002689797172219

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The alkali metal trifluorides M+F3−(M = Na, K, Rb, and Cs) are studied using density functional theory and traditionalab initiomethods. Experimental infrared and Raman spectroscopic results suggest a pronounced change in the structure of the M+F3−species with decreasing cation size. Stationary points are located on the M+F3−potential energy surfaces and harmonic vibrational frequencies computed to assess how well the variou-s theoretical methods can describe this alkali metal dependence. As with the isolated F3−anion, the theoretical results are strongly dependent on the method used, with only the B3-LYP density functional calculations reproducing correctly the structural variation inferred from the vibrational spect-ra. Additional calculations also demonstrate that this functional significantly reduces the F3 overbinding observed using B-LYP.

ISSN:0026-8976    

DOI:10.1080/002689797172228

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An electrostatic description of the effect of a proton acceptor B on the field gradient at the Cl nucleus in hydrogen-bonded complexes B···HCl of Cssymmetry is used to predict the change in the quadrupole coupling tensor of the HCl molecule due to B and the influence of this change on the nonlinearity of the hydrogen bond as determined from the quadrupole coupling in its rotational spectrum. It is found that a distributed tensor approach to the response of the HCl electron density provides more rapid convergence of the perturbation expansion. However, it is noted that the physical interpretation of these distributed quantities in terms of charge flow and polarization should be viewed with caution. For complexes of the form B···HCl, the effect of the field of B on the quadrupole coupling is found to be small, justifying its neglect by Legonet al. in recent experimental investigations of the nonlinear hydrogen bonds in these complexes.

ISSN:0026-8976    

DOI:10.1080/002689797172237

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The structure factor of liquid carbon tetrachloride has been measured up toQ= 10 Å−1at ambient temperature using the SLAD diffractometer at NFL Studsvik. The results, which are comparable with previous measurements, have been modelled by the reverse Monte Carlo method. Constraints on the CCl coordination (fourfold) and nearest neighbour distance have been applied in order to define the molecules within the model. An analysis of the particle configurations obtained, that are fully consistent with the experimental structure factor, shows no evidence for the close interlock between neighbouring molecules which has been widely accepted so far. This interlock would require the average ClCl coordination to be 6, whereas it appears to be less than 4. Our model suggests that the dominant intermolecular connection is of the corner-to-corner type, and not of the corner-to-face type.

ISSN:0026-8976    

DOI:10.1080/002689797172246

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The single and multiple quantum NMR spectra of 1,3-dichloro-2-ethenylbenzene as solute in two different nematic liquid crystals have been measured and analysed in terms of dipolar couplings and chemical shift differences. Because the single quantum spectra did not yield to straightforward analysis, normal and frequency selective multiple quantum experiments were used to aid the analysis. The results from four spectra were combined to yield structural information and the order matrix in each solution. The structure of the ring is very close to that of 1,3-dichlorobenzene, and that of the ethenyl group very close to that of ethenylbenzene. The dihedral angle between the ethenyl group and the aromatic ring is found to be 45·4±1·0°, and the C - C = C angle is found to be 127·3±0.3°.

ISSN:0026-8976    

DOI:10.1080/002689797172255

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The equilibrium structure and thermodynamics of a multicomponent primitive model mixture consisting of a single asymmetric electrolyte and a neutral particle are studied as a function of the concentration of the neutral species, and for counterion valencies − 1,− 2 and − 3. The model, relevant for 'crowding reaction' processes in many biological systems is considered within the context of the symmetric Poisson–Boltzmann, the modified Poisson–Boltzmann, and the hypernetted chain theories. An important common prediction of the theories is that, at higher concentrations of the neutral species and for all valencies, there is an attractive force between the macroions. A comparison of the pair distribution functions and thermodynamics reveals the theories to be in semiquantitative or better agreement overall, with the modified Poisson-Boltzmann and the hypernetted chain results being generally closer to each other.

ISSN:0026-8976    

DOI:10.1080/002689797172264

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Three second-order integral equation theories based on the theory of inhomogeneous fluids are studied for the bulk Lennard-Jones fluid. These second-order theories are constructed from the inhomogeneous Ornstein–Zernike equation with the well known Percus–Yevick and the hypernetted chain closures, and the recently proposed Duh–Haymet–Henderson clossure. The radial distribution functions and the cavity functions are reported and are compared with simulation results.

ISSN:0026-8976    

DOI:10.1080/002689797172273

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Monte Carlo simulations are reported for the distribution functions, the fraction of unbonded and bonded particles, and the pressure of a chemically associating fluid of hard discs of a circularly symmetric attractive square well. Depending on the choice of the bonding length, dimers or chain molecules of different length can be formed. The dependence of the structural properties of the model on the fluid density and association energy is discussed.

ISSN:0026-8976    

DOI:10.1080/002689797172282

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The potential distribution theory recasts liquid state theory as a set of sum rules for the chemical potentials. These sum rules can be interpreted as measurements of the chemical potential at a point in space, via test particles, or in terms of the free energy changes involved in inserting or removing particles. As originally formulated, the case concerning the removal of a particle, inverse potential distribution theory, is ill-defined for models involving hard-core interactions. A modified version is presented that is suitable for hard-core systems with attractive interactions, and the physics involved is illustrated in terms of computer simulation thought experiments. As a simple example, inverse potential distribution theory is used to solve the one-dimensional lattice-gas model.

ISSN:0026-8976    

DOI:10.1080/002689797172291

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The residual chemical potential of a solute, which is the main characteristic of the phase equilibrium at conditions of supercritical fluid extraction of solids, is determined from the second-order perturbation expansion proposed for mixtures of non-spherical polar molecules. Intermolecular forces are described via the generalized Kihara pair potential with a permanent point dipole or quadrupole embedded in the centre of the hard core ascribed to each molecule. General expressions for ternary systems are given first. The average and molecular distribution functions are determined on the basis of the radial distribution functions (RDFs) of the hard sphere multicomponent system; the recently proposed method for determining RDFs in the hard sphere mixture is employed. In the case of the contribution of dispersion forces, the Barker-Henderson approximation is considered for the second-order term, whereas the second-order electrostatic contributions are approximated by employing the Gaussian overlap model. Values of the calculated residual chemical potential of a solute are compared with the simulation data for the infinitely diluted binary Lennard-Jones system and infinitely diluted ternary systems; fair agreement is found. When employing parameters of the Kihara polar molecules (adjusted to experimental orthobaric data) an increase in values of the residual chemical potential is found in comparison with those for Lennard-Jones systems.

ISSN:0026-8976    

DOI:10.1080/002689797172309

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor