摘要:

Pulsed magnetic field gradient stimulated echo NMR measurements are reported for the flow and diffusion of an aqueous phase within a sample of Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area,q, for a fixed observation time Δ, is used to derive the displacement probability distributions,PΔ(X), whereX, the direction of the field gradient, is arranged to be parallel or perpendicular to the applied pressure gradient driving the flow. The shape of thePΔ(X) as a function of Δ is modelled in terms of laminar flow within a set of randomly oriented capillaries. For small Δ, molecules remain largely within the capillaries they occupy at the beginning of the time Δ and, as a consequence, thePΔ(X) for small Δ are dominated by the orientational distribution of the capillaries. For large Δ, the evolution of thePΔ(X) is reproduced only when diffusion of molecules across the velocity gradients within the capillaries is introduced and, in addition, the fluid transfers between differently oriented capillaries through the action of the overall flow.

ISSN:0026-8976    

DOI:10.1080/00268979600100171

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The Ornstein-Zernike equation for additive hard sphere mixtures is solved numerically by using the Martynov-Sarkisov (MS) closure and a recent modification of the Verlet (MV) closure. A comparison of the predictions for the equation of state and, to a lesser extent, the contact values of the radial distribution function, shows both theories to give similar, reasonably accurate, results in most situations. However, an examination of the pair cavity functions for zero separation shows the two closures to give quite different results, and the MV closure results are believed to be better. More attention should be given to the cavity function at zero separation. In addition, the MV closure satisfies known asymptotic relations for a small concentration of exceedingly large spheres, whereas the MS and Percus-Yevick closures do not satisfy these relations.

ISSN:0026-8976    

DOI:10.1080/00268979600100181

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Ermak's 1975 Brownian dynamics free-draining algorithm is extended to include the coarse grained effects of many-body hydrodynamics, using a local density approximation for the friction coefficient. The model reproduces the long term self-diffusion coefficients of model near hard sphere colloidal liquids within the experimental uncertainty over the complete solid volume fraction range. The local structural relaxation, manifested in the force autocorrelation functions, is slowed down upon the introduction of mean-field hydrodynamics. This effect is also evident in the collective diffusion coefficients, which show an enhanced relative slowing down compared with the free-draining model at wavevectorsk∼ 2π/σ.

ISSN:0026-8976    

DOI:10.1080/00268979600100191

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The rotationally resolved magnetic vibrational circular dichroism (RR MVCD) spectra of the diamagnetic molecules DCl and NH3are analysed on the basis of theoretical simulation. Basic theoretical equations are derived for the RR MVCD dispersed intensity pattern from the conventional rovibrational energy level expression. This provides a convenient method for interpretation of MVCD spectra in terms of fundamental spectroscopic parameters. Good agreement was obtained between the simulated and the experimental data measured for DCl and NH3. Minor deviations from the theory occur in the dipole strength distribution for DCl, but this is corrected by measuring theA1/D0ratio, from which the moleculargvalue can be determined for resolved transitions. Although magnetic properties of DCl and NH3are known, this study demonstrates the ability of MVCD to provide an alternate method of determining molecularg-values as compared to classical microwave studies of the Zeeman effect. The data further show that experimentally observed RR MVCD spectra are fully explained using the conventional theory for rovibrational transitions under Zeeman perturbation. Variations of magnetic parameters of the ground and excited vibrational states of these molecules cause characteristic changes of the MVCD band intensity patterns. These variations are used to evaluate approximations made in previous moment analyses of RR MVCD spectra and to delimit the sensitivity of the RR MVCD technique to differences in ground and excited stateg-values.

ISSN:0026-8976    

DOI:10.1080/00268979600100201

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A density functional (DF) method, as applied by Tarazona to classical fluids, has been used to calculate the orientation of triatomic molecule fluids near the solid-liquid interface (hard wall). The direct use of this method would result in an enormous computing time, and to avoid this a weighting function is formulated using the intermolecular potential alone. The volume-corrected hard sphere equation of state is used as the equation of state for polyatomic molecule fluids. The DF method is then used to investigate the molecular orientation in fluids near a two-dimensional solid-liquid interface. The results give valuable information about the effect of molecular shape on the orientation of real molecules such as liquid crystals near the solid-fluid interface.

ISSN:0026-8976    

DOI:10.1080/00268979600100211

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The self-consistent HMSA integral equation is derived for the square-well fluid and tested against Monte Carlo simulations. The phase diagram and mechanical properties are well reproduced, as well as radial distribution functions and structure factors. For narrow wells, the extent of vapour-liquid equilibria is reduced substantially, in qualitative agreement with results for the attractive Yukawa system. A recent sticky hard sphere mapping is tested to determine its validity.

ISSN:0026-8976    

DOI:10.1080/00268979600100221

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Monte Carlo simulations of phase equilibria for Lennard-Jones chains of intermediate length are performed in the Gibbs ensemble using configurational bias sampling. Simulations of phase equilibria for square-well chains of up to 100 segments are performed using theNPT-μmethod and newly proposed Monte Carlo moves. A two-reference-fluid equation of state is developed to describe the pressure-volume-temperature properties of square-well and Lennard-Jones chains. The phase envelopes predicted by such an equation are in good agreement with results of simulations. This equation is also shown to be superior to models derived from first-order thermodynamic perturbation theory (TPT1).

ISSN:0026-8976    

DOI:10.1080/00268979600100231

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Canonical ensemble molecular dynamics simulations of Lennard-Jones methane and ethane are conducted in an atomistic model of AlPO4-5, a molecular sieve with approximately cylindrical channels of diameter 7·3 Å. Methane molecules are able to pass each other in the nanopore and exhibit unidirectional but otherwise ordinary diffusion along the channel axis, with the mean-square displacement directly proportional to time, and a diffusion coefficient calculated at a loading of 0·7 molecules per unit cell at 295 K of 4·70 × 10-4cm2s-1. Ethane molecules cannot pass each other easily in the nanopore and for short times exhibit single-file diffusion, i.e., the mean-square displacement is proportional to the square root of the time. After longer times, contributions of ordinary unidirectional diffusion are observed due to the nonzero probability of passing. A slightly larger molecule exhibits pure single-file diffusion. The single-file mobility for the large molecule at 0·7 molecule per unit cell and 295 K is 1·57 × 10-9cm s-0·5. Effects of fluid density on absorbate mobility are also addressed. The density dependence of the single-file mobility is in agreement with one-dimensional hard rod predictions.

ISSN:0026-8976    

DOI:10.1080/00268979600100241

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Ab initioquantum mechanical methods have been applied to determine the singlet-triplet splitting of carbohydroxycarbene, which is the prototype of carbalkoxycarbenes. These methods employ basis sets up to triple-ζ plus double polarization plus f functions (TZ2Pf) in calibre and levels of correlation up to coupled cluster including single, double and perturbatively treated connected triple excitations (CCSD(T)). The triplet state is the ground state, which has the planar Cssymmetry structure, while the lowest singlet state has C1symmetry. At the highest level of theory employed, the singlet-triplet splitting is predicted to be 4·7 kcal mol-1, which falls between an early theoretical prediction of 7 kcal mol-1by Kim and Schaefer (1980,J. Amer. Chem. Soc.,102,5389) and recent experimental conclusions of Toscano, Platz, Nikolaev and Popic (1994,J. Amer. Chem. Soc.,116,8146) from solution kinetics. The harmonic vibrational frequencies and their infrared intensities are also reported.

ISSN:0026-8976    

DOI:10.1080/00268979600100251

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The series solution method used by H. Cox, S. J. Smith and B. T. Sutcliffe (1994,Phys. Rev.A,49,4520, 4533) to obtain extremely accurate eigenvalues and eigenvectors for the lowest singlet and triplet states of lowZhelium-like systems has been applied to the ground1S state of the positronium negative ion. Secular determinatnts of up to size 2847 × 2847 have been solved to give a ground state energy of 0·262 005 070 231 742 au. This agrees to 12 decimal places with the most accurate literature value reported by Y. K. Ho (1993,Phys. Rev.A,48,4780). Various expectation values involving the interparticle distances have also been evaluated. We estimate the positron-electron annihilation rate Г as 2·086 1223 ns-1. This is in agreement with the most accurate literature value of 2·086 1222 ± 5 × 10-7(Ho, 1993,Phys. Rev.A,48,4780) and the experimental value of 2·09 ± 9 × 10-2(Mills, 1983,Phys. Rev. Lett.,50,671).

ISSN:0026-8976    

DOI:10.1080/00268979600100261

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor