摘要:

Molecular dynamics simulations of poly(oxyethylene) (POE) in water are used as a reference to examine the effect of different friction coefficients in Langevin dynamics with respect to properties like the translational diffusion coefficient, the position and velocity correlation functions of the POE-chain and its centre of mass. It turns out that these dynamic properties found for the simulation of POE in water are properly described by choosing an appropriate friction coefficient in the stochastic approach. Also the transition rates between thetransandgaucheconformation of the torsional angles around the C-O bonds of POE can be described in the stochastic approach, but with a different friction constant. Distributions of torsional angles and transition rates involving the C-C bonds of POE under the influence of the explicit polar solvent are compared with those of the stochastic simulation, and a physical explanation is given for the specific interaction of POE with water causing a conformational change of POE. The consequences for the value of the characteristic ratio are analysed and used to explain contradictory measurements of the characteristic ratio. Furthermore, it is concluded that for the description of the static properties of a polymer without specific solvent interactions, stochastic or even simulationsin vacuoare appropriate.

ISSN:0026-8976    

DOI:10.1080/00268979100102531

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The structure factorsS(Q) of the molecular fluid CO2have been obtained at four different thermodynamic states, viz., CO2(l) at 220 K, 58 bar; 290 K, 60 bar; 303 K, 77 bar and CO2(g) at 303 K, 60 bar. TheS(Q) of CO2(l) at 220 K shows a shoulder on the high-Q side of the main diffraction peak which is taken by some authors as evidence of the existence of orientational correlations between neighbouring molecules. The effect of strong quadrupole-quadrupole interactions in CO2on its structure is examined in terms of orientational correlations. The real spaceG(r) results show for the first time direct evidence for the existence of orientationally correlated molecules in CO2(l) at a very short range (∼ 3·3 Å). These correlations disappear with increase of temperature. None of the models or the computer simulation data reported so far has reproduced these or similar effects.

ISSN:0026-8976    

DOI:10.1080/00268979100102541

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Molecular dynamics of methane-water systems have been studied through computer simulations (MD), nuclear spin relaxation and self diffusion methods (NMR). MD simulation results give support to the concept of hydrophobic hydration by enhancing the water structure around the methane to create a clathrate. Spontaneous contacts between methanes is observed only at high densities, while so-called solvent-separated configurations are present in all simulations. The dynamics of the solvent water molecules is slowed down. In particular, the reorientational motion is sensitive to the presence of methane in solution. The simulated translational diffusion of methanes as solutes is also slowed down, but is higher than observed experimentally. The rotational motion of methanes is very sensitive to variation of the density while, the translational motion of methane is more constant and follows that of water. Methane acts nearly as a free rotor. The simulated reorientational correlation times for methane in water solution are in good agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268979100102551

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

An affine transformation model is applied to the diffusion coefficients of perfectly aligned and partially aligned ellipsoidal particles. It is tested against molecular dynamics simulations of hard ellipsoids, free to translate and rotate, in the nematic liquid crystal phase. The theory explains the variation with order parameter of the anisotropy of the diffusion tensor, and approximately relates the average diffusion coefficient to that of an equivalent hard sphere fluid.

ISSN:0026-8976    

DOI:10.1080/00268979100102561

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A Mori-generalized Langevin method is applied to the calculation of the frequency dependent shear viscosity and thermal conductivity of a fluid of hard convex bodies (HCB's) in an uncorrelated collision (Enskog) approximation. The transport coefficients have a density dependence reminiscent of hard sphere (HS) systems. In the limit of a spherical HCB (for bodies with two rotational degrees of freedom), the shear viscosity of the fluid of HCB's reduces to the HS value, whereas the HCB thermal conductivity exceeds the HS value. Rotational energy contributions to the thermal conductivity (even to zeroth order in the particle asymmetry) are greatest at low densities and diminish with increasing density.

ISSN:0026-8976    

DOI:10.1080/00268979100102571

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Finite Brownian motion can be characterized as an effective multifractal since different spatial regions have different scaling properties. The homogenization of the spatial probability distribution as the length of the Brownian trajectory increases is reflected in an effective diffusion coefficient, which approaches the theoretical diffusion coefficient when the trajectory length is long enough to exhibit the global statistical properties of ideal Brownian motion.

ISSN:0026-8976    

DOI:10.1080/00268979100102581

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The use of a Laplace transform (LT) relationship to link time dependent kinetics and scavenger yields in homogeneous systems is examined theoretically. It is shown analytically that the connection is strictly valid only for a single pair of reactants and for first order reactions. The accuracy of the LT method is tested for multiparticle systems. For small numbers of reactive particles the true decay kinetics are compared with the inverse Laplace transform (ILT) of the scavenger yield, and the scavenged yield is compared with the LT of the kinetics. The extent of the discrepancies found depends upon the rate constants for the various possible combination reactions. The smaller the rate constant for the combination of like particles relative to the recombination of unlike particles, the more accurate the decay kinetics are described by the LT relationship. When only unlike recombinations occur the agreement is excellent. In single species systems, where only like species can combine, there is considerable disagreement: in particular the ILT kinetics show an unrealistic minimum for systems initially containing an odd number of reactants.

ISSN:0026-8976    

DOI:10.1080/00268979100102591

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Laplace transform (LT) techniques are widely used in radiation chemistry to reconstruct nonhomogeneous kinetics from product yields in scavenger competition experiments. These techniques are tested using both stochastic and deterministic methods for a variety of systems where they formally break down, either because of the time dependence of the scavenging rate constant or because of the nonlinearity of the recombination kinetics. It is shown that the method can be corrected exactly for the time dependence of the scavenging rate constant by a simple change of timescale. For a single isolated pair where both particles are scavengable another timescale change suffices to make the LT relationship exact. However, for multipair spurs the magnitude of any discrepancy is found to depend on the relative importance of the different combination reactions in the spur. If the recombination of unlike particles is relatively more important than the combination of like particles, as in the radiolysis of water, the discrepancies are small; however, if combination of like particles is important the LT method significantly underestimates the short time kinetics. The usual experimental procedure of fitting a function to the scavenged yield and inverting the LT is also investigated, and found to be generally reliable if the data extend to sufficiently high concentrations to cover the inflection in the scavenged yield, or if the initial yield is already known.

ISSN:0026-8976    

DOI:10.1080/00268979100102601

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Measurement of the birefringence induced in a gas by an applied electric field gradient has been interpreted in terms of an existing molecular theory to yield the electric quadrupole moment of its molecules, whether dipolar or not. A different expression relating the birefringence to the quadrupole moment and other molecular properties is derived in the present paper, using a recent eigenvalue theory of light propagation in matter. Such an expression is required to be independent of molecular origin, even though a quadrupole moment depends in general on the choice of origin. The new result satisfies this requirement only if the primitive quadrupole moment, as opposed to the traceless, is used. In addition, it is shown that the earlier theory is inapplicable to the quadrupole moment experiment, in that it describes an incoherent phenomenon whereas birefringence is a coherent effect.

ISSN:0026-8976    

DOI:10.1080/00268979100102611

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The relative intensities of about 90 of the more prominent vibronic bands in the2Eg→4A2gluminescence spectrum of the MnF2-6ion in Cs2SiF6have been measured at 80K. The reported intensities cover a range of more than 106. Corrected values of the important intensity ratios of thev3,v4,v6vibronic origins are given as 1:4·02:5·45. Most of the features can be assigned as progressions in thev1,v2even parity vibrational modes based on the magnetic dipole allowed electronic origin and thev3,v4,v6odd parity vibronic origins, up to four members of the progressions being observed. Prominent features also include combinations of the three odd vibrational modes. Anharmonic effects are small. The intensity mechanisms of these features are briefly discussed. It is likely that second order and third order Herzberg-Teller vibronic coupling makes a significant contribution to the intensity mechanism. These data will be used in subsequent theoretical models of the intensity mechanism.

ISSN:0026-8976    

DOI:10.1080/00268979100102621

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor