摘要:

Intermolecular dipole-dipolar relaxation is considered in the framework of Redfield equations describing the evolution of the spin density matrices for multicomponent solutions. Two types of contribution from intermolecular dipole–dipolar interactions are identified. Contributions of the first type can be emulated using the external random field (ERF) model, while contributions of the second type cannot. The latter are responsible for relaxation coupling between solute and solvent, and can be expressed in terms of cross-relaxation rates. A complete system of Redfield equations for multicomponent liquids is not practical due to its size and the multitude of unknown parameters. For this reason, interpretation of experimental data usually is limited to the ERF model, even if unreliable for experiments where solvent spins are exposed to the effect of pulses. As an alternative, it has been suggested that complete system of Redfield equations can be truncated so that the solvent part is limited to spin polarizations. Solvent multispin modes are ignored as insignificant from the point of view of solute relaxation. So formulated, this approach is referred to as the ‘coupled solute solvent relaxation’ (CSSR) model. The validity of this approximation is examined using the examples of AX–AB and AX–ABX type binary mixtures. The elements of complete Redfield matrices embracing both solute and solvent are calculated. The results include cross-correlations between intermolecular dipolar interactions, which are calculated for spherical molecules with off-centre spin sites. Simulations indicate that the CSSR approach provides a very accurate approximation, regardless of the inherently complex nature of solvent relaxation and the existence of several potential paths for intermolecular magnetization transfer. It offers improvement over the ERF model, particularly in the evaluation of intermolecular contributions, in the case when both solute and solvent spins come under the effect of RF pulses during the course of experiments.

ISSN:0026-8976    

DOI:10.1080/002689797170743

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Solute diffusion in nematic liquid crystalline fluids has been studied using Enskog kinetic theory and molecular dynamics simulation. The liquid crystalline solvent is modelled by perfectly aligned hard ellipsoids of revolution, and the solute is either a spherical or ellipsoidal particle. The diffusion coefficient is calculated for a range of solvent densities and solute and solvent aspect ratios and sizes. The kinetic theory enables us to study various parameters easily compared with simulation or experiment. The validity of the kinetic theory, and its range of applicability is tested against the computer simulations. The main focus of the study is the anisotropy of diffusion, defined as the ratio of diffusivity in directions parallel and perpendicular, respectively, to the solvent director. If the pair correlation function at contact surface is taken to be isotropic, Enskog kinetic theory finds that the anisotropy in diffusion is independent of density and collision frequency, and depends only on size and shape of colliding particles. This result is confirmed by the simulations.

ISSN:0026-8976    

DOI:10.1080/002689797170752

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The generalized collective mode approach is used for the study of longitudinal time correlation functions (TCFs) of a simple fluid. Explicit expressions for the TCFs are found and their properties are analysed. It is shown that the solution for the density–density TCF obtained in theM-mode approximation reproduces explicitly the firstMfrequency moments as well as the zero-order time moment. Similar properties also are found for all the other TCFs from theM×MTCFs considered within theM-mode approximation. Numerical calculations were made of the longitudinal TCFs (density–density, density–energy, energy–energy, current–current, and zz–stress TCFs) in a nine-mode approximation for a Lennard-Jones fluid at reduced densityn*=nσL3J= 0.845 and reduced temperatureT*=kBT/εLJ= 1·706. The results are discussed in comparison with molecular dynamics data and the results of lower-order mode approximations.

ISSN:0026-8976    

DOI:10.1080/002689797170761

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Isotopic sensitivity is reported in both the formation and the above ambient temperature annealing behaviour of lamellar microphases in C30:C36paraffin solid solutions. The time resolved small angle neutron scattering from a 1:1 C30H62:C36D74mixture and its complementary pair 1:1 C D :C H has been studied. Upon quenching such mixtures from the melt; the solid solution so formed nucleates spontaneously to a ‘long period’ microphase. The two systems differ in that for 1:1 C30D62:C36H74there is also strong diffraction atQ= 0·136 A-1associated with C36H74enrichment of the solid solution or a separate phase. Both processes are sensitive to the isotopic composition of the mixture. For both systems the long period scattering from a quenched liquid mixture develops in periods of hours at ambient temperature. Heating the aged microphase structures to temperatures above about 43°C causes them to disappear, but again isotropic sensitivity is noted. It is concluded that the effects are largely kinetic in origin, in agreement with earlier IR studies on intralamellar microaggregation in this system.

ISSN:0026-8976    

DOI:10.1080/002689797170770

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Small angle X-ray scattering from binary mixtures of deuterated and non-deuterated n-paraffins (carbon numbers n, between 18 and 36), quickly quenched from the melt, has been studied in order to understand the demixing process. For a large difference inn, such as C18/C36the components separate into two distinct phases. These are both dilute solid solutions but, for example, despite the presence of the second component, the monoclinic forms characteristic of very pure C30H62, C32H66and C36H74appear. For mixtures with a weaker demixing tendency, a number of distinct phases, typically five, have been found, reflecting the complex polymorphic behaviour of the pure phases in this region of composition. For closenvalues, solid solutions form and ‘microphase separated’ states from these have been characterized by the appearance of a set of diffraction lines which grows into the pattern over a period of days to months at 25°C. We associate this with long density fluctuations in a lamellar ‘microphase’ structure. For the case of C30H62:C36H74, the crystallographic symmetry of the system in the solid solution and microphase separated states are evaluated from the diffraction. For the closestnvalues of all, a solid solution composed of rather imperfect crystals is stable for at least 5 months.

ISSN:0026-8976    

DOI:10.1080/002689797170789

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

To understand further the mechanism of microphase separation in paraffin mixtures at room temperature the strong small angle neutron scattering from the forming and annealing microphases is reported for a range of concentrations and at a sequence of times after quenching the molten mixtures. The data are very sensitive to the longitudinal organization of the structures and thus are complementary to recent spectroscopic studies. The data for solutions of C36/C30aliphatic hydrocarbons identify scattering from a phase rich in the C36component (nearQ= ∼ 0·1 A-1) and ‘long period’ scattering whose principal wavevector is concentration dependent. Long periods between 80A and ∼ 210A are observed. The rate and extent of growth of the microphase depend on both composition and quench temperature. The rate of microphase growth at room temperature increases by a factor of about seven times in changing composition from 4 : 1 C30H62:C36D74to 1:1 C30H62:C36D74. The data are consistent with a mechanism in which the free energy is minimized by crystal vacancy (even void) elimination and molecular conformational ordering with time as the system enthalpy is minimized by the preferred packing of chains of the same length.

ISSN:0026-8976    

DOI:10.1080/002689797170798

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The second negative (A2Πu- X2Πg) system of the O2+ion was produced in a tungsten hollow cathode discharge tube using oxygen in neon. This spectrum was recorded with the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory. A total of ten bands with ν″ = 0 and 1 and ν′ = 6 to 12 in the range 15 945-30 218 cm-1were observed and rotationally analysed. The line positions of all these bands, along with those of a few additional bands available in the literature, were combined in a global least-squares fit. Many molecular parameters for the various vibrational levels and the equilibrium molecular constants for the X and A states were estimated. RKR potential energy curves were constructed and Franck-Condon factors were calculated for the vibrational bands of the second negative (A-X) system. Some additional information obtained from threshold photoelectron spectroscopy was used to extend the X2Πgpotential curve up to ν = 24.

ISSN:0026-8976    

DOI:10.1080/002689797170806

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We present the SAFT (statistical associating fluid theory) equation of state for mixtures consisting of heteronuclear hard chain molecules using Wertheim's first-order perturbation theory extended to monodisperse mixtures. The reference fluid is a hard sphere mixture. Since only pure fluid simulation results are available, comparisons of theoretical results with molecular dynamics simulation data are made for the compressibility factor of pure hard chain copolymer systems, namely, block, alternate and random copolymers. Our comparisons show a very good performance of the equation of state. The comparisons of our theory with other equations of state for heteronuclear hard chain fluids show that the SAFT equation of state is almost equivalent to the generalized Flory dimer theory, and it represents a significant improvement over the generalized Flory, Chiew-Percus-Yevick and the modified form of Chiew-Percus-Yevick theories.

ISSN:0026-8976    

DOI:10.1080/002689797170815

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Thep-xylene adduct of Dianin's compound was studied by solid state NMR and molecular modelling. Molecular potential calculations were performed considering ap-xylene guest molecule in a single host cage. These calculations confirmed the existence of six different possible orientations of the guest molecules in the cage. The orientations correspond to two types of guests, which are not related by any symmetry, each consisting of three C3symmetry related sites. The two types induce significant distortions to the local structure of the host crystal cage, and these are most pronounced at the bottle-neck of the cage. In the calculations no diffusion paths were found that transferred guest molecules of one type to the other. X-Ray studies confirmed that the dimensions of thep-xylene/Dianin's cages are essentially equal to those reported previously. In our case, the R3 space groups failed to describe the X-ray diffraction data accurately. Single crystal2H-NMR of thep-xylene/Dianin's compound, specifically deuterated in the hydroxyl groups of the cage, provided detailed information on the distortions of the hexagons of hydroxyl groups interconnecting two cages. The hexagon distortions were correlated to the orientation of the guest molecules.13C CP-MAS NMR confirmed that the methyl groups of the host, in the middle of the cage, are influenced strongly by the inclusion of thep-xylene molecules. Based on the experimental observations, a model describing the distortions in the crystal is proposed.

ISSN:0026-8976    

DOI:10.1080/002689797170824

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The coupling of rotational and vibrational motions for small clusters of argon atoms modelled in terms of pairwise additive Lennard-Jones interactions is explored by the construction of angular momentum dependent turning point surfaces and by the evaluation of angular momentum dependent harmonic vibrational frequencies. The clusters considered include the trimer Ar3and the tetramer Ar4. Insights are obtained into the transition from chaotic to non-chaotic motion as the rotational angular momentum is increased at fixed total energy.

ISSN:0026-8976    

DOI:10.1080/002689797170833

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor