摘要:

Cross-polarization from a spinI=1/2 nucleus (e.g.,1H) to a spinS= 3/2 nucleus (e.g.,23Na) or a spinS= 5/2 nucleus (e.g.,27A1 ornO) in static powder samples is investigated. The results of conventional (single-quantum), three-quantum, and five-quantum cross-polarization experiments are presented and discussed. Based on a generalization of an existing theory of cross-polarization to quadrupolar nuclei, computer simulations are used to model the intensity and lineshape variations observed in cross-polarized NMR spectra as a function of the radio-frequency field strengths of the two simultaneous spin-locking pulses. These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single- and multiple-quantum cross-polarization to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQMAS NMR experiment.

ISSN:0026-8976    

DOI:10.1080/00268970009483265

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross-couplings between thermodynamic forces and fluxes that are polar vectors and pseudo vectors respectively. This makes it possible for a temperature gradient parallel to the cholesteric axis to induce a torque which rotates the director. This phenomenon is known as the Lehmann effect. The converse is also possible: one can drive a heat current by rotating the director. In this work a recently developed non-equilibrium molecular dynamics simulation algorithm is applied to calculate the cross-coupling coefficient between the temperature gradient and the torque for a molecular model system based on the Gay-Berne fluid. According to the Onsager reciprocity relations this cross-coupling coefficient is equal to the coupling between the director angular velocity and the heat current. The cross-coupling coefficients are found to be very small but non-zero and the Onsager reciprocity relations are satisfied within the statistical uncertainty.

ISSN:0026-8976    

DOI:10.1080/00268970009483266

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The WCA fluid shows a nonlinear behaviour toward a periodic elongational flow through the presence of higher harmonics in stress responses. At the same time the liquid remains purely viscous and isotropic. The absence of numerical artefacts is checked and the physical origin of the harmonics is discussed.

ISSN:0026-8976    

DOI:10.1080/00268970009483267

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A force field has been developed to describe the phase behaviour, interfacial, and transport properties of nitrogen and hydrocarbon mixtures under conditions relevant to those found in the high pressure extraction of oil from underground reservoirs. A Gibbs ensemble Monte Carlo method is used to parametrize intermolecular potentials for the pure components by matching experimental and simulated liquid and vapour coexisting densities. Also the surface tension, diffusion coefficient and shear viscosity of nitrogen and its mixtures with butane have been determined. The latter properties were obtained by canonical molecular dynamics simulations. The diffusion coefficient and shear viscosity were calculated by a Green-Kubo method. Results for pure nitrogen are given for temperatures ranging from 70 K to 110K. For mixtures of nitrogen with butane, results are presented at 339.4 K and 380.2 K. Good agreement is found between the results of simulations and available experimental data.

ISSN:0026-8976    

DOI:10.1080/00268970009483268

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The absorption spectrum of ethane was recorded at 0.014 cm−1resolution in the range 4500–6500 cm−1using a Fourier transform spectrometer and at room temperature. Eighteen bands could be identified and their type assigned. Upper state rotational constants are provided for the band at 5948.338 cm−1and Coriolis constants are obtained for most perpendicular bands. Vibrational assignments are suggested for the bands at 5948 cm−1(v7 +v10), 5914 cm−1(v8 +v10+v11), and 5852cm−1(v5+v10). All vibrational bands reported in the literature are gathered.

ISSN:0026-8976    

DOI:10.1080/00268970009483269

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor