摘要:

The molecular constitution of viscoelastic gels is customarily described in terms of networks, which may have a transient or permanent character. Such supermolecular structures are often observed in biological or macromolecular systems, but can even occur in dilute solutions of some detergents. Surfactant molecules in solution, under suitable conditions, assemble reversibly into large aggregates of rod-like geometry. Depending on the ionic strength of the solution, these particles may be completely stiff or semiflexible. At low concentrations, when the lengths of the rods are much smaller than their mean distance of separation, there exists a sol state which is highly sensitive to shear forces.

ISSN:0026-8976    

DOI:10.1080/00268979100102721

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

NMR proton relaxation rates of normal and17O enriched water in a mixture of 68 mol% water and 32 mol% [2H6]dimethylsulphoxide were measured for temperatures between 298 K and 183 K. In the range between 240 K and 204 K the limit of fast proton-proton exchange between H162O and H172O is not obeyed, and relaxation curves deviate from mono-exponential behaviour. By fitting the relaxation curves to a model of NMR two-phase relaxation the proton-proton exchange rate within the aqueous component could be obtained. With decreasing temperature, proton-proton exchange slows down and a residence time of about 125 ms at 215 K is found, but it becomes faster again for still lower temperatures. From the phase-averaged relaxation rates of water in the17O enriched mixtures, the17O induced proton relaxation rate was derived as a function of temperature. This yields the rotational correlation times of the water molecule in the mixture and the dipolar spin-lattice coupling parameter. The latter is considerably lower than the one predicted from the geometry of water.

ISSN:0026-8976    

DOI:10.1080/00268979100102731

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Several new results in the theory of angular momentum are presented, which have their origin in NMR experiments on multiply connected spin-systems. In particular, Fano's ‘rectangular’ unit spherical tensors are used to extend Racah tensor algebra to deal with ‘non-square’ matrices. The properties of ‘non-square commutation relationships’ are examined in some detail, and illustrated using a simple three coupled spin-1/2 system. It is shown that Racah algebra is a subset of that which applies to the Fano spherical tensors.

ISSN:0026-8976    

DOI:10.1080/00268979100102741

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Unit spherical tensor operators, originally developed by Fano for use in describing radioactive decay and γ-γ correlation experiments, are used to examine a simple three coupled spin-1/2 system, evolving under a chemical shift, dipolar interactions, and hard rf fields. In particular, it is argued that the strongly coupled representation, and therefore Fano's spherical basis set, is the correct choice for the description of MQ-NMR experiments on multiply connected spin systems. Some comments are also made concerning spin systems with more than three spins. In addition, it is shown how ‘constants of the motion’ can be used to simplify the necessary tensor algebra.

ISSN:0026-8976    

DOI:10.1080/00268979100102751

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is assumed, on the basis of recent supercomputer simulations, that two electrons in the same molecular orbital attract, rather than repel, and the consequences are explored by means of three-dimensional, supercomputer, molecular mechanics type simulations of ground state H12, H22, H1H2, H32, H1H3. Using spectroscopic data, it is shown that, in every case, the resulting vibrational frequency and molecular diameter are entirely in agreement with experiment.

ISSN:0026-8976    

DOI:10.1080/00268979100102761

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The ‘cross’ spectral moments due to collision induced phenomena are considered. We show that for linear rigid rotors it is possible to obtain analytical formulae for these ‘cross’ terms. The general expressions are valid for arbitrary orders of moments and for arbitrary tensorial multipolar intermolecular interactions. Statistical averaging is performed, using the spherical componentsgl1l2α(R12) of the pair distribution function. It makes the formulae suitable for numerical computations.

ISSN:0026-8976    

DOI:10.1080/00268979100102771

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

In this paper we report a determination of the structure 2,2′-dithiophene dissolved in different nematic liquid crystal solvents. The1H-NMR spectra of this molecule dissolved in a 55% Merck ZLI 1132/45% N-(4-ethoxybenzylidene)-4′-n-butylaniline (w/w) mixture are recorded at 300 and 304 K and are analysed. The dipolar coupling constants obtained are used to show that the solute molecule exists in both the s-cisand the s-transconformations. The s-transconformer is planar and the absolute value of the dihedral angle Φ between the two thiophene ring planes for the s-cisconformer is found to be (24 ± 1)°. Using the same method of analysis we reinterpreted two additional independent sets of published dipolar coupling constants from1H-NMR spectra of 2,2′-dithiophene dissolved in Merck Phase IV at room temperature and in 80%N-(4-ethoxy-benzylidene)-4′-n-butylaniline/20%O-carbobutoxy-4-oxybenzoic acid ethoxy phenyl ester at 301 K. These additional studies yield the same results for the structure of 2,2′-dithiophene. We also estimate that in each of the three nematic solvents (61 ± 3)% of the solute 2,2′-dithiophene exists in the s-transstate.

ISSN:0026-8976    

DOI:10.1080/00268979100102781

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The polarizability and its first and second derivatives have been calculated for methane at the following levels of approximation:ab initioSCF, singles and doubles CI and averaged coupled pair functional method; correlation and basis set effects have been investigated. The calculated polarizability surface predicts to experimental accuracy several observables related to the refractive index. The correlated first derivatives provide improved predictions of Raman scattering cross sections for the fundamentals. First and second derivatives are used to calculate scattering cross sections for selected overtone and combination bands including all totally symmetric bands in the pentad region. A comparison is made between cross sections based on perturbation expressions and on variational vibrational state functions. Deficiencies in the vibrational potential derived from rovibrational spectra are pointed out.

ISSN:0026-8976    

DOI:10.1080/00268979100102791

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

It is shown that nonlocal separable potentials may be used to study bound states of particles in multicentre potentials. The binding energy is obtained in a closed form. The cases of H2+ion and polymer chains are discussed in detail and results compared to experimental data.

ISSN:0026-8976    

DOI:10.1080/00268979100102801

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A full quantum calculation of several transport properties for the helium-nitrogen molecular mixture has been carried out within the coupled state (CS) approximation using the HFD1 potential energy surface (PES) that has already been employed as a test potential in various other calculations. Comparison of the ensuing observables, both for values at given total energies and for values as function of temperature, with the essentially exact close coupling (CC) results and with infinite-order-sudden (IOS) approximate calculations (all obtained with the same HFD1 potential function) indicate an average percentage error for diffusion and viscosity ranging between 1% and 3% with respect to the CC results, while the viscomagnetic relaxation data exhibit a much more marked difference between CC and CS calculations. The physical origins of such differences, as well as explanations for those situations where close agreement is found between different dynamical approximations, are discussed in detail. Possible predictions are also made for other rare gas-N2mixtures presently under study.

ISSN:0026-8976    

DOI:10.1080/00268979100102811

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor