摘要:

Anhydrous polycrystalline cholesterol C27H46O has been investigated by pulsed proton magnetic resonance methods between 77K and the melting point (421·7K). In the low-temperature phase, measurements of the spin-lattice relaxation timeT1at 7, 14 and 28 MHz together withT1Dand second moment are ascribed to reorientation of the five methyl groups, one of which has significantly less hindrance. Values of the relaxation constants, activation energies and time factors that best characterize the motions are tabulated. Above the phase transition at 311K, reorientation of the alkyl chain about C-C bonds with activation energy 34·3 kJ mol-1is evident from the measured values ofT1Dand second moment.

ISSN:0026-8976    

DOI:10.1080/00268979000101151

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A molecular-dynamics simulation is carried out to study the changes in statistical geometry and related dynamical properties in the Li2SiO3system at the glass transition point. The number of oxygens around each cation within the first coordination shell is taken as the numberVof vertices of the polyhedron that characterizes the system. When the numberNof O-O pairs that are shorter thanRO(ROis the position where the second plateau of the running coordination numbernO-O(r) ends) is counted, the distribution ofNis found to have a maximum atN= 3V- 6 (V= 3, …, 7) below the glass transition temperature.Ncorresponds to the number of contact pairs in each polyhedron. The lifetime of the polyhedron withN= 3V- 6 is found to be longer than other polyhedra with other values ofN. The results suggest that geometrical degrees of freedom play an important role in glass formation in this system.

ISSN:0026-8976    

DOI:10.1080/00268979000101161

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The self-diffusion coefficient for a homonuclear diatomic liquid of vibrating molecules, resembling nitrogen, but with a wide range of vibration frequencies, is determined by computer simulation for a wide range of liquid states. The values are found to differ little from those for the corresponding rigid-molecule liquid and are remarkably insensitive to the vibration even for very-low-frequency and large-amplitude vibrations. We do not believe that explicit inclusion of molecular flexibility is likely to materially affect the gross dynamical properties of a molecular liquid.

ISSN:0026-8976    

DOI:10.1080/00268979000101171

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Molecular-dynamics calculations have been carried out for rigid and flexible versions of SPC water. Some evidence is found that flexibility of the molecules may enhance the rates of translational and rotational diffusion, but the effects are much weaker than those previously reported by Telemanet al.The relevance of the results for the parametrization of effective water potentials is briefly discussed.

ISSN:0026-8976    

DOI:10.1080/00268979000101181

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The equilibrium geometries for ground and low-lying excited states for silylvinylidene have been optimized at the Hartree-Fock level. Electronic excitation energies for three low-lying electronic states are reported. The results indicate that the singlet state of silylvinylidene is indeed the ground state. The energetic ordering of the other states is consistent with previous predictions for vinylidene.

ISSN:0026-8976    

DOI:10.1080/00268979000101191

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

ISSN:0026-8976    

DOI:10.1080/00268979000101201

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor