摘要:

Liquid crystals are usually not thought of as ‘simple’ liquids. Nevertheless, a surprising number of features of liquid crystals can be reproduced in computer simulations of very simple model systems, viz. hard ellipsoids of revolution and parallel spherocylinders. In particular, depending on the density and the shape of the hard-core particles, stable isotropic, nematic, smectic and crystalline phases are found. The existence of hard-core models for liquid crystals is important for two reasons. First of all, they can be used to test molecular theories for the static and dynamic properties of liquid crystals. And secondly, they may provide a starting point for thermodynamic perturbation theories. These hard-core models exhibit large pre-transitional fluctuations in the vicinity of some of the phase transitions, similar to those observed in real mesogens. Examples are given of the ‘experimental’ consequences of nematic and smectic precursor effects.

ISSN:0026-8976    

DOI:10.1080/00268978700100011

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The neutron intermolecular radial distribution functionsgm(r) of liquid CO2(atT⋍ 222 K,θ= 1·15 g cm-3) have been computed for four different potential models, namely a two-centre Lennard-Jones potential with and without point quadrupole, a three-centre Lennard-Jones with point quadrupole and the partially ‘ab initio’ potential recently proposed by Böhm and Ahlrichs.

ISSN:0026-8976    

DOI:10.1080/00268978700100021

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The vibration-rotation spectrum of CH3I in the 2020–2440 cm-1region consists of the absorption of the perpendicular systemv2+v6(E),v5+v6(E),v3+ 2v6(E). The last two bands form a strongly coupled Fermi dyad with crossing atK′ = 4 in the (+l) stack of levels. Evidence has been found for az-axis Coriolis-type contribution to their matrix elements, linear inKand with opposite signs in the (+l) and (-l) series. Thex-yCoriolis interaction ofv2+v6(E) with all components ofv5+v6(A1+A2+E) occurs as usual in methyl halides. The spectrum does not exhibit any direct absorption to the parallel systemv5+v6(A1+A2),v3+ 2v6(A1) andv2+ 2v3(A1), whose presence is revealed by extensive perturbations of the perpendicularv5+v6,v3+ 2v6system, mostly byl(2, -1) interactions, generating anomalous rotational patterns in a wideK″ΔKrange. Accurate values of the vibration-rotation parameters of the perpendicular bands were determined by a proper use of the available data.

ISSN:0026-8976    

DOI:10.1080/00268978700100031

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A new density functional formalism, that is able to describe an inhomogeneous hard sphere fluid consisting of an arbitrary number of components is presented. The theory for one component is compared with a simplified model for the hard sphere-hard wall system. This investigation shows the existence of a monatomic layer adjacent to the wall appearing as a thermodynamically separate phase, in the solid-fluid coexistence density range.

ISSN:0026-8976    

DOI:10.1080/00268978700100041

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

An expression for the off-resonance vibrational contribution to the hyperpolarizability of a homonuclear diatomic molecule is derived. This expression is written in terms of the Raman transition polarizability, and it explicitly gives the frequency dependence of all the independent tensor components of the hyperpolarizability of a randomly oriented, freely rotating molecule. The vibrational contributions to the hyperpolarizabilities of several molecules are evaluated and compared, both in the static limit, and also at optical frequencies. Several nonlinear optical processes are considered.

ISSN:0026-8976    

DOI:10.1080/00268978700100051

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The reference hypernetted-chain (RHNC) theory is solved for simple liquid crystal models and the results obtained are compared with Monte Carlo calculations. It is shown that the RHNC approximation gives an accurate estimate of the stability limit of the isotropic phase with respect to fluctuations of nematic symmetry. Furthermore, it provides a good description of the growth of long-range angular correlation in the pretransitional region. The temperature dependence of the inverse Kerr constant given by the RHNC theory is examined and qualitatively compared with mean field theory and experimental results. Finally, the reference linearized hypernetted-chain (RLHNC) theory and the mean spherical approximation (MSA) are also solved and are shown to be less accurate than the RHNC theory for the model considered.

ISSN:0026-8976    

DOI:10.1080/00268978700100061

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

In order to study exchange interactions between representative paramagnetic metal-π-complexes of the ferrocene type, bis-(benzene)-vanadium(S= 1/2) - nickelocene(S= 1) and cyclopentadienyl-cycloheptatrienyl-vanadium(S= 1/2) - nickelocene(S= 1) doublet-triplet pairs in a diamagnetic ruthenocene lattice have been investigated by low temperatureX-band E.P.R. Single crystal and powder spectra of bi-doped ruthenocene (∼1 per centS= 1/2 and ∼4–12 per centS= 1) samples clearly revealed the existence of several different ferro- or antiferromagnetically coupled molecules in neighbouring lattice sites (called ‘pairs’), and from these experiments surprisingly large isotropic exchange constantsJin the range of -4·7 to 7·9 cm-1(ℋex=JSD·ST) could be determined. Thereby a large axial zero-field splitting of 33·6 cm-1on nickelocene triplet allowed a pair ground state description in terms of an effective spin hamiltonian withS′ = 1/2,I′ = 7/2. Due to a strong dependence of the new effectiveg- andA-tensors onJ, (S= 1/2) - nickelocene doublet-triplet molecular pairs have turned out to be unique systems to measure exchange interactions with E.P.R.

ISSN:0026-8976    

DOI:10.1080/00268978700100071

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Starting from the exact solution of the mean spherical approximation for the hard core Yukawwa potential, the limit of adhesive hard spheres (i.e. an infinitely strong attractive well with a depth related to the range, which goes to zero, by a power law) is analysed. It is shown that only one choice of the scaling power leads to a model with nontrivial interactions. A critical point is obtained which has spherical model values of the critical exponents, indicating that a singular role is not played by the adhesive-hard-sphere limit.

ISSN:0026-8976    

DOI:10.1080/00268978700100081

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The perpendicularv3fundamental band of the H3O+-ion has been remeasured with a colour centre laser. Improved results have been achieved by combining our experimental data with the recently published line positions of Begemannet al. and Bunkeret al. For the first time thel-type doubling constant for the a-species has been determined, together with an additionall-type resonance parameter which describes the splitting of the (Kl) = -2 level.

ISSN:0026-8976    

DOI:10.1080/00268978700100091

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Thallium-205 spin-lattice relaxation times in the dimethylthallium cation have been measured over a range of temperature at frequencies of 21·96, 34·73 and 231·6 MHz. Although the relaxation is typically dominated by the chemical shift anisotropy (CSA) mechanism a contribution from the spin rotation (SR) mechanism has also been quantified. A value of the chemical shift anisotropy for this linear ion from other work (5550 ppm) enabled calculation of the reorientational correlation time τ⊥. An Arrhenius temperature dependence was found with a τ⊥value of 39·1 ± 0·5 ps at 298 K and an activation energy of 19·7 ± 0·7 kJ mol-1. The axial symmetry, the linear thallium environment, a knowledge of Δσ, and of the isotropic shift have allowed an absolute shift scale to be determined which is in close agreement with previous estimates by other workers. Assignment of the shift zero allowed calculation of the spin rotation constantC⊥, as 73·4 kHz. Hence values for the angular momentum correlation time τJ⊥could be determined. An Arrhenius temperature dependence was found with a τJ⊥value of 5·05 × 10-15s at 298 K and an activation energy of 17·7 ± 0·7 kJ mol-1.

ISSN:0026-8976    

DOI:10.1080/00268978700100101

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor