ISSN:0267-8292    

DOI:10.1080/02678298808086520

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

13C nuclear magnetic resonance and14N nuclear quadrupole resonance spectra of ferroelectric smectic C*liquid crystals and their non-chiral analogues allow for a microscopic determination of the polar and quadrupolar (or bipolar) biasing of rotation around the long molecular axis as well as for a determination of the anisotropy in the fluctuations of this axis. The results show that the microscopic origin of the biquadratic coupling between the polarization and the tilt, which has been recently introduced into the extended Landau model of the SA–S*Ctransition, is the quadrupolar (or bipolar) rotational bias induced by the anisotropy in the fluctuations of the long molecular axis. The tilt induced anisotropy in the fluctuations is practically identical in chiral and non-chiral smectic C phases.

ISSN:0267-8292    

DOI:10.1080/02678298808086521

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Deuterium N.M.R. of solute molecules in liquid crystal solutions with the removal of proton–proton dipolar coupling has been explored. The deuterium N.M.R. spectrum of a partially deuterated compound dissolved in a liquid crystal solvent is usually complex or unresolved because of proton–deuterium coupling and extensive proton–proton dipolar coupling. When the latter is removed by a special dipolar decoupling sequence, the deuterium N.M.R. spectrum becomes first order, and deuterium–proton dipolar coupling constants can be readily obtained from the spectrum. Results of monodeuterated hexanes and heptanes dissolved in ZLI 2142 are reported.

ISSN:0267-8292    

DOI:10.1080/02678298808086522

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Deuteron magnetic resonance spectroscopy (2H-N.M.R.) has been used to investigate the effect of the nematic environment on the flexibility and orientational order of two perdeuteriated cyanobiphenyl homologues: 4-methyl-4′-cyanobiphenyl (1CB-d11) and 4-n-pentyl-4′-cyanobiphenyl (5CB-d19). The systems studied were low concentrations of 1CB-d11and 5CB-d19dissolved in the nematic phases of 5CB,N-(-4-ethoxybenzylidene)-4′-n-butylaniline (EBBA), Merck ZLI-1132 (1132) and a 55wt% mixture of 1132: EBBA. The spectra are dramatically different in these environments. Previous studies on small solutes have suggested that in the 55wt% 1132: EBBA mixture (at 301.4 K) the dominant orienting mechanism depends on the size and shape of the molecule which suggests that it is a short range repulsive interaction. This interaction has been modelled by treating the liquid crystal as an elastic continuum and the solute as a collection of van der Waals spheres which stretch the liquid crystal in the two dimensions perpendicular to the director. The distortion of the liquid crystal depends on the dimensions of the solute, and the elastic energy is described in terms of a Hooke's law force constant,k.The model is extended to include flexible liquid crystal molecules and quadrupolar couplings are calculated for each conformation of the 5CB chain. Statistical averaging over all conformations gives an excellent fit to the experimental spectrum. The results for 1CB and 5CB show that in the other nematic phases contributions from additional mechanisms must be included. Previous studies of2H2and other solutes indicate that the additional mechanism is the interaction between the solute molecular quadrupole moment and the mean electric field gradient of the liquid crystal.

ISSN:0267-8292    

DOI:10.1080/02678298808086523

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A simple statistical model of interacting non-rigid molecules, based on a perturbation expansion of the pair correlation function and the additivity of segmental interactions, is applied to the study of orientational order as measured by N.M.R. in the nematic and SAphases of 4-n-alkyl-4′-cyanobiphenyls (N-CB, N = 5 to 8), the nematic and SCphases of 4-n-alkyloxybenzoic acids (N-OBA, N = 7, 8) and theDh0columnar discotic phase of hexa-alkyloxytriphenylenes (N-THE, N = 5 to 8). The order parameters of each homologous series are correctly described in terms of two isotropic and two anisotropic segmental coupling constants. The model predicts certain relations among coupling constants pertaining to different homogous series. These predictions are supported by the results obtained for the three types of compounds studied.

ISSN:0267-8292    

DOI:10.1080/02678298808086524

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

N-(4-alkoxybenzylidene)4′-toluidines are a family of organic compounds which present a nematic and (depending on the chain length) a smectic mesophase. We have performed proton spin echo measurements on some members of the family in these mesophases. N.M.R. spectra present well-defined structures that can be assigned to one of the two end methyl groups on the basis of phenomenological as well as analytical discussion, despite the large number of protons contributing to the total line shape. The order parameter derived from this analysis is compared with optical data on the same compounds.

ISSN:0267-8292    

DOI:10.1080/02678298808086525

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

An N.M.R. method combining the techniques of separated local field spectroscopy (SLF) and variable angle spinning (VAS) is valuable in the investigtion of nematic liquid crystals. Rapid sample spinning causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 N.M.R. spectrum. SLF is a two-dimensional N.M.R. method which produces a first order splitting pattern for each carbon signal from which C–H dipolar coupling constants can be determined. The order parameters for all segments of the liquid crystal molecule can then be calculated. Results for three 4′-cyanophenylcyclohexanes are considered here. These compounds are trans-substituted at the 4 position of cyclohexane ring withn-pentane (PCH5), 1-pentene (3d1CP) and 3-pentene (1d3CP), respectively.

ISSN:0267-8292    

DOI:10.1080/02678298808086526

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The HH and CH dipolar couplings of benzene measured in five different liquid crystal solvents are subjected to an analysis which allows for the correlation between the molecular reorientational and vibrational motions. The number of adjustable parameters is reduced by treating the CH bonds or both the CH and CC bonds as effectively cylindrically symmetric entities. In this way detailed information on the anisotropic forces acting on the bonds of benzene dissolved in liquid crystals is obtained. The behaviour of the CC bonds, but not that of the CH bonds, may be explained by anisotropic dispersion forces. There is an approximately linear relation between the torques acting on the CH bonds of benzene and methane in the same liquid crystal environment. This suggests that these forces stem from a common interaction mechanism, possibly the van der Waals interaction between the atoms of the solute molecule and the liquid crystal surroundings or the interaction between the molecular quadrupole moment and the electric field gradient due to the surrounding medium. A bond additivity model for the molecular quadrupole moment tensor is developed and discussed.

ISSN:0267-8292    

DOI:10.1080/02678298808086527

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A simple method of deformational correction for the N.M.R. spectral parameters of molecules dissolved in liquid crystals is described. The method is applicable to the different isotopically substituted derivatives of the molecules belonging to the cubic point groups. Its validity is verified for the dipolar and quadrupolar couplings observed in the molecules CH4, CD4and CH3D dissolved in the liquid crystals Merck Phase IV, Merck ZLI 1167 and their mixtures.

ISSN:0267-8292    

DOI:10.1080/02678298808086528

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The geometry of dimethylmercury was studied in several mixtures of the liquid crystals ZLI 1167 and phase 4. The variation of the geometry is not very large but systematic changes can be detected as a function of the liquid crystal composition. The anisotropics, δ[sgrave], of the1H,13C and199Hg shielding tensors were investigated by applying two methods based on mixing liquid crystals with opposite diamagnetic anisotropies and by applying various referencing methods. Apart from these, the anisotropy, δJHgC, of the199Hg–13C spin–spin coupling tensor was studied in four mixtures of these liquid crystals, and was found to vary with the solvent.

ISSN:0267-8292    

DOI:10.1080/02678298808086529

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor