J. W. Emsley

The orientational order of nematogenic molecules with a flexible core

The orientational order of the α,ω-bis(4,4′-cyanobiphenyloxy)alkanes, new nematogens with a flexible core linking two rigid groups has been studied using deuterium N.M.R. spectroscopy. The order parameter for the para-axis of the cyanobiphenyl group, determined at the nematic-isotropic transition, is found to be considerably greater for members of the homologous series with an even number of methylene groups than for those with an odd number.

Nuclear spin-lattice relaxation rates in liquid crystals

The spin-lattice relaxation rates R (i) have been measured for the deuterons in 4-n-pentyl-d11-4′-cyanobiphenyl-d4 (5CB-d15) and 4-n-pentyl-d1-4′-cyanobiphenyl (5CB-d1) at 30·7 MHz and over the temperature range 257 to 400 K which includes the nematic-isotropic transition, T NI, at 308 K. Both samples display a small biphasic region because of the presence of a low concentration of an unknown impurity. The variation of both quadrupolar splittings and R (i) values have been carefully measured at 0·1 K intervals close to and in this interesting region. A discontinuity in the relaxation rate of the deuteron at the first position in the alkyl chain has been detected at T NI. The observed discontinuity is in good agreement with the change in R (i) predicted to occur because of the onset of long range orientational order at this temperature. There are regions of temperature in both phases where in R (i) varies linearly with T -1 and from which apparent activation energies can be determined for C-D bond reorient...

Orientational ordering in uniaxial liquid crystals

Deuterium N.M.R. has revealed pronounced variations in the orientational order along the alkyl chains which form an integral part of most mesogenic molecules. The model developed by Emsley, Luckhurst and Stockley to account for such variations is found to work reasonably well for the 4-n-alkyl-4′-cyanobiphenyls. However the agreement between theory and experiment can be improved considerably if the potential of mean torque for each conformer is modified to include contributions from the C-D bonds as well as the C-C bonds in the alkyl chains and from the aromatic core. Analysis of the observed order parameters for the C-D bonds in 4-n-pentyl and 4-n-octyl-4′-cyanobiphenyl allow the temperature dependences of the parameters in the potential of mean torque to be determined. The temperature variation of these parameters is found to be consistent with the predictions of molecular field theories of orientational order in mesogens composed of non-rigid molecules.

Orientational order in the 4-n-alkyloxy-4′-cyanobiphenyls

The orientational order of rigid sub-groups for members of the homologous series of 4-n-alkyloxy-4′-cyanobiphenyls, the NOCB, has been studied experimentally and compared with the predictions of the theory proposed by Emsley, Luckhurst and Stockley, [(1982) Proc. R. Soc., A381, 117]. The experimental data were determined from the duterium NMR spectra of compounds containing fully deuteriated chains, and for 3OCB, 4OCB and 6OCB, deuterium was also included in one of the benzene rings. The good agreement obtained between calculated and observed order parameters for these mesogens supports the model proposed for the potential energy of the molecule in the liquid crystal phase. The temperature dependence of the core interaction parameter is obtained and compared with theoretical predictions. The statistical weights of conformers are calculated and it is shown that there is a change in their values on going from the isotropic to the nematic phases. The significance of the ordering matrices for individual confo...

Molecular dynamics in a liquid crystal: Measurement of spectral densities at several sites in the nematogen 4-n-pentyl-4′-cyanobiphenyl

Experiments are described for determining the auto-correlation spectral densities J 1(ω0) and J 2(2ω0) for deuterons at each position in the alkyl chain and in one aromatic ring of the nematogen 4-n-pentyl-d11-4′-cyano-2,3,5,6-d4-biphenyl (5CB-d15). The experiments involve selective population inversions followed by non-selective monitoring of the spectrum. The efficiency of the method is compared with that of the Jeener-Broekaert sequence. The results are discussed in terms of models for the molecular motion which treat the molecules either as rigid, cylindrically symmetric rods, or which assume that the internal modes of molecular motion are fast enough to be decoupled from the overall molecular rotations. Both models fail to explain the dependence of the ratio J 1(ω0)/J 2(2ω0) on position in the molecule. It is suggested that the variation of the ratio of spectral densities with position reflects the importance of both internal modes of molecular motion, which affects both J 1(ω0) and J 2(2ω0) and dire...

Solute alignment in liquid crystal solvents The Saupe ordering matrix for anthracene dissolved in uniaxial liquid crystals

Abstract We have described a theory for U, the potential of mean torque of rigid solutes at infinite dilution in a uniaxial liquid crystal phase; this may be used to calculate (S xx - S yy) and S zz, the principal elements of the Saupe ordering matrix. In its simplest form U(ω) contains only second-rank terms and the dependence of the biaxiality (S xx - S yy) is determined by ω, a parameter which describes the departure of the potential of mean torque from cylindrical symmetry, and is predicted to be temperature independent. If dispersion forces are responsible for the magnitude of the orientational order parameter then ω should be independent of the solvent and depend only on the anisotropy in the electric polarizability of the solute. Indeed, this independence should result for any pair potential which can be factorized into a product of solute and solvent properties. These predictions are tested here by determining values of S zz and (S xx - S yy) for anthracene-d 10 as a solute in several liquid cryst...

Nuclear magnetic resonance studies of magnetohydrodynamic effects in the nematic mesophase

Abstract The nuclear magnetic resonance spectrum of acetonitrile dissolved in the nematic mesophase of a liquid crystal has been measured as a function of the spinning speed. The dependence of the dipolar splitting on the rotation speed is found to be in good agreement with that predicted by continuum theory. Measurement of the rotation-speed dependence of the linewidths shows that spinning the sample below a critical speed has virtually no disrupting influence on the uniform alignment of the director.

The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d3 dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, Dij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles τ1 and τ2. Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(τ1,τ2), which is then used to define a probability density distribution, P(τ1,τ2). It is shown here that when attempting to obtain P(τ1,τ2) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(τ1,τ2). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on τ1=τ2=56.5±0.5°, which is close to the values calculated for an isolated ...

The inclusion of electrostatic and dispersion interactions into potentials of mean torque for solutes dissolved in uniaxial liquid crystal solvents

Abstract A potential of mean torque is derived for a solute at infinite dilution in a uniaxial liquid crystal solvent, which contains terms originating from the dispersion interaction, and the electrostatic interaction between quadrupole moments on both molecules. It is shown that the electrostatic term is non-zero only if the solute-solvent vectors are distributed with lower than spherical symmetry. If this distribution has cylindrical symmetry then both the electrostatic and dispersion terms in the potential of mean torque are shown to depend on order parameters for the orientational distribution of the solute-solvent vectors, as well as on the order parameters of the solvent molecules.

Computer simulation studies of the dependence on density of the orientational order in nematic liquid crystals

Abstract Simulations of the temperature dependence of the nematic orientational order parameter, [Pbar] 2, at different densities have been made using the Gay-Berne potential. The results are used to calculate Γ = −(∂In T/∂ IN V) [Pbar] 2, which is a measure of the relative sensitivity of [Pbar] 2 to temperature and density. Previous experimental measurements of Γ have obtained values in the range 2–6 for real nematogens, whilst the present simulations yield Γ = 8 ± 1. Changing the Gay-Berne potential by increasing the steepness of the repulsive term increases the calculated value of Γ, whilst changing the relative well depths for side-by-side compared to end-to-end arrangements of Gay-Berne particles leaves Γ virtually unchanged. These exploratory calculations suggest that Γ is a useful parameter for testing the parametrizations of model potentials.