G. M. Richards

Molecular order and dynamics of the nematogen MBBA: Modelling of deuterium NMR observables

We report on the measurements of quadrupolar splittings and spectral densities in the nematic phase of a partially deuterated liquid crystal p-methoxybenzylidene-p-n-butylaniline (MBBA). The Zeeman and quadrupolar spin-lattice relaxation times have been measured at 15·3 and 46 MHz using a broadband multiple-pulse sequence. Correlated internal motions of the alkyl chain may be superimposed onto the reorientation of the ‘average’ molecule. Such a model has recently been proposed and is further tested in the present study. The quadrupolar splittings are used in conjunction with the additive potential method to determine the strength of the nematic mean field. The equilibrium probability of each conformer and conditional probabilities for transitions among different conformers are evaluated to yield spectral densities of motion for deuterons at different sites of the molecule. We use ‘realistic’ geometry to generate all possible conformations in the chain. Two different motional models for molecular reorienta...

Modeling of correlated internal motions and deuteron spin relaxation in liquid crystals

Abstract Using the master-equation method, we examine the effects of correlated internal rotations in an alkyl chain on the spectral densities of motion. It is assumed that the overall reorientation of molecules is decoupled from the internal motions in their alkyl end chains. The pentyl chain that is attached to a cylindrical biphenyl core in 4- n -pentyl-4′ -cyanobiphenyl is studied. The reorientation of a symmetric top in an anisotropic potential is accounted for, using the small-step rotational diffusion model. The effects of a nematic mean field on the chain dynamics are discussed. Both J 1 (ω) and J 2 (2ω) are found to be frequency dependent, with J 2 (2ω) being weaker.

Deuterium Spin Relaxation in a Ring-Deuterated Smectic Liquid Crystal

Abstract The phase-cycled Jeener-Broekaert sequence was used to simultaneously measure the deuterium Zeeman (T 1z) and quadrupole (T 1q) spin-lattice relaxation times in the mesophases of 4-n-butoxybenzylidene-4′-n-octylaniline-2,3,5,6-d4 at a Larmor frequency of 15.4 MHz. The derived spectral densities of motion J 1(wo) and J 2(wo) increase with decreasing temperature in the nematic, smectic A and smectic B Phases and show a discontinuity similar to that of the quadrupolar splitting at the SA to SB phase transition. The angular dependence of T 1z and T 1q were measured in the smectic B phase, thus allowing us to obtain J 1(wo) and J 2(wo). Within experimental error, the spectral densities J p(w) (p = 1,2) are in the white spectrum regime. Molecular reorientation seems to be a dominant relaxation mechanism for the aniline deuterons and is discussed in terms of a small step rotational diffusion model.

Spectral Density of Motion in a Reentrant Nematic by Deuterium NMR

Abstract We report on a deuterium spin relaxation study of a 26 wt. % sample of perdeuterated hexyloxycyanobiphenyl (60CB-d 21) in octyloxycyanobiphenyl as a function of temperature at a Larmor frequency of 13.81 MHz. The binary mixture exhibits the following phase sequence: isotropic—nematic—smectic A—reentrant nematic—crystal. The DMR spectrum of this sample is relatively well-resolved and permits the measurement of the Zeeman (T 1Z) and quadrupole (T 1Q) spin-lattice relaxation times for several distinct deuterons in 60CB-d 21. The present study considers only the ring deuterons. In fact, there are two sets of ring deuterons and we restrict our study to the set with the smaller quadrupolar splitting to avoid spectral overlap from the methyl deuterons. These relaxation times were simultaneously measured by the phase-cycled Jeener-Broekaert sequence, and the spectral densities Jp (ρω) (p = 1,2) were found to increase continuously with decreasing temperature. Molecular reorientation seems to be a dominant...

Coupled motional model for spin relaxation in 5CB liquid crystal

Abstract Using the master equation method we examine the effects of correlated internal rotations in an alkyl chain on its deuterium spectral densities of motion. The overall reorientation of molecules is coupled to the internal motions in their alkyl end chains by the use of conformation-dependent diffusion axes. The pentyl chain that is attached to a cylindrical biphenyl core in 4- n -pentyl-4′-cyanobiphenyl is studied and the results are compared with the previously published decoupled diamond-lattice model.

Modeling Internal Chain Dynamics in Nematogens of Different Chain Lengths: Deuteron NMR Study

Abstract Aligned liquid crystals with deuterated chains show well-resolved deuterium NMR spectra. The site specificity of these quadrupolar doublet signals allows measurements of both static and dynamic observables, which can provide information of the rotation dynamics within the flexible end chain. We use a realistic geometry and the rotational isomeric model of Flory to generate all possible configurations and a set of C-D bond orientations in the chain. A master rate equation is used to describe transitions among all allowed configurations of the chain. In this paper, we report on motional parameters derived from quantitative fittings of quadrupolar splitting and spectral density data in three different nematogens of various chain lengths. The deuterated samples are p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline (10.4-d13), 4-n-pentyl-dn-4‘-cyanobiphenyl-d4 (5CB-d 15) and 4-n-hexyloxy-d13-4‘-cyanobiphenyl-d8 (6OCB-d21).

Molecular orientational motion in discotic liquid crystals

Abstract The spectral densities of motion for the aromatic and chain deuterons of the discotic mesogen hexahexyoxytriphenylene (THE6) have been reported in the literature for a frequency of 46 MHz. Most spectral densities Jp (pω0, 90°) have been obtained from samples consisting of a planar distribution of domains in which the directors were perpendicular to the magnetic field Limited data Jp (pω0) have also been available from single-domain samples with the director aligned parallel to the magnetic field. We have applied the small-step rotational diffusion model of Nordio et al. to the data from the aromatic deuterons of THE6-ard in its uniaxial columnar Dho phase, to describe the spinning (D ∥, rotational diffusion constant about the planar normal to the disc) and the tumbling (D⊊, rotational diffusion constant of the planar normal) motions of the molecular core. Although this model has been successfully used for rod-like nematic liquid crystals, its use has not been attempted for discotic liquid crystal...

Anisotropic reorientation and internal chain dynamics in liquid crystals: A deuterium nuclear magnetic resonance study

The spectral densities of motion J1(ω) and J2(2ω) were measured at 15.3 MHz in the nematic phase of p‐methoxy‐d3‐benzylidene‐d1‐p‐n‐butyl‐d9‐aniline for the methine and butyl chain deuterons. The present study concentrates on measurements near the clearing temperature as well as extends measurements to temperatures lower than those of an earlier report. We propose to interpret these spectral density parameters using a new model of molecular reorientation suggested by Vold and Vold. The reorientation motion of the molecules in a Maier–Saupe potential is governed by three rotational diffusion rate constants Dα, Dβ, and Dγ. The subscripts in the rate constants are the Euler angles ΩL,M which are involved in time‐dependent transformation between the molecule fixed frame and the laboratory (director) frame. Dα and Dβ are the principal components of a rotational diffusion tensor in the director frame. The motion about the molecular ZM axis (γ motion) is assumed to be independent of the α,β motions. The internal...

Chain conformation and spin relaxation in nematogens

Liquid crystals are composed of flexible organic molecules with many internal degrees of freedom; these molecules, with various alkyl chain lengths, may show different mesophases. When modelling molecular motions, the internal conformation motions of the chain may be superimposed onto the reorientation of the entire molecule. Transitions among different chain conformers have been described using a master equation. The nematic mean field, which determines the equilibrium probability of each conformer, can be modelled by fitting quadrupolar splittings observed in deuteriated liquid crystals. The above approach allows evaluation of spectral densities of motion necessary for interpreting nuclear spin–lattice relaxation times in nematogens.We report on the measurements of deuterium T1z and T1Q as a function of temperature at 15.3 and 46 MHz in 4-[2H11]pentyl-4′-cyano-[2,3,5,6-2H4]biphenyl (5CB-d15). The data are discussed based on the above motional model. In particular we focus on the site dependence of the relaxation rates along the alkyl chain, the frequency dependence of the relaxation rate, and the discontinuity of the relaxation rates at the nematic–isotropic transition.

Selective dipolar order and cross relaxation in proton NMR: application to liquid crystals

Abstract Methods to exploit the large dipole-dipole interactions among protons in magneto-aligned liquid crystals are described. These allow simultaneous determination of the spin-lattice relaxation rates of the Zeeman and dipolar reservoirs. In particular, a selected group of spins whose dipolar splitting is significantly different from those of the other spin groups may be selectively excited to monitor its relaxation rates. It is demonstrated that cross relaxation effects exist among different spin subgroups in the nematic liquid crystal MBBA.