James W. Emsley

A comparison of the conformational distributions of the achiral symmetric liquid crystal dimer CB7CB in the achiral nematic and chiral twist-bend nematic phases

The sets of residual dipolar couplings between carbon and hydrogen nuclei obtained from the proton-encoded (13)C 2D NMR experiment are used to investigate the conformational changes which occur when the achiral symmetric liquid crystal dimer CB7CB changes from the achiral nematic to the chiral twist-bend nematic phase. It is found that these changes are a consequence of the chirality of the twist-bend nematic phase, rather than being the driving force for the stability of this phase.

The relationship between molecular symmetry and second-rank orientational order parameters for molecules in chiral liquid crystalline solvents

From group theoretical arguments, we demonstrate that the effective molecular symmetry is reduced in a chiral liquid crystalline phase. This reduction changes the location of the principal axes of the orientational order matrices for four molecular point groups, namely Cs, C2v, S4, and D2d. These symmetries correspond to compounds which have prochiral faces, groups, or directions. The change in effective molecular symmetry can be detected by NMR spectroscopy, and this is illustrated by the example of acenaphthene dissolved in a chiral nematic solvent.

A comparison of proton‐detected 13C local field experiments with deuterium NMR at natural abundance for studying liquid crystals

A comparison is made between the information which can be derived on structure, conformation and orientational order of the molecules in a nematic liquid crystal 4‐pentyl‐4′‐cyanobiphenyl (5CB) from the NMR spectra of the deuterium atoms at natural abundance (NAD NMR), and the two‐dimensional proton‐detected 13C local field experiment (PDLF). The nine residual quadrupolar splittings, Δν k , obtained experimentally have been compared with quadrupolar tensors and a geometry and conformational potentials calculated by the DFT method B3LYP/6‐311G**. The PDLF experiment yielded 42 scaled 13C–1H residual dipolar couplings, kD CiHj . The scaling factor, k, is determined experimentally by comparing unscaled and scaled residual dipolar couplings in a sample of fluorobenzene dissolved in a nematic liquid crystalline solvent. The corrected residual dipolar couplings, D CiHj , are used to investigate the structure and rotational potentials about each bond in the molecule.

Computer simulation of the behaviour of a solute in a model liquid crystalline solvent

A method is described for modelling the behaviour of solute molecules dissolved in a model liquid crystalline solvent. The method is illustrated for benzene as the solute, and the solvent is a collection of particles interacting with each other via the Gay Berne potential. The interaction between the benzene and each solvent particle is modelled as a Lennard-Jones site-site potential between six united carbon and hydrogen atoms on the benzene with four interaction centres on the solvent particle. The results of the simulations are used to calculate the second-rank orientational order, the translational diffusion coefficients and second-rank rotational correlation functions of the benzene molecule in the isotropic, nematic and smectic B phases. The second-rank, orientational order parameters of the benzene [Pbar] 2 b and the solvent particles [Pbar] 2 GB are similar in magnitude, and show the same increase with decreasing temperature in the nematic phase as benzene dissolved in real nematic solvents. On en...

The orientational order and conformational distributions of the two enantiomers in a racemic mixture of a chiral, flexible molecule dissolved in a chiral nematic liquid crystalline solvent

The orientational order and conformational distributions of the two enantiomers of (±)-α-ethylhexanoic acid-d15, a flexible chiral molecule, dissolved in a chiral nematic liquid crystalline solvent made of PBLG in an organic co-solvent are obtained by analysis of NMR data. The anisotropic, NMR parameters are obtained from the separated analysis of the two enantiomers using carbon–deuterium, deuterium–deuterium 2D correlation experiments and proton-coupled carbon-13 1D experiments. The analysis of conformational distributions, and the conformationally dependent orientational order parameters are derived using the additive potential, AP, model.

The Structure and Orientational Ordering of 4−Methoxy−4'−Cyanobiphenyl in the Nematic Mesophase

Abstract The proton N.M.R. spectra of two partially deuteriated samples of the mesogen 4-methoxy-4′-cyanobiphenyl (1-OCB) have been obtained. The spectra were simplified by decoupling the interactions between protons and deuterons and analysed to yield sets of dipolar couplings, [Dtilde]ij . The [Dtilde]ij are used to investigate the angle φ between the two ring normals and the conformation of the methoxy group relative to the attached phenyl ring. If the potential governing rotation, V(φ), is assumed to have a deep minimum such that only structures with the minimum value of φ are appreciably populated, then φmin is determined to be 30·4° ± 0·4°. However, adopting a continuous form for V(φ) changes φmin to 36°. For the methoxy group the data are consistent with the potential governing rotation about the ring-oxygen bond being zero when the carbon atom is in the ring plane and infinite otherwise; in addition, we have investigated the consequences for the structure and ordering of adopting more realistic fo...

Discrimination and analysis of the NMR spectra of enantiomers dissolved in chiral liquid crystal solvents through 2D correlation experiments

Abstract The discrimination and analysis of the NMR spectra of optically active molecules dissolved in chiral liquid crystal solvents through 2D correlation experiments is studied. The technique allows the identification of the line positions of each enantiomer, thus providing a notable simplification of the spectral analysis. The 2D HOHAHA and multiple-quantum experiments are investigated and discussed. The potential of the method is illustrated using a sample of (±) 3,3,3-trichloroepoxypropane dissolved in a thermotropic cholesteric solvent. The case of chiral molecules bearing a fluorine or deuterium nucleus has also been studied. In addition, it is shown that 2D heteronuclear correlation experiments are powerful methods for correlating carbon and proton spectral data of two enantiomers. A specific example is given through (±) 2-bromopropanoic acid dissolved in a lyotropic polypeptide liquid crystal. Spectral parameters of each enantiomer are calculated for the different examples.

Structure of biphenyl in a nematic liquid-crystalline solvent

The proton NMR spectra of biphenyl dissolved in two nematic liquid-crystalline solvents have been analysed to yield 12 independent dipolar coupling constants, Dij. The Dij have been used to obtain the probability distribution, Pnem(ϕ), for the angle ϕ between the ring normals. Two methods of relating Dij to Pnem(ϕ) have been used: the ME method, which invokes the maximum-entropy principle, and the AP method, which is based on the construction of a potential of mean torque from an addition of contributions from the rigid subunits in a molecule. The value of ϕ when Pnem(ϕ) is a maximum is found to be essentially independent of the solvent, and is determined to be 34 ± 1° by the ME method, and 37.2 ± 0.1° by the AP method.

Is styrene planar in liquid phases

The proton NMR spectra of two (13)C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, phi(0), in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of phi(0) is found to be 18.0 degrees +/-0.2 degrees for the two solutions, compared with a value of 27 degrees calculated by the molecular method MP2/6-31G(*).

An investigation of the potential governing rotation about the O—CH2 bond in 4-chloroethoxybenzene by comparing observed and calculated dipolar couplings obtained for a sample dissolved in a nematic solvent

The shape of the potential function V(φ) governing rotation about the O—CH2 bond in 4-chloroethoxybenzene has been investigated by comparing partially averaged dipolar couplings obtained for a sample dissolved in a liquid-crystal solvent with values calculated by the theoretical model of Emsley, Luckhurst and Stockley (Proc. R. Soc. London, Ser. A, 1982, 381, 117).