Structure of methylenecyclopropane in a liquid crystal by 1H and 13C NMR

The ¹H and ¹³C NMR spectra of methylenecyclopropane, both in the isotropic phase and in the Merck IV nematic phase, were analysed. The relative signs of the ¹³C H scalar coupling constants were determined. The complete geometry deduced from this study was compared with that from a microwave study, and the discrepancies are discussed.     

Orientational ordering of a bent-core mesogen by two-dimensional 13C NMR spectroscopy

Various two-dimensional (2D) NMR techniques are reported on a bent-core mesogen 4,6-dichloro-1,3-phenylenebis[4'-(9-decenyloxy)-1,1'-biphenyl] carboxylate in its nematic and solid phases in order to unambiguously assign its carbon-13 NMR spectrum. The (13)C chemical shifts from the molecular core were studied as a function of temperature to extract its molecular geometry and orientational order tensor. To this end, the chemical shift anisotropy tensors of some carbon sites were measured in the solid state of this mesogen using a recent method called the separation of undistorted powder patterns by effortless recoupling (SUPER). The average bending angle subtended by the two arms of the bent-core structure is determined to be 148.7 degrees. The C-H dipolar couplings obtained from the separated local field (SLF) experiment for the aromatic rings are used to find the local order parameter tensors.     

A combined theoretical and experimental approach to determining order parameters of solutes in liquid crystals from 13C NMR data

The ordering properties of an anisotropic liquid crystal can be studied by recording 13C NMR spectra at different temperatures for a number of rigid solutes. The traditional difficulty in analyzing 13C data comes from the scarcity of experimental information about the carbon shielding tensors and from their limited transferability among different solutes. We show that these obstacles can be overcome by computing high-level ab initio shielding tensors, also including the solvent effects by the polarizable continuum model. The reliability of this combined approach is carefully verified, and the order parameters of several solutes are obtained by reanalyzing previously published spectra. The quality of the results is shown to be comparable to deuterium NMR without the need of isotopic substitution.     

Order parameters of alpha,omega-diphenylpolyenes in a nematic liquid crystal from an integrated computational and 13C NMR spectroscopic approach

The orientational order parameters and conformational behavior of five relatively large rodlike molecules, biphenyl, trans-stilbene, 1,3-diphenyl-butadiene, 1,3,5-diphenyl-hexatriene, and 1,3,5,7-diphenyl-octatetraene, dissolved in the thermotropic liquid crystal ZLI-1167, have been studied using an integrated approach combining (13)C NMR measurements and quantum mechanical computations of carbon chemical shift tensors. Besides biphenyl, the phenyl moiety of all structures has been found to have a high rotational mobility in the temperature range of the present experiments. The rank-two order parameter in the nematic phase is found to increase steadily from the shortest to the longest term of the series at any temperature within the nematic range. The molecular biaxiality order parameter is found to be small and essentially constant with temperature, giving further support to the common assumption of effective uniaxiality for these probes.     

Orientational prewetting of planar solid substrates by a model liquid crystal

We present grand canonical ensemble Monte Carlo simulations of prewetting transitions in a model liquid crystal at structureless solid substrates. Molecules of the liquid crystal interact via anisometric Lennard-Jones potentials and can be anchored planar or homeotropically at the substrates. Fluid-substrate attraction is modeled by a Yukawa potential of variable range. By monitoring the grand-potential density and the nematic order parameter as functions of the chemical potential μ, several discontinuous prewetting, wetting, and isotropic-nematic phase transitions are observed. These transitions depend on both the range of the fluid-substrate attraction and the specific anchoring at the substrate. Our results show that at substrates characterized by degenerate anchoring prewetting occurs at lower μ compared with cases in which the anchoring is monostable. This indicates that prewetting transitions are driven by orientational entropy because degenerate anchoring allows for more orientationally distinct configurations of molecules compared with monostable anchoring. In addition, by analyzing local density and various local order parameters, a detailed picture of the structure of various phases emerges from our simulations.     

Orientational order of rod- and disk-like solutes in the nematic liquid crystal 5CB

Proton and deuteron nuclear magnetic resonance has been used to investigate orientational order in binary liquid crystalline mixtures. Results obtained from small rigid molecules (2-butyne, 2,4-hexadiyne, propyne, 1,2-propadiene, acetylene, 1,3,5-trichlorobenzene and 1,3,5-tribromobenzene) dissolved in the nematic phase of selectively deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB-d^αβ₄), indicate that the solute and liquid-crystal molecules have different order parameters. Such observations are consistent with the predictions of mean-field theory, which, under certain assumptions, indicates that for a given mixture a plot of one order parameter, S⁽¹⁾, against that of the other parameter, S⁽²⁾, should yield a universal curve independent of temperature and composition. Fitting the experimental NMR results to these theoretical curves provides a virtual nematic-isotropic transition temperature, T^*_NI, for the pure solute.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4′,4″-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4',4'-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by ¹³C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A-smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X-ray and elastic characterizations of structurally similar materials.     

Orientational fluctuations in a nematic liquid crystal as seen by neutron scattering

Abstract

Real time neutron scattering is used in the study of the slow orientational fluctuations of the director in a nematic sample. A statistical analysis of the observed time series gives the Hurst exponent H and β exponent of the frequency power spectrum that satisfy the scaling relationship β = 2H + 1. In the nematic phase, but not in the solid and in the isotropic liquid phases, the exponent values are those expected for a self-organized critical state. When a magnetic field, of the order of the Freedericksz field is applied, the nematic sample is observed to display persistent oscillations of the director. We confront this observation with theoretical predictions.     

Enantiomeric differentiation of acyclic terpenes by 13C NMR spectroscopy using a chiral lanthanide shift reagent

The 13C NMR behaviour of ten acyclic terpene alcohols was examined in the presence of a chiral lanthanide shift reagent (CLSR). For each alcohol, we measured the lanthanide-induced shift (LIS) on the signals of the carbons and the splitting of some signals, which allowed the enantiomeric differentiation. As expected, the LIS decreased with the number of bonds between the binding function and the considered carbon. The enantiomeric splitting is observed for several signals in the spectrum of each compound. The influence of the hindrance of the binding function (primary, secondary or tertiary alcohol) and that of the stereochemistry of the double bonds is discussed.     

Orientational ordering of a nematic liquid crystal and its mixture with its chain-perfluorinated analogue

The orientation of different segments of 4'-cyanophenyl 4-heptylbenzoate (7CPB) has been investigated using ¹³C NMR. The method of proton-encoded local field (PELF) spectroscopy in combination with off-magic-angle spinning (OMAS) of the sample was used. High resolution 2D spectra were obtained, from which the order parameters were calculated. Linear relations between the obtained order parameters and anisotropic chemical shifts determined by 1D ¹³C NMR were established and semi-empirical parameters were achieved. A 1:2 mixture of 7CPB and its chain-perfluorinated analogue (7PFCPB) showed interesting phase behaviour with the change of temperature. It was studied by the use of ¹³C NMR and polarizing optical microscopy. The order parameters of 7CPB in the smectic A phase of the mixture were calculated using the semi-empirical parameters obtained from the 2D NMR method.     

A chiral luminescent liquid crystal with a tolane unit

We have synthesised a chiral liquid crystal with a tolane unit that exhibits intense fluorescence in both solution and the solid states. The liquid crystal can form the enantiotropic twist-grain boundary A (TGB_A*) and Blue II phases, and the helical pitch of the TGB_A* phase decreases with an increasing temperature.     

DFT/NMR integrated approach: a valid support to the total synthesis of chiral molecules

The analysis of the configuration of kedarcidin chromophore and palau'amine through quantum chemical calculation of Js and chemical shifts suggests a fast and convenient quantum chemical approach that can be applied prior to proceeding to the total synthesis of complex natural compounds in order to avoid loss of time and resources employed in the total synthesis of wrong diastereoisomers.     

Orientational ordering of a nematic liquid crystal and its mixture with its chain-perfluorinated analogue

The orientation of different segments of 4'-cyanophenyl 4-heptylbenzoate (7CPB) has been investigated using ¹³C NMR. The method of proton-encoded local field (PELF) spectroscopy in combination with off-magic-angle spinning (OMAS) of the sample was used. High resolution 2D spectra were obtained, from which the order parameters were calculated. Linear relations between the obtained order parameters and anisotropic chemical shifts determined by 1D ¹³C NMR were established and semi-empirical parameters were achieved. A 1:2 mixture of 7CPB and its chain-perfluorinated analogue (7PFCPB) showed interesting phase behaviour with the change of temperature. It was studied by the use of ¹³C NMR and polarizing optical microscopy. The order parameters of 7CPB in the smectic A phase of the mixture were calculated using the semi-empirical parameters obtained from the 2D NMR method.     

Thickness dependence of chiral smectic C liquid crystal phase transitions

In this paper we consider the influence of solid boundaries on the transition temperatures of a chiral smectic C liquid crystal. Particular attention has been paid to the S*_C-S_A transition. A simple model to explain the thickness dependence of the S*_C-S_A transition is proposed. An experimental method to determine some elastic constants and the anchoring energy of ferroelectric liquid crystal molecules is demonstrated.     

13C CP MAS NMR and DFT Studies of 6-Chromanyl Ethereal Derivatives

Vitamin E consists of a group of compounds including α- β- γ- and δ-tocopherols and α- β- γ- and δ-tocotrienols, containing the chroman-6-ol system. The recognition of the structural and dynamic properties of this system, present in all vitamers, seems to be important for the full explanation of the mechanism of the biological activity of vitamin E. This paper presents results of the structural analysis of the chosen 6-chromanyl ethereal derivatives using experimental (¹³ C NMR-in solution and solid state, as well as variable temperature experiments; single crystal X-ray diffraction) and theoretical (DFT) methods. For one of the studied compounds, 2,2,5,7,8-pentamethyl-6-((tetrahydro-2H-pyran-2-yl)oxy) chroman, the splitting of some signals was observed in the ¹³C dynamic NMR spectra. This observation was explained by the application of a conformational analysis and subsequent DFT optimization, followed by the calculation of NMR properties.