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The (super 2)H-(super 13)C heteronuclear 2D method gives a useful way to overcome the lack of (super 2)H chemical shift dispersion in titled Q-COSY 2D spectra. Also, a simple strategy is presented to attribute the quadrupolar doublets for two enantiomers and therefore obtain two sets of coherent data, one for each enantiomer, that can be used for further calculations.

In this article, we explore those factors that influence the orientational order in both nematic and twist-bend nematic phases. The systems used for our NMR experiments are binary mixtures of a dimer and a related monomer thus enabling its behaviour to be controlled continuously, through its composition rather than chemical structure. Earlier studies have revealed that as the temperature is lowered in the nematic phase, the orientational order frequently passes through a maximum prior to the formation of the NTB phase. A confirmative indication of this unusual behaviour is that the Haller function cannot fit this dependence over the entire nematic range; such behaviour is intriguing. In the NTB phase, the measured order parameter is often insensitive to decreasing temperature for systems with strong NTB-N transitions often associated with a narrow nematic range. For those mesogens with longer nematic ranges and hence weaker NTB-N transitions, the order parameter decreases more markedly with temperature. As expected for the weak transitions, the behaviour of the order parameter is observed to be essentially continuous across the transition. Associated with the orientational order in the NTB phase is the conical angle and an off-diagonal element of the Saupe ordering matrix. [ABSTRACT FROM AUTHOR]

The appearance of some special features in ²sH-NMR spectra, which could not be explained by the conventional nematic order, was crucial for the discovery of the twist-bend nematic (NTB) phase. Such features reflect the order and symmetry of the local environment of a probe and in principle their analysis can provide quantitative insights into the structural parameters of the phase. We have developed a methodology for the analysis of experimental data, different from the approaches that are normally used for two main reasons: (i) the structural features of the NTB phase, specifically its local polarity, biaxiality and chirality, are consistently taken into account; (ii) the molecular features are introduced in a detailed way, through molecular modelling. We have applied this methodology to analyse ²H-NMR quadrupolar splittings for two representative systems, CB7CB and CB6OCB. The integration of theory, computational modelling and experiment allows us to extract quantitative information from the experimental data and to predict various properties, even not directly probed by measurements. We discuss how the molecular differences between CB7CB and CB6OCB translate into a different phase behaviour and we address issues concerning the distinguishing features of the NTB phase, such as the definition of a local tilted director. [ABSTRACT FROM AUTHOR]

In this article, we explore those factors that influence the orientational order in both nematic and twist-bend nematic phases. The systems used for our NMR experiments are binary mixtures of a dimer and a related monomer thus enabling its behaviour to be controlled continuously, through its composition rather than chemical structure. Earlier studies have revealed that as the temperature is lowered in the nematic phase, the orientational order frequently passes through a maximum prior to the formation of the NTB phase. A confirmative indication of this unusual behaviour is that the Haller function cannot fit this dependence over the entire nematic range; such behaviour is intriguing. In the NTB phase, the measured order parameter is often insensitive to decreasing temperature for systems with strong NTB-N transitions often associated with a narrow nematic range. For those mesogens with longer nematic ranges and hence weaker NTB-N transitions, the order parameter decreases more markedly with temperature. As expected for the weak transitions, the behaviour of the order parameter is observed to be essentially continuous across the transition. Associated with the orientational order in the NTB phase is the conical angle and an off-diagonal element of the Saupe ordering matrix. [ABSTRACT FROM AUTHOR]

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified as a globally uniaxial twist-bend nematic phase, NTB. Within the temperature range 376–388 K, a classic uniaxial nematic phase, N, appears. The NTBphase has a helical structure and the liquid crystal director,n, is tilted with respect to the helix axis; these are formed into a conglomerate of degenerate domains having opposite handedness. In a magnetic field, the helix axis orients along the field. In the present studies, the properties of CB7CB are investigated using129Xe NMR spectroscopy of dissolved xenon and2H NMR of CB7CB-d4and the probe 4-octyl-4ʹ-cyanobiphenyl-d2also dissolved in CB7CB. In a uniaxial liquid crystalline environment, the129Xe shielding tensor is cylindrically symmetric due to the deformation of the electron cloud by anisotropic forces associated with the director. The anisotropic part of the shielding tensor depends upon the orientational order parameter of the liquid crystal with respect to the applied magnetic field and the conical (aka tilt) angle of the director. The temperature dependence of the orientational order parameter and of the conical angle has been determined independently from129Xe and previous2H NMR experiments. In the NTBphase, the averaged Saupe ordering matrix contains three off-diagonal elements. The temperature dependence of one of these, resulting from the phase chirality, has been determined from the2H quadrupolar splittings. [ABSTRACT FROM PUBLISHER]

We have investigated the phase behaviorof four glycosides (βC8OGlc, βC8SGlc, βC10OGlc,βC8OGal) in water and D2O by optical polarizingmicroscopy and deuterium NMR. Previously published phase diagramswere evaluated by deuterium NMR, via monitoring D2O spectra,and confirmed the presence of the hexagonal, bicontinuous cubic, andlamellar phases in these glycosides. We have also shown the presenceof the gel phase in (βC10OGlc) and observed the extensivesupercooling of the lamellar phase to temperatures well below theKraft line. While the main features of the phase diagrams were confirmed,some phase boundaries were found to be slightly different. Magneticallyaligned spectra were also observed for relatively dilute samples forthe hexagonal phase (βC8OGlc and βC8OGal) and the lamellar phase (βC8SGlc and βC10OGlc). The average number of bound water molecules per headgroupin the lamellar phase for the glycosides was determined by the systematicmeasurement of the quadrupolar splitting of D2O over awide range of values of the (glycoside/water) molar ratio. The numberof water molecules bound to the headgroup was found on average tobe about 1.6–1.7 water molecules with no significant differencesin this value for the different glycosides (and over the temperaturerange investigated), indicating that the bound water content is predominatelyinfluenced by the number of hydroxyl groups of the headgroup only.However, this bound water content of only 1.6–1.7 water moleculesper sugar headgroup is surprisingly low, suggesting strong intermolecularinteractions of the OH groups of headgroup sugars. The results arein line with computational results reported earlier for the octyl-β-glucosideand β-galactoside, which show the presence of strong intralayerhydrogen bonding. [ABSTRACT FROM AUTHOR]

Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phase of perdeuteriated 4-octyl-4′-cyanobiphenyl-d 25 (8CB-d 25 ) When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-on and the turn-off alignment processes as a function of time. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. For all of the experiments at different temperatures in the nematic phase of 8CB-d 25 we find that the field-induced relaxation times are independent of the group used to determine the director orientation during the alignment process. [ABSTRACT FROM AUTHOR]

The response times of liquid crystal display devices are determined by a range of factors but the most important of these is usually the rotational viscosity coefficient, γ 1 . In order to understand the relationship between molecular structure and viscosity it is of considerable interest to measure this viscosity coefficient for a variety of nematogens. Here we report the determination of γ 1 for 4-octyl-4′-cyanobiphenyl-d 2 at two temperatures using deuterium NMR spectroscopy. In these experiments the time taken for the alignment of the director can be changed by a field, either magnetic or electric. To do this the liquid crystal film was enclosed in a cell which allows the application of an electric field within the NMR spectrometer. The rate of director relaxation was followed by recording the deuterium NMR spectrum as a function of time during the process of turning the electric field on or off. We have carried out the experiments for a geometry in which the director orientation with respect to the magnetic field of the spectrometer does not exceed 45°. The alignment of the director throughout the relaxation process was observed to be uniform. The director relaxation was found to follow closely the predictions of the torque-balance equation given by the Leslie-Eriksen theory. The relaxation times for the turn-on and turn-off processes were determined from this equation and found to be of the order of 1-2 ms. A knowledge of the anisotropic electric and magnetic susceptibilities then allows the determination of the rotational viscosity coefficient. [ABSTRACT FROM AUTHOR]

The nature of the nematic-nematic phase transition in the liquid crystal dimer 1,9-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and ²H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (NTB), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the NTB-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and ²H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the NTB phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. ²H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director. [ABSTRACT FROM AUTHOR]

The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has been investigated. The intermediate states during the relaxation process were found, with the aid of time-resolved deuterium NMR spectroscopy, to be markedly nonuniform. The macroscopic order was perturbed, although the initial and final states of the director appear to be essentially uniform. However, the initial state does have a profound influence on the uniformity of the director in the intermediate states. We have developed a fundamental model based on the effect of spontaneous director fluctuations to explain these unusual NMR observations. [ABSTRACT FROM AUTHOR]