Proton-decoupled deuterium NMR study of an asymmetric liquid crystal dimer having two nematic phases.

Proton-decoupled deuterium NMR spectra were obtained for an asymmetric liquid crystal dimer 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) containing a single -CD_{2}- group. The sample has two nematic liquid crystal phases: a twist-bend nematic, N_{TB}, at the lowest temperature followed by a uniaxial nematic, N_{U}, on increasing the temperature. Proton decoupling reduces the linewidths of the peaks in the deuterium spectrum from kHz to ∼100Hz, enabling quadrupolar splittings, Δν, to be obtained with enhanced precision as well as the dipolar coupling between deuterium nuclei within the CD_{2} group, hence enhancing the information content.

Phase transitions in a high magnetic field of an odd, symmetric liquid crystal dimer having two nematic phases, N_{U} and N_{TB}, studied by NMR spectroscopy.

Both ^{1}H and ^{13}C NMR spectra have been obtained in a static magnetic field of 23.5 T on a bent-shaped dimer molecule, 1^{''},7^{''}-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB), which shows the sequence of liquid crystal phases twist-bend nematic, N_{TB}, and uniaxial nematic, N_{U}, before entering the isotropic phase. The ^{1}H spectra are used to locate the temperature at which the sample melts to form a twist-bend nematic, T_{CrN_{TB}}, and then T_{N_{U}I} when the isotropic phase is entered, both in a magnetic field of 23.5 T, and to compare these with those measured at the Earth's field. The differences between these transition temperatures are found to be zero within the error in their measurement, in stark contrast to previous measurements by Salili et al. [Phys. Rev. Lett. 116, 217801 (2016)10.1103/PhysRevLett.116.217801]. In the isotropic phase in the presence of the field the sample exists in a paranematic phase in which the molecules of CB9CB are partially ordered. The ^{1}H and ^{13}C NMR spectra in the paranematic phase are used to measure the critical temperature T* below which this phase is unstable. The spectra are also used to study the structure, molecular orientational order, and distribution of molecular conformations in the paranematic phase.

Publisher's Note:

This corrects the article DOI: 10.1103/PhysRevE.96.062702.

Similarities and differences between molecular order in the nematic and twist-bend nematic phases of a symmetric liquid crystal dimer.

The order parameter, Szz, where z is the para axis of the difluoroterphenyl groups in DTC5C9, has been obtained from chemical shift anisotropies measured by (13)C - {(1)H} NMR experiments at temperatures throughout the nematic, NU, and twist-bend nematic, NTB, phases shown by this compound. The order parameter temperature profiles are unusual in having a maximum value in the NU phase and then decreasing until the NTB phase is reached. There is a small discontinuity (∼2%) in Szz at TNNTB and then a gradual decrease until a new phase appears. This behaviour is interpreted as revealing a temperature-dependent tilting of local directors in both phases away from the applied magnetic field direction. In the enantiomorphic twist-bend phase this tilt is consistent with the structure of the phase as a helical arrangement of local directors, whilst in the high-temperature non-chiral nematic the tilt must involve a non-chiral arrangement. It is proposed that in both phases the tilting of directors has a common origin in the bent shape of the molecules.

Benzene at 1GHz. Magnetic field-induced fine structure.

The deuterium NMR spectrum of benzene-d6 in a high field spectrometer (1GHz protons) exhibits a magnetic field-induced deuterium quadrupolar splitting Δν. The magnitude of Δν observed for the central resonance is smaller than that observed for the (13)C satellite doublets Δν'. This difference, Δ(Δν)=Δν'-Δν, is due to unresolved fine structure contributions to the respective resonances. We determine the origins of and simulate this difference, and report pulse sequences that exploit the connectivity of the peaks in the (13)C and (2)H spectra to determine the relative signs of the indirect coupling, JCD, and Δν. The positive sign found for Δν is consonant with the magnetic field biasing of an isolated benzene molecule-the magnetic energy of the aromatic ring is lowest for configurations where the C6 axis is normal to the field. In the neat liquid the magnitude of Δν is decreased by the pair correlations in this prototypical molecular liquid.

The indirect through-space F-F coupling in peri-difluoronaphthalene: is it anisotropic?

The (1)H, (19)F and (13)C spectra have been obtained of a sample of peri-difluoronaphthalene dissolved in the nematic liquid crystalline solvent ZLI 1695. The (13)C satellite spectra from the six, single-(13)C isotopomers at natural abundance in both the (1)H and (19)F spectra were identified and analysed to yield a set of residual total, anisotropic spin-spin couplings, T(ij). This was achieved by first obtaining residual (13)C-(19)F and (13)C-(1)H couplings from a proton-encoded, (13)C detected, local field 2D spectrum. The 45 values of T(HH), T(HF) and T(CH) were used to obtain the structure of the molecule, and then to estimate whether there is a significant contribution from the component along the magnetic field, J, of the anisotropic, electron-mediated, spin-spin coupling tensors for (13)C-(19)F and (19)F-(19)F pairs. It is found that there is strong evidence for a significant contribution of J to T(FF) but not for the (13)C-(19)F pairs.

Tetraethyl stannane: structure, conformations, and orientational order when dissolved in a nematic liquid crystalline solvent.

Quantum chemical calculations are used to investigate the structure and the distribution of conformers of tetraethyl stannane, and these are compared with those of tetraethyl methane. The calculations predict that the most abundant conformers in tetraethyl stannane are those with symmetry C1, in contrast to tetraethyl methane, in which the D2d and S4 conformers are dominant. The structural and conformational information on tetraethyl stannane are then used, together with the additive potential model, to explain the finite quadrupolar splittings observed in the deuterium NMR spectrum of tetraethyl stannane dissolved in a nematic liquid crystalline solvent. This analysis emphasizes that to understand the orientational order of a flexible molecule in a liquid crystalline phase it is essential to consider the symmetry of individual conformers rather than that of an average structure.

An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents.

NMR spectra of 1,2-dibromo-1,1-difluoroethane and 1-bromo-2-iodo-tetrafluoroethane dissolved in nematic liquid crystalline solvents have been analysed to yield the magnitudes and signs of the scalar couplings, J(ij), and total anisotropic couplings, T(ij), between all the (1)H, (19)F, and (13)C nuclei, except for those between two (13)C nuclei. The values obtained for T(ij) in principle contain a contribution from J(ij)(aniso), the component along the static applied magnetic field of the anisotropic part of the electron-mediated spin-spin coupling. Neglecting this contribution allows partially averaged dipolar couplings, D(ij), to be extracted from the T(ij), and these were used to determine the structure, orientational order, and the conformational distribution generated by rotation about the C-C bond. The values obtained are compared with the results of calculations by ab initio and density functional methods. The differences found are no greater than those obtained for similar compounds which do not contain fluorine, so that there is no definitive evidence for significant contributions from J(CF)(aniso) or J(FF)(aniso) in the two compounds studied.

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents.

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) D(ij) can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche+/-conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and (13)C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method.

Field-induced alignment of a smectic-A phase: a time-resolved x-ray diffraction investigation.

The field-induced alignment of a smectic-A phase is, in principle, a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved nuclear magnetic resonance spectroscopy has revealed this complexity in the case of the director alignment, but provides no direct information on the motion of the layers. Here we describe a time-resolved x-ray diffraction experiment using synchrotron radiation to solve the challenging problem of capturing the diffraction pattern on a time scale which is fast in comparison with that for the alignment of the smectic layers. We have investigated the alignment of the smectic-A phase of 4-octyl-4(')-cyanobiphenyl by a magnetic field. The experiment consists of creating a monodomain sample of the smectic-A phase by slow cooling from the nematic phase in a magnetic field with a flux density of 7 T. The sample is then turned quickly through an angle phi(0) about an axis parallel to the x-ray beam direction but orthogonal to the field. A sequence of two-dimensional small angle x-ray diffraction patterns are then collected at short time intervals. Experiments were carried out for different values of phi(0), and at different temperatures. The results show that the alignment behavior changes fundamentally when phi(0) exceeds 45 degrees, and that there is a sharp change in the alignment process when the temperature is less than 3 degrees C below the smectic-A-nematic transition. The results of the x-ray experiments are in broad agreement with the NMR results, but reveal major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process.

Selective detection of the proton NMR spectra of molecules containing rare spins at natural abundance in liquid crystalline samples.

It is shown that the proton NMR spectra of molecules containing rare spins at natural abundance dissolved in a liquid crystalline solvent can be obtained free from the strong lines from the spectrum of the abundant isotopomer by the 2D HSQC NMR experiment. The technique can also give the individual chemical shifts of the rare spins, and, for a molecule containing another abundant nucleus, such as fluorine, the rare spin--(19)F total anisotropic couplings are also obtained. The usefulness of the technique is demonstrated for molecules containing (13)C as the rare spins.

Magnetic field induced alignment of the directors of a smectic-A liquid crystal.

Deuterium NMR is used to monitor the magnetic field induced alignment of the directors in the smectic A phase of 4-octyl-4(')-cyanobiphenyl. The experiments consist of first preparing the sample as a monodomain by cooling slowly from the nematic phase in the magnetic field of the NMR spectrometer. The tube containing the sample is then rotated quickly through an angle theta(+), and the subsequent field induced alignment process followed by recording deuterium spectra over a period of time. The results obtained show that the magnetic field induced alignment of the directors in a smectic-A sample is qualitatively different from that of nematic samples. Results are reported for the pure mesogen containing deuteriums at the alpha chain position, and these are compared with data obtained on the alignment process by monitoring the deuterium spectrum of a small amount of an added solute, p-xylene-d(10).

Symmetry and phase-selected NMR spectra of liquid crystalline samples.

It is demonstrated that the NMR spectra of liquid crystalline samples can be simplified by using multiple quantum filtering. In a system of N spin-12 nuclei, the N or (N-1)-multiple quantum filtered spectra (NQF or (N-1)QF) contain lines which originate only from transitions among the eigenstates belonging to the highest symmetry class of the spin permutation group. In addition the NQF spectra are divided further into two sets of lines which differ in phase by 180 degrees. A method for simulating and analysing multiple quantum filtered spectra is described, with examples from molecules with up to eight interacting spins.