Order Determinations in Liquid Crystals by Dynamic Director NMR Spectroscopy

Assignment of 13C resonances of a nematic liquid crystal using off-magic angle spinning

A novel method for assigning the resonances in the 13C NMR spectrum of a static liquid crystalline sample in its nematic phase is proposed. The method is based on the fact that the carbon chemical shifts in the isotropic phase and in the oriented phase under static and off-magic angle spinning (OMAS) conditions are uniquely related by the tensorial property of the CSA tensor, requiring just one OMAS spectrum and the assignment in the isotropic phase. A computational procedure is proposed to take into account deviations arising out of non-ideal experimental conditions and the assignments are made by identifying the minimum in the differences in the frequencies between calculated and experimental line positions. Practical implementation of the method has also been demonstrated in the case of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline.

Biaxial nematic phase in a thermotropic liquid-crystalline side-chain polymer

We report on variable-angle deuterium NMR measurements of nematic liquid-crystalline side-chain polymers, where we have found proof of the existence of a biaxial nematic phase in a system with a side-on attachment of the mesogenic group to the polymer backbone. This provides efficient coupling to the polymer backbone and thus stabilizes the as yet elusive biaxial phase. The experimental approach is validated on a uniaxial end-on nematic polymer, and the reliability of the results is investigated in detail using two-dimensional correlation spectra, providing information on director distribution effects.

Recoupling of residual dipolar couplings in single-domain polymer-stabilized liquid crystals undergoing magic-angle spinning

Measurement of dipolar couplings, chemical shift anisotropies, and quadrupole couplings in oriented media such as liquid crystals are of great importance for extraction of structural parameters in biological macromolecules. Here, we introduce a new technique, SAD-REDOR, that consists of recoupling heteronuclear dipolar couplings in molecules dissolved in a single-domain liquid crystal or other oriented medium through the combined use of magic-angle spinning and rotor-synchronized radiofrequency pulses. This application of the REDOR pulse sequence to oriented media offers several advantages such as selectivity over the type of coupling recovered and tunable scaling of the interaction. The effectiveness of the technique is demonstrated both theoretically and experimentally, using the recently developed polyacrylamide-stabilized Pf1 phage medium and 15N-labeled benzamide as the aligned molecule.

13C NMR spectral assignments for nematic liquid crystals by 2D chemical shift gamma-encoding NMR

A novel method to accomplish 13C NMR spectral assignments for nematic liquid crystals is proposed. The two-dimensional (2D) isotropic/anisotropic chemical shift correlation spectrum is observed in which the anisotropic shift parameters are represented as sharp lines by gamma-encoding. The 13C spectral assignments can be made from the 2D spectrum with the aid of the 13C isotropic shift assignments for the same compound in the isotropic liquid state. The experiments were performed on p-methoxybenzilidene-p-n-butylaniline.

Two-dimensional dynamic-director (13)C NMR of liquid crystals

A novel nuclear magnetic resonance (NMR) experiment for facilitating the resolution and assignment of liquid crystalline (13)C NMR spectra is described. The method involves the motor-driven reorientation of the liquid crystalline director, in synchrony with the acquisition of a 2D chemical shift correlation spectrum. By monitoring in this fashion the (13)C NMR evolution of spins in the liquid crystal at two different director orientations with respect to the magnetic field, the method distinguishes anisotropic from isotropic displacements and can be utilized for assigning the resonances and estimating local degrees of order. Of various potential pairs of angles suitable for such a correlation, the (0 degrees, 90 degrees ) choice was found to be most convenient, as it avoids line broadening complications that may otherwise originate from heterogeneities of the oriented phase. The technique thus derived was employed in the analysis of a series of monomeric and polymeric liquid crystal systems.