NMRSpectroscopy in Liquid Crystalline and Ordered Phases

Abstract

Applications of nuclear magnetic resonance (NMR) spectroscopy as an analytical tool in liquid crystals (LCs) are surveyed. Proton, deuteron, carbon‐13, and nitrogen‐14 are commonly used as probes in solid‐state NMR of condensed phases. Their usages in LCs are discussed in some details. Complex mathematical expressions needed to interpret NMR observables (both equilibrium and dynamic properties) are kept to a minimum, and the reader can refer to original articles and books for details. As it is impossible to provide an exhaustive coverage of the literature, selective examples are chosen to highlight areas of recent interests in the study of liquid‐crystal materials, particularly thermotropics, in terms of their physics and chemistry. Orientational/positional order parameters are readily obtained in ordered liquid‐crystal phases by means of NMR. They can serve as characteristic signatures of the studied mesophases. Liquid‐crystal ordering depends on intermolecular potentials among neighboring molecules. Solutes dissolved in LC solvents are good candidates to reveal different ordering mechanisms. Nonzero spin interactions are used to determine molecular structure of large and small molecules, and/or their conformation statistics. Nuclear spin relaxation times are readily measured by means of different NMR pulse techniques and can be powerful for revealing dynamic properties of mesogens with increasing structural complexity.

NMR of bent‐core nematogens: a mini‐review

“…3, with the new carbon peak assignments were analyzed to find S and D for the five-ring core. [33] It was found that to properly obtain molecular order from 13 C chemical shifts in a mesomorphic phase, a way to theoretically predict the relative positions of carbon peaks in the 13 C spectrum would be very useful.…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…(95) and the CSA tensors given in Ref. [228] to fit 21 smoothed aromatic < δ i > k data, the resulting fits were found to be better than previously reported fits most likely due to slightly improved peak assignments [292]. The derived order parameters S and D are plotted versus the temperature in Fig.…”

“…9 of Ref. [292]. Their derived S for the dominant Γ 1 conformer had a linear T dependence, likely owing to the aforementioned assumption, in contrast to the expected temperature trend predicted by Maier-Saupe theory [42].…”

“…The mean-field approach and the above DFT calculations have therefore provided another useful check of the 13 C peak assignments in the N u phase of A131. Instead of using local order parameters to address the molecular shape and order, the order parameters S and D for the 5-ring bent-core of A131 were derived based on an "average" molecular shape at each temperature [292]. The average geometry specifying each ring in its local (x 0 ,y 0 , z 0 ) frame w.r.t.…”

Measurement of Large Dipolar Couplings of a Liquid Crystal with Terminal Phenyl Rings and Estimation of the Order Parameters

“…NMR spectroscopy has been highly useful in the study of such systems because it has been shown to provide information with atomistic resolution. [8,9] Although nuclei like 1 H, 2 H, and 19 Fh ave been utilized for investigating such systems, [10][11][12][13] in recent years the naturalabundance 13 CNMR spectroscopict echnique is being increasingly used due to the fact that this approach does not require special sample preparation, such as isotope labeling, and provides high-resolution spectra in the oriented phase. [14][15][16][17][18][19][20][21] It is possible to obtain two anisotropic parameters from 13 CNMR spectroscopy.T he first is the alignment-induced chemical shift (AIS), which is the difference in the chemical shift of the carbon in the isotropic phase to that in the liquid crystalline phase, and arises from the alignment of the molecule in the magnetic field.…”