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

Abstract

NMR spectroscopy is a powerful means of studying liquid-crystalline systems at atomic resolutions. Of the many parameters that can provide information on the dynamics and order of the systems, (1) H-(13) C dipolar couplings are an important means of obtaining such information. Depending on the details of the molecular structure and the magnitude of the order parameters, the dipolar couplings can vary over a wide range of values. Thus the method employed to estimate the dipolar couplings should be capable of estimating both large and small dipolar couplings at the same time. For this purpose, we consider here a two-dimensional NMR experiment that works similar to the insensitive nuclei enhanced by polarization transfer (INEPT) experiment in solution. With the incorporation of a modification proposed earlier for experiments with low radio frequency power, the scheme is observed to enable a wide range of dipolar couplings to be estimated at the same time. We utilized this approach to obtain dipolar couplings in a liquid crystal with phenyl rings attached to either end of the molecule, and estimated its local order parameters.

Morphology, Mesophase, and Molecular Order of 3-Hexyl Thiophene-Based π-Conjugated Mesogens

“…Among the carbons resolved from the 2D spectrum, four carbons (A–D) showed an increase in chemical shifts, whereas the rest (10 carbons) showed a decrease in chemical shifts against solution values. Generally, in liquid crystalline molecules, the aliphatic carbons of the terminal chain show lower chemical shifts than their solution spectra, whereas the core unit carbons show an increase in chemical shifts. − This variance is due to the change in the orientational constraints of different moieties, i.e., core versus terminal chains of the molecule owing to the alignment in the magnetic field. An increase in chemical shift values of aliphatic carbons is noticed only when they are located away from the local axis of the phenyl rings. , In other words, the chains which are placed at the lateral position exhibit increase in chemical shifts, while those located at terminal position follow the reverse trend.…”