Direct method to grasp molecular topology of mesogens through C-H dipolar couplings.

A prime factor in determining liquid crystalline phase formation is the overall molecular shape since molecules undergo rotational motion about the long axis. Molecular topology deals with the connectivity of atomic centers in a given molecular architecture, ultimately giving rise to the gross molecular shape. C NMR has emerged as the most important technique in establishing the molecular topology of mesogens in the liquid crystalline phase. In this work, we demonstrate the utility of C-H dipolar couplings determined from 2D separated local field NMR for finding the topology of three different mesogens in the liquid crystalline phase. The core unit of the investigated mesogens fundamentally differs, which may be categorized as rod-like, laterally substituted, and bent-core shapes. 1D and 2D C NMR measurements in the liquid crystalline phase revealed fascinating information. The C-H dipolar couplings extracted from 2D NMR are found to be sensitive to topologically variant core units. This permitted us to establish the molecular topology just by looking at the C-H dipolar couplings of the protonated carbons of the constituent phenyl rings. By considering the dipolar couplings of rod-like mesogens as a reference, the large variation in the magnitude of C-H dipolar couplings of the laterally substituted and bent-core mesogens is attributed to changes in the topology of their core units. The order parameters estimated from C-H dipolar couplings enabled visualization of the ordering array of phenyl rings of the mesogens. Interestingly large C-H dipolar couplings are observed for mesogens in which (a) laterally located phenyl ring and (b) central phenyl ring of bent-core mesogens exhibited different trends as revealed by the orientational order parameters.