Orientational Ordering of a Bent-Core Mesogen by Two-Dimensional ¹³C NMR Spectroscopy

Abstract

Various two-dimensional (2D) NMR techniques are reported on a bent-core mesogen 4,6-dichloro-1,3-phenylenebis[4'-(9-decenyloxy)-1,1'-biphenyl] carboxylate in its nematic and solid phases in order to unambiguously assign its carbon-13 NMR spectrum. The (13)C chemical shifts from the molecular core were studied as a function of temperature to extract its molecular geometry and orientational order tensor. To this end, the chemical shift anisotropy tensors of some carbon sites were measured in the solid state of this mesogen using a recent method called the separation of undistorted powder patterns by effortless recoupling (SUPER). The average bending angle subtended by the two arms of the bent-core structure is determined to be 148.7 degrees. The C-H dipolar couplings obtained from the separated local field (SLF) experiment for the aromatic rings are used to find the local order parameter tensors.

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete2H and13C NMR Study

“…To apply Equation (7) to the various carbon sites of the aromatic core, we have used the CSA tensors of similar compounds given in the literature; [34,35,37,38] these are tabulated in Table 1. Only the 13 C data of the unwound TGBA* phase is analyzed.…”

Intercalated Smectic A Phases in Banana‐Shaped Liquid Crystals with Carbonate End Groups

“…Thus, the differences of carbon chemical shifts in the aligned phase from their isotropic shifts can be used to determine the molecular structure and order parameters. [20] The principal values of the chemical shift anisotropy (CSA) tensor for each carbon site are measured using a two-dimensional technique known as SUPER. [21] The molecular structure of I with carbon site labels is shown in Figure 4 a.…”

“…The powder patterns of carbons z 1 and z 2 are significantly different from those of the other carbon sites, and can be used for their assignments, as demonstrated in ref. [20].…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…To do this, separated local field (SLF) experiment was performed on aligned LCs [229]. Later proton-detected 13 C local field (PDLF) experiment designed for solids is often preferred [230][231][232], as it exploits the high 1 H sensitivity to acquire the NMR signal, while C-H dipolar interaction is detected as a modulation of the proton signal in the second dimension of a 2D spectrum [233].…”

“…As shown in the SLF work of Xu et al [229], the Indian group has used C-H dipolar couplings in a rigid fragment to derive the local order parameters. These local order parameters would be identical to those from analyzing the corresponding 13 C anisotropic chemical shifts data [65].…”