¹³C NMR investigations and the molecular order of 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT)

Abstract

The static 1D (13)C and 2D Proton Encoded Local Field (PELF) NMR experiments are carried out in the nematic phase of a less viscous liquid crystal 4-(trans-4'-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) with a view to find orientational order. The PELF spectra provide better resolution which facilitates the assignment of cyclohexyl and phenyl ring carbons relatively easy. For the cyclohexyl unit, four pairs of dipolar splitting are clearly noticed in contrast to earlier reports on structurally similar mesogens where only two pairs of doublets are seen. The linear relationship between anisotropic chemical shifts and orientational order is established and semi-empirical parameters are obtained to aid the study of the order behaviour of 6CHBT over the entire nematic range. The data further fitted to the Haller equation and a reasonably good agreement is observed. The temperature dependence trends of orientational order parameters extracted for various carbons using (13)C-(1)H dipolar couplings with those of (13)C chemical shifts are compared. A gradual decrease in the order parameter is noticed for different molecular segments while traversing from the core to the aliphatic chain via the cyclohexyl ring. The notable decreasing trends of order parameters along the chain are observed similar to those of the corresponding phenyl cyclohexanes reported earlier.

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation

“…These investigations unveiled that short-range nematic order fluctuations and order director fluctuations effectively influence the intramolecular dipolar interactions among nuclear spins ( 1 H– 1 H) in the isotropic and nematic phases, respectively . Furthermore, variable-temperature 1 H FFC NMR relaxometry analysis points out the correlation between molecular reorientations around the short axes and DF modes in the nematic phase of 6CHBT. , These studies enabled the extraction of the correlation times and amplitudes of molecular reorientations due to the short-range nematic order, director fluctuations, and the short axes, occurring on different time scales. , The static 1D 13 C and 2D proton-encoded local field (PELF) NMR experiments conducted in the nematic phase of 6CHBT monitored the trends of orientational order parameter vs temperature for various carbons from the knowledge of 13 C– 1 H dipolar couplings and 13 C chemical shifts . However, to the best of our knowledge, a detailed investigation concerning the dynamics of various molecular motions occurring on different time scales in the crystal phase of 6CHBT exhibited by elongated (rod-like) molecules is scanty.…”

“…10,11 The static 1D 13 C and 2D proton-encoded local field (PELF) NMR experiments conducted in the nematic phase of 6CHBT monitored the trends of orientational order parameter vs temperature for various carbons from the knowledge of 13 C− 1 H dipolar couplings and 13 C chemical shifts. 48 However, to the best of our knowledge, a detailed investigation concerning the dynamics of various molecular motions occurring on different time scales in the crystal phase of 6CHBT exhibited by elongated (rod-like) molecules is scanty. In this regard, we herein employ a combination of relaxometry approaches based on FFC 1 H NMR and high-field pulsed NMR (10 kHz−400 MHz) to unravel complex molecular dynamics in the crystal phase of 6CHBT.…”

13C NMR Investigations of Hairy-Rod-Like π-Conjugated Mesogens

“…The 1D 13 C NMR spectrum in mesophase was obtained by ramped cross-polarization (CP) scheme with a 50 kHz radio frequency (RF) field strength on both 1 H and 13 C channels during the contact time of 3 ms, number of scans 128 and recycle delay of 8 s. High resolution 2D-SLF spectra were recorded by using the SAMPI-4 and Proton Encoded Local Field (PELF) , pulse sequences (Figures S1 and S2 of the Supporting Information). The utility of SAMPI-4 and PELF scheme for a wide variety of mesogens is reported in our earlier work. ,− For the evaluation of scaling factor, 1D proton coupled and 2D SLF experiments were recorded on a 13 C-labeled CHCl 3 oriented in the nematic liquid crystal sample of 4- n -pentyl-4′-cyanobiphenyl (5CB). The 13 C– 1 H splitting obtained in the 1D experiment was used to estimate the scaling factor of 2D SLF experiments.…”

2-Octyl thiophene based three ring mesogens: solid state 13C NMR and XRD investigations

“…15,16 Conversely, if C 2V symmetry is broken by introducing two different units at 2, organization and orientational order since they are polymesomorphic exhibiting good mesophase range. [25][26][27] Hence, in this work, we characterize the structures, molecular packing as well as orientational order of 4-n-alkoxyphenyl 4-(5-n-octyl-2-thienyl) benzoates in which thiophene is part of the core unit. [21][22][23][24] This is attributed to the ability of these techniques to probe molecular organization by variable temperature X-ray diffraction (VT-XRD) and the molecular dynamics as well as orientational order (solid-state 13 C NMR) in different phases.…”

“…Further, the recent past witnessed the utility of 13 C-1 H dipolar couplings determined by 2D separated local field (SLF) experiments for finding the orientational constraints and molecular topology of mesogens in their liquid crystalline phases. [25][26][27] Hence, in this work, we characterize the structures, molecular packing as well as orientational order of 4-n-alkoxyphenyl 4-(5-n-octyl-2-thienyl)benzoates in which thiophene is part of the core unit. Since these mesogens show polymesomorphism with nematic and a wide range of smectic phases, the structural characterization of them is attempted by making use of VT-XRD and high-resolution solid-state 13 C NMR in liquid crystalline phases.…”