Proton and Deuterium NMR Study of the CBC9CB Dimer System.

J Carvalho,A Kohlmeier,G H Mehl,C Cruz,J L Figueirinhas,M G Tamba

doi:10.1021/acs.jpcb.8b11526

J Carvalho, A Kohlmeier + Show 4 more

Open Access

https://doi.org/10.1021/acs.jpcb.8b11526

Copy DOI

Abstract

Using proton and deuterium NMR spectroscopy, this work provides a description of the molecular orientational order of the mesogenic groups in both the Ntb and the classical nematic phase (N) of the liquid crystal dimer CB-C9-CB. The proton NMR data were collected at high field (7 T) with the nematic domains aligned by the field. Deuterium NMR data obtained from aligned samples at 11.7 T, published by Hoffmann , A. Soft Matter 2015 , 11 , 850 , were also considered in this study. Using the first-order perturbation theory, we carried out detailed simulations of the proton spectra from the terminal mesogenic groups along with the quadrupolar splittings from the carbon-deuterium bonds in the first chain positions, which allow for the determination of the Saupe order tensor for the mesogenic groups. We show that the main mechanism that induces the change of the 1H NMR spectrum and the quadrupolar splittings at the N-Ntb phase transition is the change of the orientation of the most ordered molecular frame (eigenframe of the Saupe tensor), along with the onset of the molecular biaxiality parameter, D. This orientation change is associated with the achiral symmetry breaking at the N-Ntb phase transition.

Highlights

Up to 2010 only four distinct nematic ground states had been found,[1] for achiral molecular systems, the uniaxial nematic and the biaxial nematic; for chiral molecules the helical nematic and the blue phases
The results presented in figures 5 show that the biggest contribution to the profile of the spectra comes from the mesogenic group, while the flexible chain predominantly accounts for the lower frequencies and the deeps at higher frequencies
The angle between the z axis of the most ordered molecular frame and its projection on the symmetry plane of the carbon-proton bonds of the first element in the linking chain, changes from zero in the N phase to values in the range of 0.009 to 0.3 rad in the new twist-bend phase (Ntb) phase, according to the results presented in figure 8

Summary

Introduction

Up to 2010 only four distinct nematic ground states had been found,[1] for achiral molecular systems, the uniaxial nematic and the biaxial nematic; for chiral molecules the helical nematic and the blue phases. An alternative model for the molecular organization in the low temperature nematic phase (labeled Nx) exhibited by these materials, supported on deuterium NMR measurements, has been proposed.[17] The authors allege that if the helical distribution of the director was present in the Nx phase, the deuterium spectrum, after a sample rotation of 900 about an axis perpendicular to the magnetic field, should be a superposition of peaks corresponding to the continuous helix Instead what they obtained was a typical spectrum of the perpendicular orientation relative to the magnetic field of a uniformly oriented director.

EXPERIMENTAL DETAILS

Vzz can be related to the quantity νQ

DATA AND MODEL RESULTS

Digitised Splttings Interpolation Results

CONCLUDING REMARKS