  Infrared spectroscopy (IR) and quantum chemistry calculations, based on
density functional theory (DFT) were used to study the structure and the
molecular interactions in the nematic (N) and twist-bend (NTB) phases of
thioether-linked dimers. Infrared absorbance measurements were conducted in a
polarized beam for homogeneously aligned samples to measure and understand the
orientation of the vibrational transition dipole moments in the liquid crystal
states. Revealing temperature-dependent changes in the mean IR absorbance in
the twist-bend nematic phase were observed. In the transition from the N to the
NTB phase, is was found to be associated with a decrease in absorbance for the
longitudinal dipoles. This is a result of the antiparallel axial interactions
of the dipoles, while the absorbance of the transverse dipoles remained
unchanged up to 340 K and then increased and the dipoles correlated in
parallel. In order to account for the molecular arrangement in the nematic
phase, DFT calculations were conducted for the system nearest to the lateral
neighborhoods. Changes in the square of transition dipoles were found to be
quite similar to the mean absorbance that was observed in the IR spectra.
Interactions of molecules that were dominated by pair formation was observed as
well as a shift of the long axes of the molecules relative to each other. The
most important observation from the spectroscopic measurements was the sudden
increase in the intermolecular interactions as the temperature decreased from
the N to the NTB phase. This was evidenced by a significant increase in the
correlation of the dipoles that were induced in the dimer cores.
