Probing molecular ordering in the nematic phases of para-linked bimesogen dimers through NMR studies of flexible prochiral solutes

The quadrupolar splittings of perdeuteriated n-decane dissolved in nematic phases formed by mesogenic dimers of the CBnCB series, for n = 7,9,10,11, are measured throughout the entire temperature range of these phases. These results are reported together with related measurements using the common nematic phase of 5CB as a solvent for n-decane. The data obtained from the 13C spectra of the cyanobiphenyl mesogenic units of the monomeric and dimeric solvent molecules yield the order parameter of those units. The information obtained from this set of experiments is used to elucidate the structure of the low temperature (NX) and the high temperature (N) nematic phases of CBnCB dimers with n = 7,9,11. The polar twisted nematic (NPT) model is found to provide a consistent description not only of our experimental results but also of NMR measurements previously reported in the literature for these phases. These findings suggest that the high temperature nematic (N) is not a common, locally uniaxial and apolar nematic, but rather a nematic phase consisting of NPT clusters. The twist-bend (NTB) model, often identified with the NX phase, is shown to be inadequate to account even qualitatively for crucial features of the experimental findings

Liquid-State Structure via Very High-Field Nuclear Magnetic Resonance Discriminates among Force Fields

Deuterium nuclear magnetic resonance (²H NMR) spectra of labeled molecular liquids obtained at high fields, for example, |<b>B</b>| = 22.3 T (950 MHz proton NMR), exhibit resolved quadrupolar splittings that reflect the average orientation of the molecules relative to <b>B</b>. Those residual nuclear spin interactions exhibited by benzene and chloroform provide an experimental determination of the leading tensor component of the pair correlation function for these two molecular liquids. In this way, very high-field ²H NMR may be used to extract unambiguous information about liquid-state structure. Additionally, replicating the experimentally derived pair correlation function using molecular dynamics simulations provides a critical test of simulation force fields