The application of chiral liquid crystal solvents to the NMR of polymers

Abstract

In recent years there has been a significant amount of research into the uses of chiral liquid crystal polymers as solvents in solution state Nuclear Magnetic Resonance (NMR).The NMR spectra of molecules in liquid crystalline solvents can be markedly different to the spectra observed for the isotropic solution. In these anisotropic environments which are neither isotropic liquid phase or crystalline, some of the NMR parameters do not average to zero. Chemical shift anisotropy, dipolar couplings and quadrupolar interactions become a feature of the spectra and in certain circumstances can be measured. Dipolar couplings, if introduced to a spectrum, can make the results very complicated as there can be a great many interactions. Quadrupolar couplings are often larger than dipolar couplings and therefore easier to observe, however this is dependent on the nuclei under investigation having a spin > 1/2.In these studies, it was the observation of chemical shift anisotropy which was to be the aim of the investigation and in particular the hypothesis that the technique could be used for enantiomeric purity determination. It is assumed that the R and S species will be oriented slightly differently in the solvent and so produce slightly different spectra. Once the method of sample preparation had been perfected the aim was to apply this technique to the elucidation of the stereochemistry of polymer samples. The purpose being to establish a protocol to assign polymer tacticity without the need for isotopic substitution, such as deuterium labelling.