NMR studies of Liquid Crystals
School of Chemistry, University of Southampton, UK.
Lundi 25/02/2008, 11h00
Bât. C5 P.421A, CEA-Grenoble
NMR and liquid crystals were made for each other. The molecules in such samples move rapidly, as in normal liquids, but not randomly, with the result that at any point in the sample there is a preferred direction of orientation of the molecules, which is described by a unit vector, the local director. The average orientation of the molecules with respect to the local director may be described by sets of orientational order parameters. In the magnetic field of an NMR spectrometer it is possible to align all the individual directors to produce a uniformly aligned sample. Such samples give NMR spectra which are rich in information, and their analysis yields anisotropic interactions such as dipolar couplings, quadrupolar splittings and chemical shift anisotropies, whose partially-averaged magnitudes allow us to investigate the structure, conformational fluidity, and orientational order of the molecules in a liquid crystalline sample in unique detail. Examples will be described of the application of NMR to studying samples at almost negligible molecular, orientational order to ones in which the molecules are almost completely ordered.
The process of alignment of the directors by a magnetic field can also be studied by NMR to give an insight at the molecular level of this collective process. Liquid crystalline samples which also have some translational order, known as smectic phases, undergo an alignment process in a magnetic field which is particularly challenging to understand. For these samples it has proved to be an advantage to use data obtained from x-ray scattering as well as NMR to try to understand the alignment process.