Masters Group Current Research

NMR spectroscopy in liquid-crystal solvents

The molecules of liquid crystals are locally aligned relative to one another. In a magnetic (or electric) field, the direction of alignment becomes the same for all regions of the sample, and as the molecules align, they force any dissolved molecules to become aligned, too. This then allows observation of direct, through-space couplings between nuclei. These coupling constants depend on the degree of alignment of the molecules, and on the distance between the nuclei - structural information.

This is a powerful method for structure determination, particularly for small, fairly rigid, organic molecules, especially rings:

• it is very precise
• it is best for hydrogen, then for carbon, fluorine and other light elements
• it gives ratios of distances rather than absolute values
• it gives separate, uncorrelated, measurements of distances, for example the three different C-H distances in chlorobenzene.

But, all methods have their limitations:

• it is limited to nuclei with spin ½
• at least one distance must be assumed - the full structure is never given
• there are difficulties with heavy nuclei (due to anisotropy of indirect coupling, J)
• there can be problems allowing for vibrational effects.

In our work, we use the fact that information given by this technique complements that given by gas electron diffraction, so that by performing a combined analysis of the data from both experiments, we can derived structures which are much more accurate and complete than can be obtained using one technique by itself.