Abstract
Proton and fluorine NMR spectra of 2,6-difluoro- and 2,4,6-trifluoropyridine dissolved in different nematic phases have been recorded and analyzed. Sample rotation was used to determine relative signs of isotropic coupling constants. The analyses of the spectra of EBBA solutions of the trifluoro compound was carried out with the aid of an iterative computer program which does not rely on the assignment of lines in observed and computed spectra. The anisotropic coupling constants obtained from the analysis are consistent with a negligible anisotropic contribution to J 26 FF in 2,6-difluoropyridine, but two, or more, J ij values in 2,4,6-trifluoropyridine contain small but significant anisotropic contributions. These conclusions were reached after allowing for the effect of vibrational motion on the dipolar couplings, which necessitated computation of normal coordinates for these molecules. Vibrational corrections were also computed for dipolar couplings in pentafluoropyridine.
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