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A method for determining the relative signs of scalar and di-polar couplings between two spin-12 nuclei in a liquid-crystal system is presented. It is shown theoretically that the mixture of dipolar and scalar contributions to constants which determine the rate of Hartmann-Hahn transfer between two spins in spin-locked experiments and the splitting in directly observed 1D spectra are different. Comparison allows separation and determination of the relative signs of these contributions. Experimental results acquired on the H5,H6 pair of spins of uracil in a liquid-crystal solvent agree with theory only if a positive dipolar coupling is assumed. Such knowledge about the signs of coupling places useful restrictions on the average geometry of molecules in liquid-crystal media.
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Tolman et al. (1994) studied this question.
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