13C NMR spectroscopy of unsaturated long-chain compounds: an evaluation of the unsaturated carbon signals as rational functions

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Abstract
Literature data on the 13C NMR spectra of unsubstituted monounsaturated fatty acids and esters as well as fatty acids and esters with allylic hydroxy groups between the double bond and the terminal methyl group have been evaluated. The signals of the unsaturated carbons between the functional group at C-1 and the position of shift equivalence (POSE; defined as the location of the double bond where the separations of the olefinic carbon signals becomes zero in unsubstituted compounds) are of special interest because they depend on the distance of the unsaturation from C-1. The resonances of both unsaturated carbons as well as their difference (the shift value of the carbon closer to C-1 is subtracted from that of the C closer to the terminal methyl group) are rational functions. The equations for the s are the difference of the equations for the individual unsaturated carbons. When comparing Z and E unsaturation, the equations for s do not differ but those for the individual olefinic carbons differ. Some octadecadienoic acids are also discussed. The results generally agree with the electric field model presented by other authors for explaining the decreases in shift separations.

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