G. Knothe et Mo. Bagby, C-13 NMR-SPECTROSCOPY OF UNSATURATED LONG-CHAIN COMPOUNDS - AN EVALUATION OF THE UNSATURATED CARBON SIGNALS AS RATIONAL FUNCTIONS, Perkin transactions. 2, (3), 1995, pp. 615-620
Literature data on the C-13 NMR spectra of unsubstituted monounsaturat
ed fatty acids and esters as well as fatty acids and esters with allyl
ic hydroxy groups between the double bond and the terminal methyl grou
p have been evaluated. The signals of the unsaturated carbons between
the functional group at C-1 and the position of shift equivalence (POS
E; 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 uns
aturation 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 s
ubtracted 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 i
ndividual 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 separ
ations.