Deuterium isotope effects and assignment of 13C chemical shifts in spectra of methyl octadecanoate and the sixteen isomeric oxooctadecanoates

Abstract

13C nmr spectra of the 16 isomeric gem-dideuterooctadecanoates were measured and second and third atom deuterium isotope effects calculated for most of the affected carbons of 12 of the isomers. The average value for the second atom effect was −0.20 ppm and for the third atom effect was −0.05 ppm. From these effects and the changes in spectra caused by introduction of two deuterons at positions along the fatty acid chain, chemical shifts were determined for all carbons of methyl octadecanoate. Spectra of seven dideuterooxooctadecanoates, with both deuterons attached to a carbon γ to the oxo group, were measured, and using isotope effects, unambiguous assignments of chemical shift were made. Chemical shifts were thus assigned to all carbons in the 16 isomeric oxooctadecanoates and these show the effect of the carbonyl group at different positions in the chain. The results indicate that, in addition to large effects on carbons α and β to the oxo group, the γ to θ carbons are all shielded with displacements: γ −0.46, δ −0.30, ε −0.27, ζ −0.13, η −0.09, and θ −0.06 ppm. The effects of the oxo and ester carbonyls extend over seven methylene groups, but shielding due to ester carbonyl is a little smaller. Spectra of 2- to 9-oxo esters showed that interaction between the two carbonyl groups causes relatively greater shielding of carbons situated between the groups.