Effect of cis–trans Isomerism on the Carbon-13 Chemical Shifts of Some Unsymmetrically Substituted Ethylenes

Abstract

The difference in shielding between the 13C nuclei of cis and trans isomers CHX = CHX for the series X = Cl, Br, I, CO2C2H5 was previously attributed to the inhibition of contributing resonance structures by bulky substituents in cis isomers. The present work, which includes the determination of 13C chemical shifts of cis and trans isomers with nonequivalent substituents, CHX = CHY, indicates that the previous interpretation must be modified considerably and that the tendency toward charge separation and reduced bond order in the C = C π bond of cis isomers may be the major factor in these differential shieldings. An application of the Pople–Karplus equations for 13C shielding is consistent with this view.