Geometrical isomerism and 13C spectra of the β‐substituted Enones R1C(1)OC(2)HC(3)HR2

The 13C chemical shifts of the unsaturated carbons were measured in 31 cis and trans pairs of β‐substituted enones R1C(1)OC(2)HC(3)HR2. In these polarized ethylenes the chemical shifts of the olefinic carbons are simply related by the equation δc=δt+A. The steric and electronic effects introduced by the R1 and R2 substituents influence the chemical shifts of C‐2 and C‐3 in both isomers. It is shown that the sign and magnitude of the intercept A mainly reflect the π‐charge electronic density changes which arise in the cis isomer and are transmitted via the π‐framework. The effect of the steric interaction on the chemical shift of C‐3 in the cis isomers is postulated to be related to the symmetry of the substituents. Therefore, the differential shielding of C‐3 is indicative of the conformational structure of the cis molecule.