Hyperconjugation Involving Strained Carbon–Carbon Bonds. Structural Analysis of Ester and Ether Derivatives and One-Bond 13C–13C Coupling Constants of α- and β‑Nopinol

σC–C–σ*C–O interactions involving the strained carbon–carbon bonds of α- and β-nopinol, and their ester and ether derivatives have been demonstrated in the solid state using the variable oxygen probe. These hyperconjugative interactions are manifested as a strong response of the C–OR bond distance to the electron demand of the OR substituent. Although the effects upon the donor C–C bond distances are not large enough to be measurable by X-ray crystallography, they do result in systematic and measurable effects on the 13C–13C one-bond coupling constants. For the donor C–C bond, coupling constants decrease, consistent with weakening of this bond, while the intervening C–C bond coupling constants increase, consistent with bond strengthening, as the electron demand of OR increases.