Carbon-13 Chemical Shifts of the Carbonyl Group. II. A Correlation with CO Pi-Bond Polarity

Abstract

The 13C chemical shift of the carbonyl group in molecules of the type XCOY is considered in terms of the theory for the local paramagnetic term σpAA for 13C shifts presented by Karplus and Pople. On the assumption of (i) an invariant, nonpolar sigma network, (ii) a constant average excitation energy, and (iii) neglect of neighbor anisotropy and intramolecular dispersion effects, a formula is derived which relates the chemical shifts to the polarity of the carbonyl pi bond. When the limitations imposed by Assumptions (i) and (iii) are minimized by an adjustment based on the 13C shifts of symmetrical disubstituted ethylenes, the resultant semiempirical theory predicts that the ``corrected'' carbonyl shifts should correlate with the CO pi-bond polarity, or substituent parameters to which the latter is related. The experimental data are found to correlate with Taft's reactivity parameter σI, in accord with the interpretation of substituent effects in terms of π-bond polarity in the carbonyl group.