Origin and Nature of Transmission Modes of Anomalous Effects of meta-Substituents on the 13C Chemical Shift of the Carboxyl Carbon (δCO) of Benzoic Acid

Studies of substituent effects on NMR chemical shifts are of great benefit in determining fine details of electron distribution in molecules. Interestingly, NMR substituent effects are often different and even opposite to those associated with chemical reactivity. Among molecules exhibiting anomalous (reverse) substituent effects is benzoic acid, the standard model for studying substituent effects. The substituent effect on the C-13 chemical shift of its carboxyl carbon (delta(CO)) is just the opposite of that on its acid strength or reactivity. To develop insights into the origin of the anomalous effect of a substituent on delta(CO), occupancies of natural atomic orbitals at the carboxyl and ring carbons of a set of 10 meta-substituted benzoic acids have been calculated at the density functional theory level using the B3LYP function with split valance 6-311G++** basis set. Statistical correlations obtained for the C-13 chemical shifts, delta(CO) and delta(C-ring) of these benzoic acids with the natural atomic orbital occupancies calculated for respective carbon atoms on one hand and with Taft's inductive and resonance parameters (sigma(I) and sigma(BA)(R)) of the substituents on the other hand have been critically analyzed. The findings have established firmly that a meta-substituent's anomalous effect on delta(CO) is caused by the substituent-induced changes in the total occupancy of only the p(z) natural atomic orbitals at the carboxyl carbon. The study has demonstrated further that the transmission of the anomalous effect can be successfully interpreted by a 5.5:-2.5:1 combination of the localized, extended, and resonance-induced delta-polarization effects.