Heteronuclear ¹³C, ¹⁵N and ¹⁷O NMR cross-correlations of 4-substituted benzamide derivatives: Importance of the average excitation energy term ΔE in NMR substituent effects

Heteronuclear ¹³C, ¹⁵N and ¹⁷O NMR cross-correlations of 4-substituted benzamide derivatives were carried out and an excellent ¹⁵N-¹⁷O cross-correlation was found (slope = 2.45, r = 0.996, n = 7). Very poor ¹³C-¹⁵N and ¹³C-¹⁷O correlations were observed. The substituent interaction mechanism of the carbony! carbon differs with respect to that experienced by the carbonyl oxygen and amidic nitrogen. Previous studies have reported that the benzamide ¹⁵N and ¹⁷O chemical shifts obey the Hammett equation but ¹³C shifts do not. It is concluded that ¹³C chemical shifts do not correlate with the substituent constant σ_x because the ΔE term of the Karplus-Pople equation does not obey the Hammett equation. It is proposed that, in the localized excited state, the mechanism of interaction of substituents on the aromatic ring with the carbonyl carbon differs from that of the carbonyl oxygen and the amide nitrogen in their respective localized excited states. Whenever a non-linear Hammett plot is obtained, in an NMR substituent study, the possibility that substituents interact differently with the ground and excited states should be considered.