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 σx 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.
|Original language||English (US)|
|Number of pages||4|
|Journal||Journal of the Chemical Society. Perkin Transactions 2|
|State||Published - Aug 1997|
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