13C NMR spectra of some amides and imides. Effect of inductive and mesomeric interactions, cyclization and hydrogen bonding on 13C NMR chemical shifts

Abstract Carbon‐13 NMR chemical shifts in various amides and imides are examined. The compounds include mono‐ and diamides, acyclic imides and cyclic 5‐ and 6‐membered ring imides. The observed shifts can be satisfactorily rationalized on the basis of various electronic and steric factors which contribute to the chemical shifts in these systems. The 13 C shifts in amides and imides are further compared with the shifts in corresponding oxygen analogues, i.e. esters and anhydrides. The saturated and α, β‐unsaturated amide carbonyls absorb in the range of 173–178 ppm and 167–170 ppm, respectively. These shifts are downfield by 2–3 ppm in comparison with the corresponding esters. The carbonyl shifts in cyclic imides appear in the range 170–180 ppm. In comparison with the corresponding anhydrides and amides, these shifts are downfield by 5–7 ppm and 3–4 ppm, respectively. These differences are discussed in terms of electronic interactions and steric effects.