Conformational Analysis by Nuclear Magnetic Resonance. Nitrogen-15 and Carbon-13 Spectra of Lactams

Abstract

The 5-, 6-, 7-, 8-, 9-, and 13-numbered lactams have been studied as simple models for the amide linkage. ^(15)N and ^(13)C spectra have been determined in various solvents and assignments made for the majority of the resonance lines. The ^(15)N and ^(13)C carbonyl chemical shifts show no apparent relationship to the amide ring size or to each other. The 9-membered lactam, azacyclononanone, exists as an equilibrium mixture of cis and trans conformers, the relative amounts of which depend on solvent and temperature. Carbon-13 NMR spectra of pure trans-zacyclononanone have been obtained from solutions of the solid at -45 ºC in CDCl_3. At -35 ºC two trans conformers are seen; at room temperature approximately equal quantities of the cis and trans isomers are present in ethanol, dimethyl sulfoxide, and tetrachloroethylene, whereas the cis conformer predominates in chloroform. Further increases in temperature cause rapid (on the NMR time scale) equilibrium between the two isomers such that a single averaged spectrum is seen at 120 ºC. From the coalescence temperature of the carbonyl peaks the free energy of activation for the interconversion of cis- and trans-azacyclononanone is found to be 17 kcal/mol. This value is discussed in terms of the twofold periodic potential function for the distortion of amides from a coplanar conformation.