Rate Processes and Nuclear Magnetic Resonance Spectra. II. Hindered Internal Rotation of Amides

Mathematical methods are presented for calculating rate constants of processes which narrow nuclear magnetic resonance absorption lines having discrete components. High resolution proton spectra show that the R1CO-NR2R3 skeletons of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) are planar and suggest that N-methylformamide, N-methylacetamide, N-methylformanilide and N-methylacetanilide exist predominantly in one configuration. The presence of a significant amount of double bond character in the C–N amide bond is proved by the temperature dependent coalescence observed for the chemically shifted proton doublet of the N(CH3)2 groups in DMF and DMA, which gives values of about 22 and 19 kcal respectively for the free energy of activation required for reorientations about the bond.