Abstract: A series of amides of piperidin were synthesized and used for studies on the partial doublebond feature of the amide bonds. ¹H and ¹³C NMR spectra of 2-(3, 5-dinitrophenyl)-1-(piperidin-1-yl) ethanone (compound 1) were acquired at various temperatures. The results showed that the C-N bond of the piperidin amides exhibited partially double-bond character at 20 ℃ (293 K) , suggesting that the chemical shifts of a and b, c and d in compound 1 were very- different due to the two rotameric states. With increasing temperature, the two peaks of non-equivalent -CH₂- groups collapsed into one peak due to the fast rotation of the C-N bond. The rate constant for the dynamic process detected for compound 1 was 380 s^-1, and the free energy of activation for compound 1 at coalescence temperature (343 K) was 67 kJ · mol^-1.

Key words: NMR, variable-temperature experiment, amide bond, isomerization