Direct Evidence for Synchronicity between Rotation along C-C and Pyramidalization of C in Amides

In amides, the presence of sinusoidal synchronicity between rotational motion along C-alpha-C ' sigma-bond (tau rotation) and the concomitant pyramidalization of C ' (theta) is evidenced for the first time in solution. Temperature-dependent H-1 NMR and DFT simulations on suitable amide models (iBu(i-)-Pro(i)-Xaa(i+1)-OMe; iBu is isobutyryl) reveal the perturbation in synchronicity when tau-rotation is 'locked' with interactions, resulting in a strain on the amide bond, which influences a decrease in energy barrier for the amide bond cis/trans isomerism by similar to 1.68 kcal/mol.