The origin of rotational barriers in amides and esters

Abstract

We discuss in this article the origin and magnitude of the single bond rotational barrier in amides and esters. The high rotational barrier of amides is biochemically manifested in the limited conformational freedom of proteins, Since there are only two instead of three bonds to rotate about per arnino acid residue. On the basis of thermochemical estimates with model compounds, we find that the resonance energy of esters is somewhat higher than that of amides. However, the experimental rotational barrier for the former is considerably lower than the latter. We suggest esters have lower rotational barriers than the corresponding amides because they retain a large fraction of the resonance energy in the transition state. Justification is offerred using an orbital delocalization argument.