Electronic against steric effects in distorted amides

The equilibrium structures and the barriers to rotation about the C-N bond of a series of N,N-dialkylsubstituted formamides and acetamides have been computed by means of the hybrid QM/MM IMOMM (MP2:MM3) method. Both the structural parameters and rotational barriers calculated with the hybrid QM/MM method agree with experimentally determined and previously calculated values. Moreover, the IMOMM computational scheme gives a quantitative separation between electronic and steric effects and a clear analysis of its origin. The distortion of the amide linkage from the planarity is due to the balance of electronic and steric contributions. Steric effects introduced by bulky substituents decrease the rotational barrier around the amide bond when they are able to compensate the electronic destabilisation of the amide linkage introduced by such substituents. © 2003 Published by Elsevier B.V.