Stabilization and destabilization energies of distorted amides

Summary In brief, ab initio molecular orbital calculations using the 6-31G * basis set appear to predict good molecular geometries and reasonable experimental resonance energies. The “pure” resonance energy of a tertiary amide or lactam as predicted by a) the loss of resonance in 2-quinuclidone (3) or the rotational barrier of an amide to yield a hypothetical orthogonal transition state maintaining planarity at nitrogen is 21–22 kcal/mol. Molecular mechanics (MM2 in the Spartan 3.1 Program) provides reasonable overall conformational structures and surprisingly decent strain energies. However, the N-CO bond length is, not surprisingly, insensitive to distortion. The semi-empirical techniques do a surprisingly poor job of calculating distortion energies in this series.