Distortion of the amide bond in amides and lactams. Photoelectron-spectrum and electronic structure of 3,5,7-trimethyl-1-azaadamantan-2-one, the most twisted amide

We report the He(I) photoelectron spectrum of 3,5,7-trimethyl-1-azaadamantan-2-one (20). Ionization potentials have been assigned to molecular orbitals on the basis of quantum chemical calculations. Compared with other lactams (1–19), the first ionization, IP[n(N)], of 20 has an usually low energy, while the second, IP[n(O)], appears at a “normal” value. For undistorted lactams, there is a linear correlation of IP[n(O)] values and normalized carbonyl frequencies ν120(CO): the data for 20 fall well off this correlation. The deformations of the amide linkage in 20, in the essentially undistorted δ-lactam 9, the moderately distorted α-lactam 17 and in the bicyclic lactam 19 were investigated by B3LYP/6-31+G* calculations. The deviation from the linear IP[n(O)]/ν120(CO) correlation can be related to the overall distortion parameter θ (defined as the sum of the absolute values of the C–N torsional angle τ and the out-of-plane bending deformations at the amino and the carbonyl group χN and χC, respectively). This allows θ for amides and lactams to be estimated from the observed IP[n(O)] and ν(CO) values.