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

We report the He(I) photoelectron spectrum of 3,5,7-trimethyl-1-azaadamanta n-2-one (20). Ionization potentials have been assigned to molecular orbital s on the basis of quantum chemical calculations. Compared with other lactam s (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 la ctams, there is a linear correlation of IP[n(O)] values and normalized carb onyl frequencies v(120)(C=O): the data for 20 fall well off this correlatio n. The deformations of the amide linkage in 20, in the essentially undistor ted delta-lactam 9, the moderately distorted alpha-lactam 17 and in the bic yclic lactam 19 were investigated by B3LYP/6-31+G* calculations. The deviat ion from the linear IP[n(O)]/v(120)(C=O) correlation can be related to the overall distortion parameter theta (defined as the sum of the absolute valu es of the C-N torsional angle tau and the out-of-plane bending deformations at the amino and the carbonyl group chi(N) and chi(C), respectively). This allows theta for amides and lactams to be estimated from the observed IP[n (O)] and v(C=O) values.