Enamines. II. A theoretical and photoelectron spectroscopic study of the molecular and electronic structures of aliphatic enamines

Comparison of PE. spectroscopic data for four series of enamines (including azetidine and some aziridine derivatives) for studying the influence of amine‐ring size on electronic structure show the pyrrolidino group to exhibit the strongest amine/double bond coupling in sterically unconstrained enamines. However, the azetidino group accommodates best steric congestion due to dialkyl substitution at the β‐position of the enamine unit. Quantum‐chemical calculations of equilibrium structures and energy profiles for amine rotations in model enamines by the PRDDO SCF method agree satisfactorily with experimental results. Notable exceptions are pyrrolidine derivatives for which PRDDO overestimates the amount of N‐pyramidality.