Pseudo Jahn-Teller effect in oxepin,azepin, and their halogen substituted derivatives

Oxepin and azepin are heterocyclic compounds with a seven-membered ring, which are present in the main skeleton of many anti-depressive drugs. Planar configuration instability due to the pseudo Jahn-Teller effect (PJTE) in oxepin, azepin and six their halogen substituted derivatives were investigated as an original PJTE study. Optimization and the following frequency calculations in these two series illuminated that all of these eight compounds were unstable in high-symmetry planar (with C _2v symmetry) configuration and their structures were puckered to lower C _s symmetry stable geometry. Moreover, the vibronic coupling interaction between ¹ A ₁ ground and the first ¹ B ₁ excited states via (¹ A ₁ + ¹ A ₁ ^’ + ¹ B ₁) ? b ₁ and (¹ A ₁ + ¹ B ₁ + ¹ A ₁ ^’ ) ? b ₁ PJTE problems were the reasons for the symmetry breaking phenomenon and non-planarity of the seven-member ring in those series. Finally, numerical fitting of the adiabatic potential energy surface (APES) cross-sections along the b ₁ puckering coordination was employed to estimate the vibronic coupling constants of PJTE problems for all the considered compounds.