Die Photoelektronen‐Spektren des Tricyclo[4.2.1.0^2,5]nonadiens und seines 3,4‐Diaza‐Analogons. Ein Beitrag zur Kenntnis der Wechselwirkung zwischen den einsamen Elektronenpaaren der cis‐konfigurierten Azogruppe

A comparative photoelectron spectroscopical investigation of the title compound (11) and its 3,4‐ and 7,8‐dihydro derivatives (9 and 10) indicates that a considerable ‘through bond’ interaction exists between the π‐orbitals in 11. The PE. spectra of the 3,4‐diaza‐analogue of 10 and 11, which contain a cis azo group in a four‐membered ring, yield a splitting Δ_n (4‐memb. ring) = 1.55–1.60 eV between the nitrogen lone‐pair orbital energies. This value contrasts with those obtained for a three‐membered ring analogue (3,3‐dimethyldiazirine (5), Δ_n (3‐memb. ring) = 3.55 eV) and for a five‐membered ring analogue (2,3‐diazanorbornene (7) Δ_n(5‐memb. ring) = 3.10 eV). The sequence Δ_n (3‐memb. ring) > Δ_n (4‐memb. ring) Δ_n (5‐memb. ring) is satisfyingly reproduced by MINDO/2‐ and EHT‐calculations for the model systems with n = 3,4,5. A similar trend can be deduced from MINDO/2‐calculations for cis‐diimid where Δ_n becomes minimal for a NNH angle φ ≈ 100°, whereas Δ_n for the corresponding trans‐structure goes through a maximum in this region. The experimental finding as well as the calculated results confirm the predictions made by Gimarc [15] who attributes the behaviour of Δ_n for cis‐azo groups to a ‘through‐bond’ interaction of the n₊‐orbital with a lowerlying NN σ‐orbital; this interaction becomes maximal for NNR angles of the size present in a fourmembered ring, e.g. in 12 or 13.