F. Brogli1,2,3, Wolfgang Eberbach1,2,3, Edwin Haselbach1,2,3, E. Heilbronner1,2,3, Volker Hornung1,2,3, David M. Lemal1,2,3
1Chemisches Laboratorium der Universität Freiburg, D–78 Freiburg i.Br., Albertstrasse 21
2Department of Chemistry, Dartmouth College, Hannover, New Hampshire 03755
3Physikalisch-Chemisches Institut der Universität Basel, CH–4056, Basel, Klingelbergstrasse 80
Tóm tắt
AbstractA 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 Δn (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), Δn (3‐memb. ring) = 3.55 eV) and for a five‐membered ring analogue (2,3‐diazanorbornene (7) Δn(5‐memb. ring) = 3.10 eV). The sequence Δn (3‐memb. ring) > Δn (4‐memb. ring) Δn (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 Δn becomes minimal for a NNH angle φ ≈ 100°, whereas Δn 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 Δn for cis‐azo groups to a ‘through‐bond’ interaction of the n+‐orbital with a lowerlying NN σ‐orbital; this interaction becomes maximal for NNR angles of the size present in a fourmembered ring, e.g. in 12 or 13.
