SYNTHESES AND PHOTOELECTRON-SPECTRA OF 7-AZANORBORNADIENE AND RELATED-COMPOUNDS - AN ANALYSIS WITH FRAGMENT ORBITALS

The He(I) photoelectron spectra (PE) of 7-azanorbornane (5), 7-azanorbornene (6), and 7-azanorborandiene (7) as well as of related urethanes have been recorded. The syntheses of these bicyclic compounds are described in detail. A most convenient analysis of the PE spectroscopic results is based on the procedure of Heilbronner-Schmelzer on an ab initio STO-3G level. This method allows to construct fragment orbitals elucidating the orientation of the nitrogen lone pair, the through-space interaction with pi-bonds, and the participation of sigma-bond orbitals. The result is remarkable: whereas the direct interaction of localized lone-pair and pi-orbitals is significant in both syn- and anti-orientation, the interaction of a localized lone pair with a precanonical fragment pi-orbital is completely different in the two geometries. In 6-syn a considerable interaction matrix element F-psi,n-pi-syn = - 0.71 eV comes to the fore, whereas the corresponding parameter in 6-anti turns out to be almost zero, F-psi,n-pi-anti = + 0.09 eV. Since 6-syn is calculated to be more stable than 6-anti, it is this invertomer, 6-syn, which is likely to be responsible for the main bands in the PE spectrum of 6. The comparatively large experimental split between the first two PE bands of 6 (0.98 eV) is in accord with the 6-syn geometry and compares well with the calculated band separation of 1.26 eV for 6-syn.