Small and Medium Rings, 74. 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‐azanorbornadiene ( 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 π bonds, and the participation of σ bond orbitals. The result is remarkable: whereas the direct interaction of localized lone‐pair and π orbitals is significant in both syn ;‐ and anti ‐orientation, the interaction of a localized lone pair with a precanonical fragment π orbital is completely different in the two geometries. In 6;‐ syn a considerable interaction matrix element F anti Ψnπ =−0.71 eV comes to the fore, whereas the corresponding parameter in 6 ;‐ anti turns out to be almost zero, F anti Ψnπ =+ 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 .