The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.