PHOTOELECTRON-SPECTRA, AB-INITIO SCF-MO, AND NATURAL BOND ORBITAL STUDIES ON STELLENES - LONG-RANGE PI SIGMA INTERACTIONS/

The He I photoelectron spectra of 2,6-dimethylenetricyclo[3.3.0(1,5).0 (3,7)]octane (stella-2,6-diene) (6), stella-2,6-dione (8), stella-6-en -2-one (10), distella-2,2',6,6'-triene (7), distella-6,6'-en-2,2'-dion e (9), and distella-2,2',6'-dien-6-one (11) have been recorded. The en ergy differences between the pi ionizations arising from the terminal pi bonds amount to 0.9 eV (6) and 0.4 eV (7). An energy difference of 1.1 eV has been found between the 2p lone-pair ionizations of 8. Using Hartree-Fock SCF calculations with a 3-21G basis, the geometries of 6 -11 have been calculated. For all six molecules long central sigma bon ds (1.58-1.60 Angstrom) were predicted. By means of the Weinhold natur al bond localization procedure, the interactions between the a frame a nd the in orbitals as well as between the sigma frame and the n orbita ls have been probed. It has been concluded that the large energy split ting between the pi MOs mainly localized at the terminal pi bonds in 6 and 7 is mediated by the ribbon orbitals of the twisted cyclohexane r ing.