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'-dione (9), and distella-2,2',6'-dien-6-one (11) have been recorded. The energy 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 bonds (1.58-1.60 Angstrom) were predicted. By means of the Weinhold natural bond localization procedure, the interactions between the a frame and the in orbitals as well as between the sigma frame and the n orbitals have been probed. It has been concluded that the large energy splitting 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 ring.