Photoelectron Spectra, Ab Initio SCF MO, and Natural Bond Orbital Studies on Stellenes. Long-Range π/σ Interactions

Abstract

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 π ionizations arising from the terminal π 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 σ bonds (1.58−1.60 Å) were predicted. By means of the Weinhold natural bond localization procedure, the interactions between the σ frame and the π orbitals as well as between the σ frame and the n orbitals have been probed. It has been concluded that the large energy splitting between the π MOs mainly localized at the terminal π bonds in 6 and 7 is mediated by the ribbon orbitals of the twisted cyclohexane ring.