Linear and angular interactions in distellenes: Photoelectron spectra and ab initio SCFMO studies

The photoelectron spectra of rac-6,6'-diisopropylidene-2,2'-bis(tricyclo[3.3.0.0(3,7)]octylidene) (12), syn- and anti-2,2'-bis(tricyclo[3.3.0.0(3,7)]octane)-4,4'-dione (13 and 14), syn- and anti-4-methylidene-2,2'-bis(tricyclo[3.3.0.0(3,7)]octylidene)-4'-one (15 and 16), and anti-4,4'-dimethylidene-2,2'-bis-(tricyclo[3.3.0.0(3,7)]octylidene) (18) have been recorded. The energy difference between the 2p-type lone pairs of the angular oriented oxygen atoms of 13 and 14 amounts to 0.6 eV, while the corresponding energy difference in 4, where the oxygen atoms are linearly orientated, is 0.2 eV. For 3 and 12,in which the terminal pi-bonds are arranged linearly, the difference between the first two peaks in their spectra is 0.4-0.5 eV, while the angular arrangement of the terminal pi-bonds in 18 gives a splitting of only 0.2 eV. Hartree-Fock SCF calculations, using a 6-31G* basis set, have been used to interpret the photoelectron spectra of 12-16, 18, and 19. The interactions between the sigma frame and the n and pi orbitals in these molecules have been investigated and analyzed in terms of the through-space through-bond concept. It is concluded that a maximum interaction between pi bonds is attained, if they are arranged in a linear fashion, while the interaction between pi bonds and keto groups reaches its maximum in an angular orientation.