SMALL AND MEDIUM RINGS .77. NONCONJUGATED PI,PI INTERACTIONS APPROACHING CLASSICAL PI,PI CONJUGATION - PE-SPECTROSCOPICAL INVESTIGATION OF THE 7,8-12,19-DISECO-1,16-DODECAHEDRADINE SYSTEM

Transannular (orbital) interactions in compounds 1-9 are studied by means of He(I(alpha))-PE spectroscopy. Unusually strong through-space-dominated pi,pi interaction is found to be operative in the nonconjugated 1,6-dienes 1, 2, 7, and 8. The pi,pi splitting of 1.9 eV for disecododecahedradiene 1 is even surpassed by the record value of 2.2 eV for disecodienedione 2, which almost equals that in 1,3-butadiene (2.46 eV). The experimental results are explained by computational methods, especially by analyses of the precanonical MOs.