SINGLET AND TRIPLET EXCITED-STATES OF A PYRAMIDALIZED ALKENE - ELECTRON-ENERGY-LOSS SPECTRA, PHOTOELECTRON-SPECTRA, AND CALCULATIONS OF THEEXCITED-STATES OF TRICYCLO[3.3.3.0(3,7)]UNDEC-3(7)-ENE

Spectroscopic consequences of alkene pyramidalization have been invest igated by measuring the tripler and singlet excitation energies of the pyramidalized alkene, tricyclo[3.3,3.0(3,7)]undec-3(7)-ene (1), and t he unpyramidalized reference alkene, bicyclo[3,3.0]oct-1(5)-ene (2), u sing electron-energy-loss spectroscopy. The experimental excitation en ergies and intensities are compared with the results of density functi onal calculations that include configuration interaction and also with the results of CASPT2 calculations. The valence pi,pi triplet and si nglet states of 1 were found to be lower in energy than those of 2, by 0.7 and 1.1 +/- 0.2 eV, respectively. In general, pyramidalization in 1 results in smaller decreases in the energies of the Rydberg states than of the valence states. However, pyramidalization was found to hav e dramatic consequences on the relative ordering and intensities of th e singlet Rydberg states, due to lower symmetry of 1, the change in th e nature of the pi and pi MOs in 1, and resulting changes in the mixi ng between valence and Rydberg excitations (C) 1997 Elsevier Science B .V.