Singlet and triplet excited states of a pyramidalized alkene:: Electron-energy-loss spectra, photoelectron spectra, and calculations of the excited states of tricyclo[3.3.3.03,7]undec-3(7)-ene

Spectroscopic consequences of alkene pyramidalization have been investigated 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 the unpyramidalized reference alkene, bicyclo[3,3.0]oct-1(5)-ene (2), using electron-energy-loss spectroscopy. The experimental excitation energies and intensities are compared with the results of density functional calculations that include configuration interaction and also with the results of CASPT2 calculations. The valence pi,pi* triplet and singlet 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 have dramatic consequences on the relative ordering and intensities of the singlet Rydberg states, due to lower symmetry of 1, the change in the nature of the pi and pi* MOs in 1, and resulting changes in the mixing between valence and Rydberg excitations (C) 1997 Elsevier Science B.V.