A THEORETICAL DETERMINATION OF THE ELECTRONIC-SPECTRUM OF METHYLENECYCLOPROPENE

The vertical electronic spectrum of methylenecyclopropene, the prototy pe of the nonalternant hydrocarbons known as fulvenes, has been studie d using multiconfigurational second-order perturbation theory. The cal culations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first pi ionization limit. Ve rtical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation fro m the highest occupied sigma orbital to a pi orbital, while the other two are pi --> pi excitations. The third transition gives rise to th e most intense feature in the electronic spectrum. The results are rat ionalized within the scheme of two interacting double bonds. Compariso ns are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the availab le experimental data of methylenecyclopropene in solution.