Excited states and photochemical reactivity of fulvene. A theoretical study

Abstract

Semiempirical and ab initio calculations including electron correlation have been carried out for the ground and excited states of fulvene. Vertical singlet and triplet excitation energies including several Rydberg transitions, which have been calculated using an extended basis set augmented with even tempered diffuse functions, are in good agreement with experiment. Spectroscopic minima were obtained for the 1A1(S2) and 3B2(T1) states of planar fulvene, and on the semiempirical level also for the 1A″(S1) state, which is only of Cs symmetry. Twisting of the exocyclic double bond was studied by means of correlation diagrams as well as by calculating potential energy hypersurfaces. At 90° twist all stationary points are identified as transition states at the ab initio level, while two minima (S0 and S2) and one transition state (T1) result from the semiempirical calculations. Taking into account pyramidalization of the exocyclic methylene group yields a minimum for the T1 state and a conical intersection of the S1 and S0 states, which is not identical with the one that is to be expected for a critically heterosymmetric biradicaloid. The photochemistry of fulvene is being discussed in the light of the present results.