Conical intersections induced by repulsive 1πσ* states in planar organic molecules:: malonaldehyde, pyrrole and chlorobenzene as photochemical model systems

Ab initio calculations of reaction paths and potential-energy profiles of excited states are reported for the model systems malonaldehyde, pyrrole and chlorobenzene. In all three cases, optically dark singlet states of pi sigma* character have been found, which are repulsive with respect to an appropriate reaction coordinate. The resulting surface crossings with (1)pi pi* states and/or the electronic ground state, which are symmetry allowed for the planar systems, are converted into conical intersections by out-of-plane modes of appropriate symmetry. It is argued that conical intersections of this type are a common phenomenon in planar organic molecules with heteroatoms and may dominate the photochemistry of these systems. (C) 2000 Elsevier Science B.V. All rights reserved.