STRAINED DELOCALIZED CARBENOID RING-SYSTEMS - A THEORETICAL INVESTIGATION

A number of ring compounds containing a divalent carbon center (carben es) have been studied using ab initio quantum chemical methods. The st udied systems include: imidazol-2-ylidene, 4-pyranylidene, 9-xanthylid ene, cyclohexa-2,5-dienylidene and 4-oxocyclohexa-2,5-dienylidene. Ext ended ANO type basis sets were used. Wave functions and energies were obtained with a multiconfigurational approach (CASSCF), where dynamic correlation effects are treated by using second-order perturbation the ory (CASPT2). The singlet-triplet splitting has been found to depend l inearly on the energy separation between the two carbene orbitals. All systems, where this splitting is larger than about 10 eV have been fo und to have a singlet ground state, while those with a smaller gap hav e a triplet ground state. A number of excited states have been charact erized. Computed excitation energies are in agreement with experiment in cases where such information is available.