THE IMPORTANCE OF ELECTRON CORRELATION FOR THE GROUND-STATE STRUCTUREOF PORPHYCENE AND TETRAOXAPORPHYRIN-DICATION

The influence of electron correlation on two of the new porphyrinoids porphycene ([18]porphyrin-(2.0.2.0)) and tetraoxa[18]porphyrin-(1.1.1. 1)dication is investigated by comparison of RHF, UHF, MP2, and density functional calculations. Semiempirical as well as ab-initio RHF metho ds, the latter independent of the size of the applied basis sets, pred ict structures with pronounced bond localization and low symmetry. The inclusion of electron correlation leads to delocalized structures wit h high symmetry. Experimental data confirm these structures. An additi onal examination on the tautomerism of porphycene predicts the trans t automer to be 4-8 kJ mol(-1) more stable than the energetically lower of the two possible cis forms. The observed importance of electron cor relation for the prediction of correct ground state structures agrees with earlier findings on [18]annulene and porphyrin. It is highly prob able that it is in general impossible to obtain reliable geometries of porphyrinoids at the restricted Hartree-Fock level. (C) 1998 Elsevier Science B.V.