The importance of electron correlation for the ground state structure of porphycene and tetraoxaporphyrin-dication Original Research Article

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 methods, the latter independent of the size of the applied basis sets, predict structures with pronounced bond localization and low symmetry. The inclusion of electron correlation leads to delocalized structures with high symmetry. Experimental data confirm these structures. An additional examination on the tautomerism of porphycene predicts the trans tautomer to be 4–8 kJ mol−1 more stable than the energetically lower of the two possible cis forms.