Abstract
An ab initio study of the influence of electron correlation on the relative stability of the C 2v and D 2h ground states of porphin is presented. Multiconfigurational CASSCF and RASSCF methods, as well as single- and multi-reference CI approches, are employed to analyze the π-electron correlation effects. In addition, σσ and σπ correlation contributions are accounted for by means of multiconfigurational second-order perturbation (CASPT2) and MP2 methods. It is found that the electron correlation effects of the σ electrons, in addition to the π-correlation contributions, have to be taken into account in order to give a balanced treatment of the relative stability of the two structures. Theoretical treatments based on only π-electron correlation might not be reliable for this purpose. The CASPT2 results obtained with an ANO basis set of valence DZ quality yields a D 2h ground state, which is about 1 eV more stable than the C 2v structure, in agreement with a recent MP2 study. The best estimate of the energy difference between the C 2v and D 2h structures obtained in the present work is 0.8 eV.
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