title: DFT and Ab Initio Study of Iron-Oxo Porphyrins: May They Have a Low-Lying Iron(V)-Oxo Electromer?
abstract: The energetics of various electromeric states for two heme complexes with an iron-oxo (FeO(3+)) group, FeO(P)(+) and FeO(P)Cl (P = porphin), have been investigated, employing DFT and correlated ab initio methods (CASPT2, RASPT2). Our interest focused in particular on tri- and pentaradicaloid iron(IV)-oxo porphyrin radical states as well as iron(V)-oxo states. Surprisingly, the iron(V)-oxo ground state is predicted for both models in vacuo. However, the presence of a polarizable medium, such as a solvent or a protein environment, favors the iron(IV)-oxo porphyrin radical cation, which is predicted to be the actual ground state of FeO(P)Cl under such conditions. Nonetheless, the iron(V)-oxo electromer is still expected to lie only a few kcal/mol above the ground state-a conclusion coming from both CASPT2 and RASPT2 calculations with a very large active space and further supported by a calibration with respect to coupled cluster CCSD(T) calculations for a simplified small model. The DFT results turn out to be strongly functional-dependent and thereby inconclusive. The widely used B3LYP functional-although correctly predicting the iron(IV)-oxo porphyrin radical ground state for FeO(P)Cl-seems to place the iron(V)-oxo states much too high in energy, as compared to the present CASPT2, RASPT2, and CCSD(T) results.
authors.affiliation: Faculty of Chemistry, Jagiellonian University , ul. Ingardena 3, 30-060 Kraków, Poland.
authors.forename: Mariusz
authors.initials: M
authors.lastname: Radoń
authors.affiliation: Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences , ul. Niezapominajek 8, 30-239 Kraków, Poland.
authors.forename: Ewa
authors.initials: E
authors.lastname: Broclawik
authors.affiliation: Department of Chemistry, University of Leuven , Celestijnenlaan 200F, B-3001 Heverlee-Leuven, Belgium.
authors.forename: Kristine
authors.initials: K
authors.lastname: Pierloot