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Detailed Assignment of the Magnetic Circular Dichroism and UV−vis Spectra of Five-Coordinate High-Spin Ferric [Fe(TPP)(Cl)]

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journal contribution
posted on 02.06.2008, 00:00 by Florian Paulat, Nicolai Lehnert
High-spin (hs) ferric heme centers occur in the catalytic or redox cycles of many metalloproteins and exhibit very complicated magnetic circular dichroism (MCD) and UV−vis absorption spectra. Therefore, detailed assignments of the MCD spectra of these species are missing. In this study, the electronic spectra (MCD and UV−vis) of the five-coordinate hs ferric model complex [Fe(TPP)(Cl)] are analyzed and assigned for the first time. A correlated fit of the absorption and low-temperature MCD spectra of [Fe(TPP)(Cl)] lead to the identification of at least 20 different electronic transitions. The assignments of these spectra are based on the following: (a) variable temperature and variable field saturation data, (b) time-dependent density functional theory calculations, (c) MCD pseudo A-terms, and (d) correlation to resonance Raman (rRaman) data to validate the assignments. From these results, a number of puzzling questions about the electronic spectra of [Fe(TPP)(Cl)] are answered. The Soret band in [Fe(TPP)(Cl)] is split into three components because one of its components is mixed with the porphyrin A2u⟨72⟩ → Eg⟨82/83⟩ (π → π*) transition. The broad, intense absorption feature at higher energy from the Soret band is due to one of the Soret components and a mixed σ and π chloro to iron CT transition. The high-temperature MCD data allow for the identification of the Qv band at 20 202 cm−1, which corresponds to the C-term feature at 20 150 cm−1. Q is not observed but can be localized by correlation to rRaman data published before. Finally, the low energy absorption band around 650 nm is assigned to two P → Fe charge transfer transitions, one being the long sought after A1u(HOMO) → dπ transition.