Not Innocent: Verdict from Ab Initio Multiconfigurational Second-Order Perturbation Theory on the Electronic Structure of Chloroiron Corrole

Abstract

From a suitably broad perspective, transition metal corroles may be viewed as stable, synthetic analogues of high-valent heme protein intermediates such as compounds I and II. Against this backdrop, the electronic structure of chloroiron corrole has provoked a lively debate in recent years. Thus, whereas NMR spectroscopy and DFT calculations suggest an S = 3/2 Fe(III) corrole (*2-) radical description, certain researchers have favored an Fe(IV) formulation. These two descriptions are indistinguishable as far as DFT calculations are concerned. Ab initio CASSCF/CASPT2 calculations provide unambiguous support for the former description. In addition, they rule out any Fe(IV) state, whether high- or low-spin, within 1.5 eV of the ground state.

The ab-initio density matrix renormalization group in practice

“…We thus restrict ourselves here to active-space DMRG calculations. In more realistic studies, such activespace calculations would be augmented by a further more approximate dynamical correlation treatment using perturbation theory, 8,92,93 configuration interaction, 55,94 or canonical transformation theory. 38 We first consider oxo-Mn(salen).…”

Progress and Challenges in the Calculation of Electronic Excited States

“…[142] Particularly in the field of bioinorganic chemistry DFT methods should be carefully applied. There are many troublesome aspects one needs to face, for instance the antiferromagnetic interactions in metal clusters, the problems with non-innocent ligands (such as NO and corrole ligands [143][144][145][146] ) or the complications due to densely lying electronic states [such as in the biologically relevant Fe II /Fe III chemistry, [147][148][149][150] in which complexes can access three different spin states: singlet/doublet low-spin complexes, triplet/quartet intermediate spin complexes as well as high-spin complexes (quintet or sextet)]. In many iron complexes multiple states are close-lying and spin flips may easily be provoked, by for example thermal treatment, as in spincrossover systems or along a biochemical reaction pathway, as in heme proteins.…”

Spin in density‐functional theory

“…In particular, the complete active space self-consistent field method, usually in combination with second-order perturbation theory (CASPT2), has been employed to study transition metal complexes (for examples, see Refs. [13][14][15][16][17][18]). However, the factorial scaling with the size of the active space puts rather severe limits on the size of the active space, which prevents most applications to polynuclear transition metal complexes and clusters.…”