Characterization of charge transfer excited states in [2Fe\u20132S] iron\u2013sulfur clusters using conventional configuration interaction techniques

Adam Kubas

doi:10.1007/s00214-020-02635-7

Abstract

The experimental UV–Vis spectra of the biologically relevant [2Fe–2S] iron–sulfur clusters feature typically three bands in the 300–800 nm range. Based on ground-state orbitals and using the one electron transition picture, these bands are said to be of charge transfer character. The key complication in the electronic structure calculations of these compounds are the antiferromagnetic coupling of the iron centers and high covalency of Fe–S bonds. Thus, the examples of the direct computations of electronically excited states of these systems are rare. Whereas low lying electronic excited states were subject of recent studies, higher energy states computed with many-body theories were never reported. In this work we present, for the first time, calculations of the electronic spectra of [Fe2S2](SMe)42− biomimetic compound. We demonstrate that spin-averaged restricted open-shell Hartree–Fock orbitals are superior to high-spin orbitals and are convenient reference for subsequent configuration interaction calculations. Moreover, the use of conventional configuration interaction methods enabled us to study the nature of the excited states in details with the difference density maps. By systematic extension of the donor orbital space we show that key excitations in the 300–800 nm range are of Fe 3d ← (μ-S) character.

Highlights

While only limited number of states can be computed at low-spin state, we show that by virtue of intrinsic electronic properties of iron–sulfur clusters the nature of key CT states can be approximated by assuming ferromagnetic coupling
We note that spin-averaged Hartree–Fock (SAHF) procedure yields virtually the same orbitals as CAS self-consistent filed (CASSCF) method with maximal number of roots for a given multiplicity and initial orbital space
We confirm the equivalence by computing the total SAHF and state-averaged CASSCF (5 states) energies of high-spin F e2+ ion with the active space of 5 3d orbitals and 6 associated electrons

Summary

Introduction

Their key biological role is to mediate the electron transfer (e.g. in hydrogenases [1]) but they serve as the enzymatic reaction centers like in S-adenosyl-l-methionine reducing proteins that are involved in the DNA repair [2]. The key feature that makes the iron–sulfur clusters efficient electron mediators is the dense ladder of excited states that presumably allows the system to switch between them upon geometry change in a nonadiabatic process [5, 6]. The key issue is that iron–sulfur clusters are known to posses unusually high density of states that is a consequence of low-spin,

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Conclusion