An examination of the electronic structure and spectroscopy of high- and low-spin model ferredoxin via several SCF and CI techniques

We examine the electronic structure and spectrosopy of [Fe2S2(SH)4]2−, a simple model for ferredoxins, utilizing unrestricted Hartree-Fock (UHF), projected unrestricted Hartree-Fock (PUHF), restricted open-shell Hartee-Fock (ROHF) with configuration interation (CI), and configuration averaged Hartree-Fock (CAHF)-CI procedures. Our lowest-energy calculation, PUHF, predicts a ground state which is diamagnetic (antiferromagnetic) in agreement with experiment, with a calculated Heisenberg exchange splitting value JH of −161 cm−1 and a Mössbauer quadrupole splitting constant (ΔEQ) of 1.04 mm/s, in reasonable agreement with experimental observations that these compounds are diamagnetic with JH values of between −54 and −183 cm−1 and ΔEQ values of 0.72 mm/s. Although the PUHF model gives the most systematic results for the ground state, ROHF-CI calculations using localized orbitals are also quite good. The use of delocalized ROHF orbitals gives a strong bias to the high-spin 11-tet (ferromagnetic) situation, which is very difficult to correct through CI. CAHF-CI calculations that recognize the local nature of the d-orbitals also yield results in good agreement with experiment and are simple to perform. We tried to calculate the charge-transfer spectrum of these complexes assuming a diamagnetic ground state and found this difficult to do for any reasonably sized CI. Rather, we used a perturbative approach which allows the calculation of the low-spin case from the far simpler high-spin calculation. Using this, our estimated charge transfer is, on average, calculated 1600 cm−1 too high in energy when compared to experiment.