Ab initio and post-ab initio quantum chemical study of the heme spin states in electron transfer reactions

Quantum-chemical calculations of neutral and charged ironporphyrin (FeP, FeP⁺¹ and FeP⁻) systems were performed using B3LYP and MP2 methods. It was shown that all ground states of FeP (S = 1), FeP⁺¹ (S = 3/2) and FeP⁻ (S = 1/2) systems have C_2v symmetry. During the first step of electron transfer process an electron goes to β-LUMO − 1 Fe d_yz-orbital of FeP⁺¹. The second electron goes to β-LUMO of FeP which is attributed to π-system of porphyrin ring. The 3s- and 3p-orbitals do not play a significant role in the electron transfer process. The ability of FeP⁻¹ system to form π-dative chemical bond is low. The formation of π–π-complexes is preferable.