Ground and excited states of oxyheme: SAC/SAC-CI study

The SAC(symmetrie adapted cluster)/SAC-CI(configuration interaction) method is applied to the ground and low-lying excited states of oxyheme (FeC23N6O2H16). The ground state (1A′) is suitably represented by the Pauling model, Fe(II) (S = 0) + O2(1Δg). The SAC-CI result reproduces well the lower excitation spectrum of oxyhemoglobin. The lowest peak observed at 1.34 eV is assigned to the 1Δg → 1Σg+ excitation of the O2 ligand. Many transitions originating from the iron d orbitals are calculated, although their intensities are very small. The lowest triplet state (13A″), which is due to the 1Δg → 3Σg− transition around the O2 ligand, is calculated at 0.47 eV above the ground state, but its level is sensitive to the electron correlations included. In the geometry of the deoxy form, this 13A″ state becomes more stable than the closed-shell singlet state (1A′), indicating a geometrical control of oxygen affinity of heme.