Electronic ground states and vibrational frequency shifts of diatomic ligands in heme adducts

DFT calculations were carried out to study heme complexes with diatomic ligand (CO, NO, or O(2)) and trans-imidazole ligand. The optimized electronic ground states of CO, NO, and O(2) adducts are singlet, doublet, and open-shell singlet, respectively. For O(2) adduct, the open-shell singlet is slightly lower in energy than the close-shell singlet. However, important differences are found in optimized structures and vibrational frequencies. Particularly, the trans-imidazole-induced frequency up-shift of the Fe-O(O) stretching mode can be predicted only with the open-shell singlet as ground state. An analysis of normal modes confirms that the up-shifts in the bent (NO and O(2) ) adducts are mainly due to mixing of Fe-X(O) stretching mode with Fe-X-O bending coordinate. Our study of binding mechanism indicates that a secondary source of the upshifts is the diminished weakening of the Fe-X(O) bonds. The Fe-X(O) bond strengths are modulated by σ competition mechanism, which weakens the Fe-X(O) bond and σ-π cooperation mechanism, which only exists in the bent adducts and enforce the Fe-X(O) bond. -