Heitler-London Calculation on the Bonding of Oxygen to Hemoglobin. II. Explanation of the Diamagnetism of Oxyhemoglobin for the Parallel Arrangement of O2 to the Heme

Calculation is carried out for all the triplet and quintet states arising from the low lying terms of the ferrous heme iron atom and those of the oxygen molecule. The charge transfer configurations of ionic structures, Fe 3+ –O 2 - and Fe + –O 2 + , are also taken into account. The results are compared with those for the singlet and septet states calculated previously. If the value of an unknown splitting energy between u and υ orbitals is estimated to be 4,000 cm -1 or more, the singlet state composed mainly of 3 E and 3 Σ g - terms (triplet-triplet binding) becomes the lowest in energy at short distances between the heme and the oxygen molecule. Thus, the diamagnetism of oxyhemoglobin can be explained theoretically on the basis of the electronic structures of the hemes determined by the ligand field theory.