HEMOCYANIN MODEL COMPOUNDS - A QUANTUM-MECHANICAL CALCULATION OF GEOMETRY-DEPENDENT EXCHANGE INTERACTION

Strong antiferromagnetic coupling between the two copper(II) ions of oxyhemocyanin with -2J > 600 cm-1 (H = -2J12S1S2) was observed a long time ago. SCF-MP (self-consistent field-Moller Plesset) calculations have been performed to understand this high value. Five geometrically different model molecules were constructed to determine the contributions to J, the J values, partial charges and the involved orbitals. Information about the exchange pathway has been derived. Strong antiferromagnetic exchange interactions of 2J = - 6600 to -17 800 cm-1 have been calculated. The exchange pathway includes the peroxo bridge via its p(x) and p(y) atomic orbitals, which are responsible for the high antiferromagnetic exchange. It has been shown that geometric variations have a strong effect on the J value, but the involved orbitals, d(xy) of Cu and p(x), p(y) of O, remain the same. The terminal NH3 ligands are not involved in the exchange interaction.