LCAO-X-ALPHA CALCULATION OF THE MAGNETIC EXCHANGE INTERACTIONS IN A MN(IV)MN(III)3 CUBANE COMPLEX - RELEVANCE TO THE WATER OXIDATION CENTER OF PHOTOSYSTEM-II

The results of density functional calculations of the LCAO X-alpha-type which employed the broken symmetry method are presented for the experimentally known distorted cubane complex [Mn(IV)Mn(III)3O3Cl7(O2CCH3)3]3-, complex 1. Of all the spin states possible, the ground state was calculated to have S = 9/2, in agreement with magnetic susceptibility results. Magnetic exchange parameters (-2J(ij)S(i)S(j) Hamiltonian) characterizing the Mn(III)-Mn(III) (J33) and Mn(III)-Mn(IV) (J34) interactions were calculated. ValueS Of J33 = 19.3 cm-1 (ferromagnetic) and J34 = -46.6 cm-1 (antiferromagnetic) were calculated employing the X-alpha potential with Becke gradient energy and the larger basis set. These values are comparable to exchange parameters experimentally evaluated from susceptibility data for similar Mn(IV)Mn(III)3O3Cl cubane complexes. Net charge and spin densities were also calculated for the three (M(s) = 15/2, 9/2, or 1/2) broken symmetry states. From the calculated net charges and compositions of molecular orbitals it is clear that for most molecular orbitals there is extensive mixing of metal and ligand atomic orbitals. Only small changes in atomic charges are found for the three broken symmetry states. From the net spin densities it was found that the mu-3-Cl atom is a net beta-spin donor toward the three Mn(II) atoms. On the other hand the three mu-3-O atoms are net beta-spin donors to the Mn(IV) ion, and with respect to the Mn(III) ions, they are net a spin donors. An analysis is presented to show that spin polarization of the bridging and terminal ligands is likely the major contributor to the metal-metal spin coupling. There are probably cooperative effects among the different ligands with respect to spin donation with corresponding effects on the magnitude of the pairwise magnetic exchange interactions in [Mn4O3Cl7(O2CCH3)3]3-. A comparison of the nature and origin of exchange interactions and ground states is made between the Mn4 complex 1 and reduced Fe4S4(1+) and Fe4Se4(1+) complexes.