Calculating the electron paramagnetic resonance parameters of exchange coupled transition metal complexes using broken symmetry density functional theory:: Application to a MnIII/MnIV model compound

The capability of the density functional broken symmetry approach for the calculation of various EPR parameters of exchange coupled metal clusters is demonstrated by studying the experimentally well-investigated [(MnMnIV)-Mn-III(mu-O)(2)(mu-OAc)DTNE](2+) complex. Geometry optimizations of the complex in its broken symmetry and high spin states yielded structures with two distinct manganese sites and geometrical parameters in good agreement with the X-ray structure. Exchange coupling constants were calculated from the energy differences between the high spin and broken symmetry states using the Heisenberg spin Hamiltonian. Very good agreement between theory and experiment was achieved with the B3LYP hybrid functional. The g-tensor calculations were performed employing the coupled perturbed Kohn-Sham equations. A strategy for the computation of g-tensor site values is presented and provides single-site g-tensors that are in good agreement with the expectations for Mn-III and Mn-IV, respectively. Spin projection gave the g-tensor of the coupled manganese complex in very good agreement with the experimental results. Complete Mn-55 hyperfine tensors, including spin-orbit contributions, were calculated and spin-projected. The source of anisotropy in this system could be traced back to the Mn-III ion in line with the experimental results. The isotropic manganese hyperfine coupling constants were underestimated by factors between 1.4 and 2.5. It is shown that this deficiency is systematic in character and not anchored in the broken symmetry approach. Nuclear quadrupole splitting of the Mn-55 nuclei is shown to be small in this system. In addition, N-14 and H-1 ligand hyperfine data were calculated and compared well with the experimental results. The quality of the extended point-dipole model was demonstrated in application to H-1 anisotropic hyperfine coupling constants.