Validation of broken-symmetry density functional methods for the calculation of electron paramagnetic resonance parameters of dinuclear mixed-valence MnIVMnIII complexes

The EPR parameters of a series of dinuclear manganese(III,IV) complexes with mono(mu-oxo), bis(mu-oxo), bis(mu-oxo)(mu-carboxylato), bis(mu-oxo)(mu-carboxylato), and (mu-oxo)bis(mu carboxylato) bridges were studied by broken-symmetry density functional (DFT) methods. The influence of the exchange-correlation functional on the agreement with experiment has been evaluated systematically for g tensors; Mn-55, N-14, and H-1 hyperfine coupling tensors; and Heisenberg exchange couplings. N-14 and H-1 hyperfine couplings, Mn-55 hyperfine anisotropies, g tensors, and exchange couplings are well described by hybrid functionals with moderate exact-exchange admixtures such as B3LYP. The isotropic Mn-55 hyperfine couplings require larger exact-exchange admixtures. However, the errors of the B3LYP calculations are systematic and may be corrected by a constant scaling factor, providing good predictive power for a wide range of EPR parameters with broken-symmetry DFT and standard functionals. The influence of terminal and bridging ligands on structure, spin-density distributions, and EPR parameters are evaluated systematically. Computed hyperfine and g tensors are not covariant to each other. This may have consequences for spectra simulations. The nature of the broken-symmetry state and the origin of its spin contamination were analyzed by an expansion into restricted determinants, based on paired orbitals.