Prediction of EPR g tensors in simple d1 metal porphyrins with density functional theory

Electron paramagnetic resonance (EPR) g tensors of 20 five- or six-coordinated d(1) metal porphyrins following the [M=E(P)]-L structural motif (M = V(IV), Nb(IV), Cr(V), Mo(V); E = N, O, S, Se; P = porphyrin dianion; L = F-, Cl-, Br-, ClO4-, OH-, OCH3-, H2O, or not present) were computed using density functional theory (DFT). For all complexes, the singly occupied molecular orbital (SOMO) is dominated by the metal d(xy) orbitals. Qualitative trends in Delta g components are determined by magnetic-field-induced coupling of the SOMO with three classes of molecular orbitals (MOs): (a) beta-spin sigma MOs formed by the metal d(x2-y2) atomic orbital (AO) and the porphyrin ligand; (b) the corresponding vacant alpha-spin sigma* MOs; and (c) pairs of unoccupied alpha-spin pi* MOs formed between the metal d(xz) (d(yz)) AOs, p(x) (p(y)) AOs of the axial ligands, and the porphyrin pi system. The rich orbital system of the porphyrin ligand usually gives rise to multiple contributions of each type. As a consequence, electronic structure of the entire porphyrin ligand must be taken into account for the analysis of experimental g tensors. Values of the theoretical Delta g tensor components are systematical too positive compared to experiment. Once the systematic errors are accounted for, changes in the calculated g tensor components for complexes of metals from the same transition row are in good quantitative agreement with experiment. In oxomolybdenum porphyrinates [Mo=O(P)]-L, the sixth ligand L influences g tensors both through geometrical distortion of the invariant part of the complex and by direct electronic interactions. Changes in the orientation of g tensors upon coordination of the sixth ligand arise mostly due to the electronic effects. The importance of the direct contribution increases for more covalent ligands L. The S tensor components of the isolated [Cr=O(P)](+) cation, which has not been characterized by EPR so far, are predicted to be Delta g(parallel to) = -15 and Delta g(perpendicular to) = -20 ppt. The Delta g(parallel to) and Delta g(perpendicular to) values for the [Mo=O(P)I](+) complex are predicted to be -29 and -35 ppt.