Prediction of EPR g tensors in simple d(1) metal porphyrins with density functional theory

Electron paramagnetic resonance (EPR) g tensors of 20 five- or six-coordina ted 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 densi ty functional theory (DFT). For all complexes, the singly occupied molecula r orbital (SOMO) is dominated by the metal d(xy) orbitals. Qualitative tren ds in Delta g components are determined by magnetic-field-induced coupling of the SOMO with three classes of molecular orbitals (MOs): (a) beta-spin s igma 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 ri ch orbital system of the porphyrin ligand usually gives rise to multiple co ntributions of each type. As a consequence, electronic structure of the ent ire porphyrin ligand must be taken into account for the analysis of experim ental g tensors. Values of the theoretical Delta g tensor components are sy stematical too positive compared to experiment. Once the systematic errors are accounted for, changes in the calculated g tensor components for comple xes of metals from the same transition row are in good quantitative agreeme nt with experiment. In oxomolybdenum porphyrinates [Mo=O(P)]-L, the sixth l igand L influences g tensors both through geometrical distortion of the inv ariant part of the complex and by direct electronic interactions. Changes i n the orientation of g tensors upon coordination of the sixth ligand arise mostly due to the electronic effects. The importance of the direct contribu tion 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 fa r, 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) valu es for the [Mo=O(P)I](+) complex are predicted to be -29 and -35 ppt.