A Density Functional Study of EPR Parameters for Vanadyl Complexes Containing Schiff Base Ligands

Abstract

Deviations of the coordination arrangement of vanadyl complexes from a regular square pyramid are thought to influence, among other things, their biological function. Such structural distortions have been found to be reflected characteristically in EPR spectra (Cornman et al. Inorg. Chem. 1997, 366, 6401). In this work, density functional calculations of electronic g tensors and metal hyperfine coupling tensors have been carried out for a series of four of these vanadyl complexes with structures ranging from nearly trigonal bipyramidal (TBP-5) to nearly square pyramidal (SQP-5). The EPR spectroscopic parameters have been rationalized in terms of electronic and geometrical structures. Using all relevant perturbation operators together with local or gradient-corrected density functionals, Δg-tensor components are underestimated systematically by ca. 40%. Good agreement with experiment is obtained for hyperfine tensor components calculated with hybrid functionals (B3PW91 and BHPW91), which account better for the spin polarization of the core orbitals than GGA functionals such as BP86. The rhombicity of the hyperfine tensor is reproduced well at all levels of theory applied. It is mainly determined by the SOMO composition. The latter explains the increasing rhombicity of the A tensor with increasing distortion of the SQP-5 structures along the series of complexes studied. The orientational dependence of the principal tensor components on the local vanadium coordination is much more pronounced for the g tensor than for the A tensor. The principal axes of the g and A tensors are found to be rotated with respect to each other by as much as 41°.