Theoretical Study of Ligand Superhyperfine Structure. Application to Cu(II) Complexes

A theoretical and computational study of the nitrogen superhyperfine structure in Cu(II) complexes is reported. The determination of hybridization parameters for nitrogen donor orbitals from the data is examined. For most Cu(II) complexes the results deviate substantially from pure "sp2" or "sp3" hybridization. Semiempirical INDO/S calculations for five Cu(II) complexes were carried out at the UHF and ROHF level. The results suggest that the small anisotropy in the nitrogen hyperfine parameters is caused by spin polarization of the nitrogen valence shell orbitals. A simple, approximate way for the determination of the π-spin density from experimental data is outlined. A density functional study using various basis sets and functionals is reported for the same five complexes. Hybrid functionals, such as B3LYP and PWP1, give better predictions than functionals based on the generalized gradient approximation like BP or BLYP. Provided that at least a polarized triple-ζ basis is used, the hybrid functionals B3LYP and PWP1 give good predictions for the isotropic couplings but overestimate the anisotropic part by almost a factor of 2. The computational results are further analyzed in terms of local versus nonlocal contributions, influence of scalar relativistic and spin−orbit coupling effects and the transferability of quasi-atomic hyperfine couplings.