Spectroscopic and Theoretical Studies of the Unusual EPR Parameters of Distorted Tetrahedral Cupric Sites: Correlations to X-ray Spectral Features of Core Levels

X-ray spectral data and self-consistent field-Xα-scattered wave (SCF-Xα-SW) calculations are presented for D_4h and D_2d CuCl₄^2- in order to evaluate specific contributions to the small hyperfine in distorted tetrahedral copper sites and to compare the mechanism of hyperfine reduction in these complexes with that for the blue copper site in plastocyanin. Comparison of the 2p_3/2 XPS data for the two geometries indicates that the extent of delocalization of the d_x2-y2 ground state in the D_2d salt is slightly less than in the D_4h complex. Multiplet splitting of satellite structure from Cu 3s photoemission shows no change going from D_4h to D_2d CuCl₄^2-, indicating that the indirect Fermi contact contribution to the hyperfine from the core Cu 3s level does not change between the two complexes. Analysis of X-ray edge data for Cs₂CuCl₄ (D_2d) indicates that there is at most 5.8% 4p_z mixed into the ground-state wave function, an amount that is insufficient to explain the reduced hyperfine in this distorted T_d complex. SCF-Xα-SW calculations performed with sphere radii adjusted so that the ground-state wave function fits the experimental g values indicate that ∼70% of the reduction in A_∥ between D_4h and D_2d comes from increased orbital angular momentum in the ground state of the D_2d salt arising from decreased ligand field transition energies. In contrast, increased delocalization relative to D_4h CuCl₄^2- accounts for most of the reduction in the blue copper proteins. The remaining 30% of the reduction in A_∥ is associated with a ∼50 × 10^-4 cm^-1 reduction in Fermi contact between the two salts. Xα calculations of a number of Cu complexes with <D_2d symmetry indicate that this reduction is not associated with direct 4s mixing into the half-occupied ground state but is most likely due to increased polarization of the filled totally symmetric valence levels in D_2d CuCl₄^2- resulting from increased 4s mixing as compared with D_4h CuCl₄^2-. These studies are then extended to include copper sites exhibiting rhombically split g and A values. Single-crystal optical and EPR studies on copper-doped bis(1,2-dimethylimidazole)zinc(II) dichloride (Zn[Cu] (dmi)₂Cl₂) combined with ligand field and SCF-Xα-SW calculations indicate that the rhombic features in these complexes can be explained through admixture of ∼3% d_z2 character in the ground-state wave function. Finally, the structurally uncharacterized blue copper protein stellacyanin, which also shows a rhombic EPR spectrum similar to that in Zn[Cu](dmi)₂Cl₂, is considered. In contrast to the C_3v effective symmetry found in the structurally defined site in plastocyanin, stellacyanin is predicted to have C_2v effective symmetry. A ligand field calculation using the plastocyanin site as a starting point indicates that stellacyanin requires a stronger field ligand along the Cu-methionine coordinate to produce the observed d_z2 mixing.