Xα multiple scattering calculations on copper, silver, and gold porphines

Xα calculations are presented for copper, silver, and gold porphines, and the results are compared to other calculations and to experimental data. Particular attention is paid to comparisons of electron spin resonance and ENDOR data on the copper and silver complexes. These results show that the nominally d-like odd electron has much more ligand character in the silver than in the copper complexes. The anisotropic parts of the hyperfine tensors for the metal, nitrogen, and hydrogen nuclei are in excellent agreement with measured values; spin-polarization effects give a good qualitative account of the nonaxial character of the nitrogen hyperfine tensor. The calculations underestimate the Fermi contact contributions for the metal and pyrrole hydrogens by 30-60%; smaller errors in the opposite direction are found for the nitrogens. The calculated ¹⁴N quadrupole coupling constants are also in poor agreement with experiment. Orbital energy trends (including relativistic corrections) for the series copper-silver-gold can be used to rationalize electrochemical and spectroscopic studies, but there is evidence that the d-orbital energies in these calculations are too low relative to the porphyrin orbital energies. Finally, the details of the calculated spin distribution are used to indicate the limitations and ambiguities inherent in the usual ligand field interpretations of ESR and ENDOR spectra.