14N,1H, and metal ENDOR of single crystal Ag(II)(TPP) and Cu(II)(TPP)

Abstract

The results of ENDOR investigations of magnetically dilute single crystals of Ag(II) and Cu(II) tetraphenylporphyrins (TPP) are presented and discussed. The complete hyperfine tensors, and quadrupole tensor for nitrogen, have been determined for the metal ions, nitrogens and pyrrole protons. The non-first order ¹⁴N ENDOR spectra resulting from multiple equivalences among the nitrogen nuclei have been completely analysed by exact diagonalization. The standard ligand-field treatment of electron-nuclear coupling parameters can largely accommodate the ¹⁴N results; among other conclusions, it is shown that the nominally d-like odd-electron is only ⩽ 62 per cent 3d in Cu(TPP) and in fact is only ⩽ 38 per cent 4d in Ag(TPP). This treatment cannot describe the observed degree of σ-π mixing manifested in the significantly non-axial ¹⁴N hyperfine tensors observed. The ¹⁴N nuclear quadrupole couplings are analysed to obtain the total electronic distribution at nitrogen; in addition, the assignment of quadrupole tensor orientation and sign is discussed. The pyrrole proton hyperfine couplings are indicative of both the M(TPP) site geometry and the extent and mode of delocalization of the unpaired electron. The predictions of recent X_α calculations on Cu (porphine) are compared to the experimental results.