Detection of electron paramagnetic resonance signals from three different isotope combinations 63Cu/63Cu, 63Cu/65Cu and 65Cu/65Cu in stable dicopper(I) radical complexes

Abstract

The new stable paramagnetic cations [(R′R₂P)₂Cu(adcOR″)Cu(PR₂R′)₂]⁺, adcOR″= azodicarboxylic ester, have been characterized by high-resolution EPR spectroscopy. Two equivalent copper centres within one molecule give rise to isotope combinations ⁶³Cu/⁶³Cu (47.9%), ⁶³Cu/⁶⁵Cu (42.6%) and ⁶⁵Cu/⁶⁵Cu (9.5%); both isotopes have I= 3/2, but the nuclear magnetic moment is 7.1% higher for ⁶⁵Cu. A theoretical analysis of the hyperfine splitting pattern, including a stick spectrum and actual computer simulations, is given, and experimental spectra could be successfully reproduced in this way. Factors favouring the detection of individual isotope combination lines, particularly in the outer spectral regions, are the small number of coupling nuclei (¹⁴N, ³¹P) in the ligands, the rather narrow EPR lines of these ligand-centred radical complexes and the relatively large isotropic copper coupling constants of ca. 1.6 mT.