Broadening by strains of lines in the <i>g</i>-parallel region of Cu2+ EPR spectra

Abstract

Linewidths of the mI=−3/2 and −1/2 hyperfine lines in the g-parallel region of several square-planar copper complexes with oxygen ligands in frozen solutions have been measured at 2.6, 3.8, and 8.9 GHz. A theory has been developed in terms of distributions of molecular bonding parameters that satisfactorily accounts for the observed dependence of linewidths on mI and the microwave frequency. It has been found that distributions of the bonding parameters are strongly correlated, indicating that copper complexes when subjected to strains during freezing distort in a well-defined, predictable manner. It is found that the mI=−3/2 line generally is narrowest near 6 GHz and the mI=−1/2 line near 2 GHz. By proper selection of the microwave frequency, improved spectral resolution can be obtained. Complexes with nitrogen ligands also have been examined. Proper selection of microwave frequency often permits observation of superhyperfine structure, and even when it cannot be seen, analysis of the linewidth variation with microwave frequency yields information on nitrogen ligation. Octave bandwidth S-band EPR spectroscopy is emphasized as a preferred method for studying copper complexes.