High-Frequency Electron Paramagnetic Resonance Studies of VO²⁺ in Low-Temperature Glasses

Abstract

First-derivative electron paramagnetic resonance (EPR) absorption spectra of the vanadyl ion (VO 2+ ) in glasses of five different organic−aqueous cosolvent mixtures at 10 K are reported over the 9.4−376 GHz microwave frequency range. Whereas absorption features arising from the g -parallel and g -perpendicular components of the g e tensor are overlapping at low frequencies, it is shown that they are completely separated at microwave frequencies >110 GHz. The analysis of the EPR spectra revealed that line-broadening is due to g -strain over the 9.4−376 GHz range. EPR spectra of VO 2+ bound to bovine transferrin in a 1:1 molar ratio were also compared at 9.4 and 110 GHz. The results for the VO 2+ −transferrin complex showed that absorption features corresponding to the g -parallel and g -perpendicular components of the g e tensor were similarly separated at high frequency and that line-broadening was attributable to g -strain. We show that line-broadening characteristic of g -strain under conditions of low g -anisotropy can be explained by an S = 1 / 2 spin system in which the principal g values are themselves the random variables described by a normal distribution. On this basis, it is shown that line-broadening is proportional to the microwave frequency and the spread of each of the principal values of the g e tensor.