High-frequency electron paramagnetic resonance studies of VO2+ in low-temperature glasses

First-derivative electron paramagnetic resonance (EPR) absorption spectra of the vanadyl ion (VO2+) 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 VO2+ bound to bovine transferrin in a 1:1 molar ratio were also compared at 9.4 and 110 GHz. The results for the VO2+-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.