Until recently the study of paramagnetic resonance in liquid solutions did not attract much attention, although this effect was discovered by Zavoisky for solutions of Mn2+ salts as early as 1944.1 The author of this paper and also his coworkers, N. S . Garifyanov and A. I. Rivkind, carried out studies of electron and proton paramagnetic resonance in a number of solutions of VO2+, Cr3+, Mn2+, Fe3+, Fe2+, Cozf, Ni2+, Cu2+ and Gd3+ salts in water and in other solvents (ethyl alcohol, glycerol, acetone). Measurements at high frequencies (-10,000 Mc/s) were carried out by the passing wave method using modulation of the magnetic field and at lower frequencies by the grid-current method of Zavoisky.1 A measurable electron paramagnetic effect 2 was found in solutions of salt of VO2+, Cr3+, Mn2+, Cu2+ and Gd3f.Of the ions mentioned, sufficiently dilute aqueous solutions of V02+ and Mn2+ salts exhibit a hyperfine structure of absorption lines at room temperature.Solutions containing the remaining ions yield at room temperature absorption curves with a single maximum. Absorption curves with hyperfine structure will be considered first.Of the solutions of VO2+ salts the sulphate and the chloride solutions were studied by Garifyanov and the author of this paper.3 In these, resolution of the hyperfine line structure begins at V02+ ion concentrations of about 2 to 3 moles/l. The effect is observable up to 0.01 mole/l. The position of the lines of the hyperfine structure is excellently described by the equation A2 Azm H* = HO -Am --[I(I + 1)mz] --(2M -l),
2HO* Former observations,6 which showed the presence of very weak absorption maxima at low frequencies, attributed to the effect of the spins of the copper nuclei, were not confirmed in later experiments.
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