Proton Magnetic Resonance Study of Polycrystalline HCrO2

A proton magnetic resonance study of polycrystalline HCr${\mathrm{O}}_2$ as a function of magnetic field and temperature is presented. HCr${\mathrm{O}}_2$ is paramagnetic, and electron paramagnetic dipole as well as nuclear dipole effects lead to line broadening. The lines are asymmetric and over the range of field $470\le H_0\le 9400$ gauss and temperature $77°\le T\le 294°$K the asymmetry increases with increasing $H_0$ and decreasing $T$. An isotropic resonance shift of ∼0.03% to lower applied fields indicates a weak isotropic hyperfine contact interaction. The general theory of resonance shifts is used to derive a general expression for the second moment $M_{2p}$ of a polycrystalline paramagnetic sample and is specialized to HCr${\mathrm{O}}_2$. The theory predicts a linear dependence of $M_{2p}$ on ${\left[\frac{H_0}{(T+\theta )}\right]}^2$, where $\theta$ is the experimentally determined Curie-Weiss constant. The experimental second moment ${M_{2p}}^{\prime }$ conforms to the relation ${M_{2p}}^{\prime }=(3.96\mathrm{}\pm 0.06)+(0.084\pm 0.002){\left[\frac{H_0}{\mathrm{}(T+276\mathrm{})\mathrm{}}\right]}^2$ in agreement with theory. Hence, the electron paramagnetic effects (slope) can be separated from the nuclear effects (intercept). The paramagnetic dipole effects provide some information on the particle shapes. The nuclear dipole effects provide some information on the motions of the hydrogen nuclei, but the symmetry of the O-H-O bond in HCr${\mathrm{O}}_2$ remains in doubt.