Fluorine-19 Nuclear Magnetic Resonance of the Liquid and Solid Phases of Fluorine

Fluorine‐19 NMR has been studied from 105 to 4.2°K in elemental fluorine. In the liquid phase the spin–rotational interaction determines the spin–lattice relaxation time ${\mathrm{(T}}_1)$ except within about 17°K of the melting point, where the effect of a small amount of dissolved molecular oxygen becomes evident. Self‐diffusion coefficients have been measured in the liquid from 105°K to the melting point (53.5°K) and are well represented by the expression ${\text{(6.6\hspace{0.17em}±\hspace{0.17em}0.3)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}\exp \text{[−\hspace{0.17em}(510\hspace{0.17em}±\hspace{0.17em}30)\hspace{0.17em}/\hspace{0.17em}RT]}{\mathrm{cm}}^2{\sec }^{\text{−1}}$ . Spin–rotational ${\mathrm{(\tau }}_1)$ correlation times have been calculated from the experimental relaxation times. Nuclear dipolar ${\mathrm{(\tau }}_2)$ correlation times have been estimated from the quasilattice‐random flight model. The product ${\tau }_1{\tau }_2$ is a factor of 20 longer than predicted by the continuum theory of rotational diffusion. A root mean square angular jump of 2.6 rad is estimated from comparison of the experimental data with the large angular jump model of molecular rotation. $T_1$ , rotating frame ${\mathrm{(T}}_{\text{1ρ}})$ , and spin–spin relaxation times were measured in the $\beta$ phase from 53.5 to 45.5°K. The $\beta$ phase is a plastic crystal with relatively rapid translational diffusion and very rapid, but anisotropic, rotational diffusion. $T_1$ is essentially unchanged from the liquid to the $\beta$ phase. The self‐diffusion coefficients are accounted for entirely by an increase in the work required to create a vacancy in going from the liquid to the $\beta$ phase. In the $\alpha$ phase, below 45.5°K, a small amplitude mode of molecular motion persists. A 15° “tilt” motion of the molecules is assigned to the minimum observed in $T_{\text{1ρ}}$ in the $\alpha$ phase. A shallow minimum in $T_1$ is assigned to spin diffusion to molecular oxygen impurities and occurs due to the proximity of the oxygen electronic spin–lattice relaxation rate to the fluorine larmor frequency.