NMR Relaxation Study of Liquid CCl3F. Reorientational and Angular Momentum Correlation Times and Rotational Diffusion

Abstract

Using pulsed NMR techniques, values of the self‐diffusion constant D_s and the ¹⁹F spin‐lattice and rotating frame relaxation times, $T_1^F$ and $T_{\text{1ρ}}^F$ have been obtained for CCl₃F over its entire liquid range. (∼150–450°K). The dependence of $T_{\text{1ρ}}^F$ on the rotating field strength ${\omega }_1$ has been used to derive temperature‐dependent values of the ³⁵Cl spin‐lattice relaxation time $T_1^{\mathrm{Cl}}$ and the chlorine to fluorine spin‐spin coupling constant $J_{{}^{19}{\mathrm{F–}}^{35}\mathrm{Cl}}\text{(=11.9± 0.4}\mathrm{Hz}$ , independent of temperature). Except at low temperatures where the intermolecular dipole‐dipole relaxation mechanism is important, $T_1^F$ is dominated by the spin‐rotation interaction ${\mathrm{(T}}_1^F{)}_{\mathrm{sr}}$ . Using D_s data to separate the dipole‐dipole contribution from $T_{1\mathrm{sr}}^F$ allows us to estimate values of the angular momentum correlation time τJ over a 300° temperature range. Over the same temperature range, values of $T_1^{\mathrm{Cl}}$ give the correlation times for molecular reorientation ${\tau }_{\text{θ,2}}$ . Although possible anisotropy in molecular motion and in the spin‐rotation interaction preclude rigorous quantitative comparisons with rotational diffusion theory, the results for τJ and ${\tau }_{\text{θ,2}}$ are shown to be consistent with Gordon's extended J diffusion model. In particular, at high temperatures the molecular reorientation is no longer described by the small angular steps implied in classical theory: near the critical temperature τJ and ${\tau }_{\text{θ,2}}$ become of comparable magnitude and correspond to angular steps approaching 1 rad.