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 Ds and the 19F spin-lattice and rotating frame relaxation times, T1F and T1ρF have been obtained for CCl3F over its entire liquid range. (∼150–450°K). The dependence of T1ρF on the rotating field strength ω1 has been used to derive temperature-dependent values of the 35Cl spin-lattice relaxation time T1Cl and the chlorine to fluorine spin-spin coupling constant JF19–Cl35(=11.9± 0.4 Hz, independent of temperature). Except at low temperatures where the intermolecular dipole-dipole relaxation mechanism is important, T1F is dominated by the spin-rotation interaction (T1F)sr. Using Ds data to separate the dipole-dipole contribution from T1srF allows us to estimate values of the angular momentum correlation time τJ over a 300° temperature range. Over the same temperature range, values of T1Cl give the correlation times for molecular reorientation τθ,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 τθ,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 τθ,2 become of comparable magnitude and correspond to angular steps approaching 1 rad.