CROSS RELAXATION IN NMR-STUDIES OF CRYSTALLINE SYMMETRICAL TRIFLUOROBENZENE (C6H3F3)

Crystalline symmetrical trifluorobenzene (C6H3F 3) was studied by 1H and 19F broadline and pulsed NMR techniques between ∼189°K and its melting point (267.6°K). The 1H and 19F static frame spin-lattice relaxation is nonexponential in the entire temperature region, and can be described as a sum of two exponential terms. The same pair of time constants were observed for both nuclei at each temperature. This behavior is interpreted in terms of cross relaxation between proton and fluorine nuclei, which originates from fluctuations of the H-F dipolar interactions, which are due to in-plane rotation of the molecule about its triad symmetry axis. The rotating frame spin-lattice relaxation was found to be exponential for both nuclei in the entire temperature range of investigation. A decrease in the 1H and 19F second moments and line widths is obvserved on heating. This is consistent with the onset of the in-plane threefold rotation. In accordance with this line narrowing, a minimum in the T1 vs T-1 curves were obtained for 1H and 19F nuclei along with corresponding minima in the T1ρ vs T-1 plots. Calculations of the T1 and T1ρ temperature dependence using an activation energy (Ea) of 9.0±0.4 kcal/mole and a pre-exponential factor (τ0) of 2.4(±0.2) × 10 -15 sec for the motion is in agreement with our experimental results. © 1979 American Institute of Physics.