Cross relaxation in NMR studies of crystalline symmetrical trifluorobenzene (C6H3F3)

摘要

Crystalline symmetrical trifluorobenzene (C6H3F3) was studied by1H and19F broadline and pulsed NMR techniques between sim;189thinsp;deg;K and its melting point (267.6thinsp;deg;K). The1H and19F static frame spinndash;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 Hndash;F dipolar interactions, which are due to inhyphen;plane rotation of the molecule about its triad symmetry axis. The rotating frame spinndash;lattice relaxation was found to be exponential for both nuclei in the entire temperature range of investigation. A decrease in the1H and19F second moments and line widths is obvserved on heating. This is consistent with the onset of the inhyphen;plane threefold rotation. In accordance with this line narrowing, a minimum in theT1vsTminus;1curves were obtained for1H and19F nuclei along with corresponding minima in theT1rgr;vsTminus;1plots. Calculations of theT1andT1rgr; temperature dependence using an activation energy (Ea) of 9.0plusmn;0.4 kcal/mole and a prehyphen;exponential factor (tgr;0) of 2.4(plusmn;0.2) times;10minus;15sec for the motion is in agreement with our experimental results.