Pulsed nuclear magnetic resonance and molecular reorientation in solid 1,3,5-trifluorobenzene

Single and double-pulse transient experiments have been used for the direct measurement of the resonant and non-resonant spin contributions to the ¹H and ¹⁹F second moments in polycrystalline 1,3,5,-C₆H₃F₃. The applicability of these experiments to molecular solids is examined. The temperature dependence of the experimental proton second moment has been investigated between 130 K and the melting point at 267·7 K; a transition centred on 200 K is observed. It is shown that the molecules have an ordered arrangement in the crystal at all temperatures and that this transition is due to thermally activated reorientational jumps of the molecules through 120° about their triad axes. The activation energy for this motion is 42·7 ± 2·9 kJ mol^-1 as determined from the temperature dependence of the ‘apparent’ second moment in the transition region.