NMR Study of Molecular Reorientation in Solid Hexafluorobenzene (C6F6)

Abstract

Continuous wave and pulsed NMR studies were carried out on solid hexafluorobenzene (C₆F₆) from 270 to 77°K. The temperature dependence of the second moment exhibits two regions of motional narrowing and that of the spin‐lattice relaxation time shows minima at 142.5 and 260°K. The spin‐lattice relaxation in the rotating frame has a minimum at 172°K and decreases toward a second minimum at low temperature. Although the line shape is asymmetric in the entire temperature range, it differs between 100 and 145°K from that at higher and lower temperatures. This intermediate region line shape is interpreted as a superposition of two components from motionally inequivalent hexafluorobenzene molecules. The second moment and relaxation time data are consistent with a model in which two different sets of C₆F₆ molecules reorient around their sixfold axis at different frequencies. Using BPP expressions, we obtain from our T₁ and T_1ρ data the following parameters for the low‐ and high‐temperature motions, respectively, $$\begin{array}{c}{E}_a\text{=2.85± 0.15\hspace{0.17em}}\mathrm{kcal/mole}{\mathrm{,\; \tau }}_0{\text{=(1.60± 0.1)× 10}}^{\text{−13}}\sec ,\\ E_a^{\prime }\text{=7.00± 0.40\hspace{0.17em}}\mathrm{kcal/mole}{\mathrm{,\hspace{0.17em}\tau }}_0^{\prime }{\text{=(5.00± 0.3)× 10}}^{\text{−15}}\hspace{0.17em}\sec .\end{array}$$