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 T1ρ data the following parameters for the low- and high-temperature motions, respectively, Ea=2.85± 0.15 kcal/mole, τ0=(1.60± 0.1)× 10−13sec,Ea′=7.00± 0.40 kcal/mole, τ0′=(5.00± 0.3)× 10−15 sec.