NMR Study of Molecular Motions and Phase Transitions in Cyclohexene

NMR measurements of the mean-square width and the spin–lattice relaxation time have been made in polycrystalline cyclohexane from 65°K to the melting point 169.7°K. The results show that cyclohexane has three solid phases and two forms of molecular reorientations. On cooling the specimen, phase III (169.7°–128.5°K) is obtained first, followed by phase I (128.5°–65°K). On warming the specimen to 110°K an irreversible transition from phase I to phase II takes place. Another irreversible transition from phase II to phase III takes place at 138.8°K. In phase I, the lattice is essentially rigid below about 100°K, while above this temperature, classical rotation about an axis normal to the “plane” of the molecule takes place. Above 115°K ring inversion of the half-chair form of the molecule is believed to take place, and in phase III this motion becomes effective in reducing the mean-square width. Phase II, which persists below 138.8°K, is characterized by the absence of molecular motion.