NMR Study of Molecular Motions and Phase Transitions in Cyclohexene

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Abstract
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.

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