NMR study of solid cholesterol

Abstract

Anhydrous polycrystalline cholesterol C₂₇H₄₆O has been investigated by pulsed proton magnetic resonance methods between 77K and the melting point (421·7K). In the low-temperature phase, measurements of the spin-lattice relaxation time T ₁ at 7, 14 and 28 MHz together with T _1D and second moment are ascribed to reorientation of the five methyl groups, one of which has significantly less hindrance. Values of the relaxation constants, activation energies and time factors that best characterize the motions are tabulated. Above the phase transition at 311K, reorientation of the alkyl chain about C-C bonds with activation energy 34·3 kJ mol^-1 is evident from the measured values of T _1D and second moment.