Study of Phase Transitions by Means of Nuclear Magnetic Resonance Phenomena

Abstract

Although the nuclear magnetic resonance condition generally depends only on the gyro-magnetic ratio of the resonating nuclei, the width and amplitude of the resonance depend critically on the substance containing the resonating nuclei. A basic factor affecting the line width is the characteristic flipping time for the substance, i.e., the average time it takes a molecule to change its orientation appreciably. This fact has been applied in a study of a group of molecular solids which exhibit two or more phases in order to investigate the mechanisms of these transitions. Of the two mechanisms proposed, rotational and order-disorder, the former was eliminated in favor of the latter for HCl, HBr, HI, ${\mathrm{H}}_{2}$S, and ${\mathrm{H}}_{2}$Se, while free molecular rotation is shown to be plausible in C${\mathrm{H}}_{4}$ and C${\mathrm{H}}_{3}$D.