19F N.M.R. and127I N.M.R. and N.Q.R. study of the molecular motions and deformations of iodine heptafluoride in its solid phases

Abstract

In solid IF7, 19F and 127I N.M.R. and 127I N.Q.R. absorption line shapes, frequencies and relaxation times have been studied from the melting temperature down to 56 K. Two new solid-solid transitions have been found at 180 K and 96 K in addition to the one already known at 153 K. The two high temperature phases are shown to be plastic. The fast molecular rotations and the slow molecular diffusion are studied from 19F and 127I relaxation times. In the ordered phases, an analysis of the 19F second moments and longitudinal relaxation times shows the existence of a fast intramolecular exchange between axial and equatorial fluorine atoms, together with a much faster reorientation about the D 5 axis. The characteristic times of these two motions are obtained and a mechanism for the exchange process is proposed. The chemical shift values of 127I, the temperature dependence of the N.Q.R. frequencies and of the N.Q.R. relaxation times of 127I, as well as the dipolar energy relaxation times have been measured and are discussed.