Nuclear magnetic resonance inestigations at high....pressure

Abstract

In this review it has been attempted to give a summary of the MR experiments which have been used to investigate high pressure effects on the molecular motion, diffusion and phase transitions in solids. These experiments show that the effect of hydrostatic pressure is more on diffusive motion than on reorientational motion. To achieve measurable effect on the reorientational motion either very high pressures at room temperature or moderately high pressures at low temperatures are required. The effect of pressure on the phase transition temperature in solids is marked and in some cases it produces new phases. In the later chapters of this thesis the effect of moderately high pressures on molecular motion in solids will be presented. Experiments have been performed on methyl ammonium halides and N?deuterated methyl ammonium bromide and iodide as a function of pressure and temperature. It is observed that under the application of pressure, the barrier for the NH? motion increases, whereas the barrier for the CH? motion shows less observable change. It is concluded that changes in cell volume have an appreciable effect on the barriers to the motion of the groups, through changes in hydrogen bond lengths. Proton and fluorine resonance spectra have been recorded between 77°K and room temperature and also at pressures between 1 kb and 14 kb. It has been observed that the application of hydrostatic pressure has no observable effect on the motion of the tunneling ion. On the other hand, a pressure of 14 kb shifts the second moment transition temperature of F¹? resonance to a higher temperature by approximately 15°K and increases the barrier for the PF? reorientational motion to 4.96 Kcals/mole. The fluorine resonance line has been observed to be asymmetric close to 77°K in MPF? and ND?PF? and this has been explained on the basis of chemical shift non?equivalence of the PF? fluorine nuclei.