Pressure Dependence of the NMR Chemical Shifts in Cesium and Rubidium Halides

The chemical shifts, line shapes, linewidths, and line intensities have been measured as a function of pressure and crystal structure from 1 to 104 kg/cm2 at room temperature for both metal and halide nuclei in alkali halide pressed powder pellets. Measurements were made on Rb87, Cs133, Cl35, Br81, and I127 in RbCl, RbBr, RbI, CsBr, and CsI. The chemical shifts become more paramagnetic with increasing pressure within any one phase. At the transition from NaCl to CsCl-type crystal structure exhibited by the rubidium halides, the Rb87 chemical shifts become less paramagnetic, and the halide chemical shifts become more paramagnetic. As usual, the magnitude of the effects increases with increasing atomic number. The correction of the chemical shifts for second-order quadrupole interactions is discussed. The data are compared to chemical shifts calculated from a covalent model of Yosida and Moriya, and an overlap model of Kondo and Yamashita. The overlap model is found to give reasonably good agreement with the data if it is modified to include next-nearest neighbor effects in order to explain the behavior of the chemical shifts at the transition. The covalent model is by itself inadequate to explain the data, but an improvement in the agreement with the data is possible if the covalent model is used to modify the overlap model. However, the predicted chemical shifts and their changes with pressure for the CsCl-type crystal structure are still too large compared to the data. The absolute values of the chemical shifts are in doubt; Itoh and Yamagata have advanced the possibility that the ion in dilute aqueous solution, which is used as the reference from which the chemical shift is measured, has a nonzero chemical shift with respect to the isolated ion. This difficulty is avoided by comparing with the models only relative changes in the chemical shifts. Further evidence is advanced for the existence of this shift.