Nuclear Magnetic Shielding of Alkali Ions in Crystals and Dilute Aqueous Solutions

The formulation for the Kondo-Yamashita overlap mechanism for the nuclear magnetic shielding in alkali halides developed earlier, is applied to the rubidium nucleus in RbCl, RbBr, and RbI crystals. The pertinent average energy denominators of perturbation theory are obtained using experimental pressure data on the magnetic shielding. Using these energy denominators, the calculated values of sigma(Rb), the chemical shifts for the rubidium nucleus in the crystal with respect to the free ion, are -2.17X10(-4), -2.14X10(-4), -2.11X10(-4) for RbCl, RbBr, and RbI, respectively. When combined with Baron's experimental data on the chemical shifts with respect to dilute aqueous solutions, we obtain for sigma(aq)(Rb), which represents the chemical shift between the Rb(+) ion in aqueous solution and the free ion, the values -0.88X10(-4), -0.63X10(-4), -0.60X10(-4) from the RbCl, RbBr, and RbI data, respectively. These values are in fair agreement with the result sigma(aq)(Rb) = -0.65X10(-4) that we have derived using a model of six oriented water molecules around the Rb(+) ion in dilute aqueous solutions.