Alkali metal NMR chemical shielding as a probe of local structure: An experimental and theoretical study of Rb+ in halide lattices

When Rb+ is incorporated into different cubic halide lattices at very low c oncentrations, the Rb-87 NMR chemical shift becomes more shielded by as muc h as 282 ppm when the cation is changed from Na+ to Cs+. Similarly, in mixe d crystals of KCl and RbCl, the average resonance frequency shifts with the degree of incorporation. The Rb-87 chemical shift data for each halide sho w a near-linear correlation with the average alkali metal-halide interionic distance in the different crystals studied, in good agreement with ab init io calculations which show that indeed the chemical shielding has a strong dependence on the nearest neighbor shell. of halide anions and their distan ce from the Rb+. When the Rb concentration is increased as in KCl/ RbCl sol id solutions, the Rb-87 NMR spectra are influenced by a large distribution of K/Rb configurations in the next-nearest-neighbor shell of 12 cations. Wh ile the total shift is still largely dependent on the Rb-Cl distance, shell s containing both K and Rb atoms give rise to second-order quadrupolar effe cts and an offset from the isotropic chemical shift. This wort; is a remark able demonstration of the sensitivity of the Rb-87 nucleus to its physical environment.