Ac. De Dios et al., Alkali metal NMR chemical shielding as a probe of local structure: An experimental and theoretical study of Rb+ in halide lattices, J PHYS CH A, 104(5), 2000, pp. 908-914
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.