Detection of rubidide anion (Rb-) in crystalline rubidides by rubidium-87 nuclear magnetic resonance

The rubidide anion, Rb/sup -/, was detected in crystalline alkalides by /sup 87/Rb magic-angle-spinning (MAS) NMR spectroscopy. The compounds Rb/sup +/ (15-crown-5)/sub 2/Rb/sup -/, Cs/sup +/(15-crown-5)/sub 2/Rb/sup -/, and Cs/sup +/(18-crown-6)/sub 2/Rb/sup -/ were found to be pure rubidides with the same isotropic chemical shift of -186 +/- 2 ppm relative to Rb/sup +/(aq). The frequency dependence of the chemical shift and line width correspond to a quadrupole coupling constant of 1.2 +/- 0.2 MHz for Rb/sup -/. The systems RbK(15-crown-5)/sub 2/, RbK(18-crown-6), and Rb(18-crown-6) are mixtures. Samples of Rb/sub 2/(cryptand 2.2.2) and Rb/sub 2/(18-crown-6) that were expected to contain Rb/sup -/ showed no signal. Although the signal of Rb/sup +/ could be detected in the simple salts RbCl, RbI, and RbSCN, the complexed rubidium cation could not be detected in the solid state. Signals from both Rb/sup +/(15-crown-5)/sub 2/ (..delta.. nu/sub 1/2/ approx. = 5000 Hz) and Rb/sup -/ (..delta.. nu/sub 1/2/ = 150 Hz) were detected in solutions of Rb/sup +/(15-crown-5)/sub 2/Rb/sup -/ in dimethyl ether in the presence of excess 15-crown-5. This work represents the first observation of the NMR spectrum of Rb/sup -/ in solids and of complexed Rb/sup +/ in solution. 32 references, 3 figures,more » 1 table.« less