87Rb, 85Rb, and 39K NMR Studies of Alkalides, Electrides, and Related Compounds

Abstract

Alkali metal anions generally have narrow NMR lines that are only slightly shifted from those calculated for the gaseous anion. When, however, the anions form contact dimers, as in K+(cryptand[2.2.2])K- and Rb+(cryptand[2.2.2])Rb-, or chains, as in Rb+(18-crown-6)Rb-, the NMR peak of the alkali metal anion is significantly broadened and shifted paramagnetically. In order to obtain reliable quadrupole coupling constants, asymmetry parameters, and chemical shifts from polycrystalline samples of alkalides and electrides, spin−echo 87Rb, 85Rb, and 39K static NMR spectra were obtained for 9 alkalides, 3 electrides, and 11 model compounds. A puzzling distortion of the powder line shapes of two alkalides, an electride, and a model salt was traced to orientation-dependent transverse relaxation times (T2) by studies of the single-crystal 87Rb NMR spectra of Rb+(cryptand[2.2.2])Cl-. To analyze the NMR powder patterns, a general computer program was developed and coupled to a nonlinear least-squares fitting program.