ALKALI-METAL OXIDES, PEROXIDES, AND SUPEROXIDES - A MULTINUCLEAR MAS NMR-STUDY

The many roles of alkali metal-oxygen phases as catalyst components an d promoters motivated a survey of the NMR properties of these compound s. Solid state Li-7, Na-23, K-39, Rb-87, and Cs-133 magic angle spinni ng NMR spectra are reported for the following alkali metal oxides, per oxides and superoxides: Li2O, Li2O2 Na2O, Na2O2, NaO2, KO2, Rb2O2, RbO 2, Cs2O2, and CsO2. Na2O, Na2O2, NaO2, KO2, Rb2O2, RbO2, Cs2O2, and Cs O2. Commercial samples labeled as ''Cs2O'' proved to be mixtures of Cs 2O2 and CsO2. The superoxide (O-2(-)) anion is paramagnetic, and this property leads to highly temperature-dependent chemical shifts for the alkali metal nuclei in all of the superoxides. All four superoxides a lso show phase transitions between polymorphic forms of cubic and lowe r symmetry, and these transitions profoundly affect the alkali metal N MR spectra. For CsO2 and RbO2, purely ionic bonding restricts the para magnetic shift to the through-space dipolar mechanism, and large tempe rature-dependent shifts in the tetragonal phases vanish upon transitio n to the cubic phases. C-13 in situ MAS NMR experiments show that the oxide (M2O) compounds react as bases while the peroxides and superoxid es are both bases and oxidizers. Possible roles for peroxide and super oxide phases should be considered for catalysts formulated using alkal i metals, especially cesium.