Alkali Metal Oxides, Peroxides, and Superoxides: A Multinuclear MAS NMR Study

摘要

The many roles of alkali metal-oxygen phases as catalyst components and promoters motivated a survey of the NMR properties of these compounds. Solid state ~7Li, ~(23)Na, ~(39)K, ~(87)Rb, and ~(133)Cs magic angle spinning NMR spectra are reported for the following alkali metal oxides, peroxides and superoxides: Li_2O, Li_2O_2, Na_2O, Na_2O_2, NaO_2, KO_2, Rb_2O_2, RbO_2, Cs_2O_2, and CsO_2. Commercial samples labeled as "Cs_2O" proved to be mixtures of Cs_2O_2 and CsO_2. 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 also show phase transitions between polymorphic forms of cubic and lower symmetry, and these transitions profoundly affect the alkali metal NMR spectra. For CsO_2 and RbO_2, purely ionic bonding restricts the paramagnetic shift to the through-space dipolar mechanism, and large temperature-dependent shifts in the tetragonal phases vanish upon transition to the cubic phases. ~(13)C in situ MAS NMR experiments show that the oxide (M_2O) compounds react as bases while the peroxides and superoxides are both bases and oxidizers. Possible roles for peroxide and superoxide phases should be considered for catalysts formulated using alkali metals, especially cesium.