Electron spin resonance studies of the solvation of superoxide ions. Radiation generation of O2– at 4.2 K

Abstract

Exposure of glassy solutions of molecular oxygen to X-rays at 4.2 K gave a marked inhibition of the formation of e_t^– centres indicating the formation of O₂^–. For methanol and aqueous methanolic solutions a very broad parallel feature centred at ca. g= 2.095 was detected at 4.2 K, but for ethanol and other solvents no such feature could be detected. On warming to temperatures between 4 and 90 K the parallel feature for methanolic systems became far better defined and moved to the equilibrium value of g= 2.078. For ethanol solutions a broad feature at ca. g= 2.103 appeared at ca. 40 K and this gave way at ca. 90 K to the normal equilibrium feature at g= 2.081.

These results suggest that the environment of O₂ in methanol is such that initial hydrogen-bond transfer to O₂^– is facile even at 4.2 K, probably via a bifurcated hydrogen-bond mechanism. However, for ethanol and higher alcohols the O₂^– is too far from hydrogen-bonded OH groups for such a mechanism to operate.

Our results are compared with similar data for the solvation of trapped electrons.