Electron spin resonance analysis of motional effects of hyperoxide (O2-) absorbed in calcium-strontium- and barium-Y zeolites

Abstract

Electron spin resonance (ESR) spectra of ultraviolet photogenerated O2 in alkaline-earth-Y zeolites have been studied from 4 K to room temperature. A temperature-dependent ESR spectrum is observed for O2 only in Ba-Y zeolite. Analysis of the g tensor of O2 in Ba-Y, Ca-Y, and Sr-Y zeolites indicates that the crystal field interaction is weakest for Ba-Y zeolite and suggests that this is the reason why only this zeolite shows a temperature-dependent spectrum for O2 . The temperature-dependent ESR spectra were simulated by the modified Bloch equations in terms of g-anisotropy averaging of O2 . The biggest change occurs in the g1 tensor component which lies along the O-O bond direction. The motional model that fits the temperature-dependent spectra best corresponds to 180 rotational jumps about an axis between BaS and one oxygen of O2 . The O-O bond of O2 is nearly parallel to the interior zeolite surface, making an angle with the surface of only 16. The activation energy for the motion is 25 kJ mol . Other motion models are also discussed. 17 references, 10 figures, 2 tables.