Properties of methyl radical trapped in amorphous SiO2 and in natural SiO2-clathrate Melanoflogite

We report an experimental investigation by electron paramagnetic resonance (EPR) on methyl radical (CH3 • )observed in γ-ray irradiated high-purity amorphous silicon dioxide (a-SiO2) and in a polycrystalline sample ofMelanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlationtime of the hindered rotational motion of CH3 • molecules at T=77 K in the two different materials. Thisphysical quantity gives a quantitative measure of the freedom of motion of CH3 • molecules trapped in the twosolid systems, putting forward relevant information on the properties of the cavities/interstices inwhich the radicalsare confined. In particular, our data suggest that in a-SiO2 the CH3 • molecules are trapped in interstices withdiameter significantly lower than about 5.7 Å, which is the size of the smaller cavity involved in the crystallinestructure of Melanophlogite.