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

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 of Melanophlogite, a rare natural form of SiO2-clathrate. From the analysis of the EPR spectra we estimate the correlation time of the hindered rotational motion of CH3 molecules at T = 77 K in the two different materials. This physical quantity gives a quantitative measure of the freedom of motion of CH3 molecules trapped in the two solid systems, putting forward relevant information on the properties of the cavities/interstices in which the radicals are confined. In particular, our data suggest that in a-SiO2 the CH3 molecules are trapped in interstices with diameter significantly lower than about 5.7 Å, which is the size of the smaller cavity involved in the crystalline structure of Melanophlogite.