Carbon in silica

Abstract

The structure of paramagnetic centers (PMC) in carbon-doped silica is studied by the EPR and quantum chemistry methods. Three types of radicals, ≡Si-·CH2, (≡Si-)2·CH and (≡Si-)3C·, are identified in which the free valence is localized on the impurity carbon atom. Their structure is determined by using samples enriched by isotopes 2D, 29Si, and 13C. The assignment of the obtained radiospectroscopic parameters of radicals is confirmed by quantum-chemical calculations of model systems. Based on the obtained data, a conclusion is made that paramagnetic centers (the so-called EX-centers) discovered earlier by the EPR method in silicon oxidation products have the structure (≡Si-)3C·, i.e. are not the intrinsic defects of the material but are related to impurity carbon atoms. Differences in the spectral characteristics of the (≡Si-)3C· groups observed in experiments are caused by the amorphous structure of silica. The kinetic nonequivalence of the (≡Si-)3C· radicals in the reaction of the hydrogen atom detachment from the H2 molecule is established (the activation energies for different PMC fractions lie in the range from 10 to 17 kcal/mol). The quantum-chemical calculations of model systems performed in the paper suggest that the differences observed in the reactivity of radicals are related to their spatial structure. It is found that the high-temperature pyrolysis of the (≡Si-)3C-H and (≡Si-O-)3Si-H groups is accompanied by the quantitative regeneration of free radicals (≡Si-)3C· and (≡Si-O-)3Si·. The probable mechanism of carbon atom embedding from. (≡Si-O-)3Si-O-CH3 groups to silica accompanied by the formation of (≡Si-)4C groups is analyzed.