ESR Studies of Methyl Radicals Stabilized on Silica-gel Surfaces

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The methyl radicals produced on silica-gel surfaces by the photolysis of methyl iodide or methane have been investigated by ESR. When the silica gel is preheated at high temperatures (above ca. 300°C), two methyl radicals which have abnormally small hfs constants are trapped on the surfaces. In the case of the photolysis of methyl iodide, these two radicals disappear by continuing uv-irradiation, and instead an ordinary type of methyl radical is produced. The former two radicals have nearly isotropic g-values, and these values are approximately equal to that of a free electron and are considered to have a non-planar structure. The adsorption sites for these radicals were estimated to be siloxane groups. On the other hand, when the silica gel is preheated at low temperatures (below ca. 300°C) the methyl radicals produced have hfs constants of the same magnitude as that of the ordinary type of methyl radical, but each proton line of the radicals is accompanied by four satellite lines. The outer satellite lines have been confirmed by measurements of the X- and K-band spectra to be a concurrent spin-flip transition of an odd electron and a nearby proton of the silica-gel surface. The models of the adsorption state of the radicals are considered.
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