| Résumé | An experimental and theoretical study is reported of the abstraction of hydrogen atoms by methyl radicals from organic glasses, in particular methanol and several of its deuterated analogues. Rate constants are obtained for hydrogen and deuterium transfer as a function of temperature for a distribution of trapping sites of the radical in the glass. The site dependence is investigated by an analysis of the non-exponential decay of the methyl radical concentration based on the use of Laplace transforms. With the help of a recently developed theoretical model for hydrogen tunnelling, a relationship between tunnelling rate and tunnelling distance is established on the basis of these observations. The results yield a detailed picture of the structure of the trapping sites: roughly spherical cages in which the tumbling radical is surrounded on average by eleven rotating methyl groups, with the closest of which it reacts. The model yields a quantitative description of the rate of this reaction, expressed in terms of spectroscopic, thermodynamic and quantum-chemical input parameters. |
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