Isotope effect in the reaction of methyl radicals with methanol at 77-110 K

The mass-spectrometric method has been applied to study the isotope composition of methane in the CD3 radical reaction with matrix of glass-like methanol CH3OH+CD3OH. The isotope effect has been determined by the formula eta=(1-c)[CD3H]/c[CD4] where C is the fraction of CH3OH in the mixture. The isotope effect at 77 K is practically independent of the composition and is found to be: eta=75. With temperature increase the isotope effect begins to depend on the matrix composition decreasing with increase in the CH3OH content. The temperature dependence of eta has its maximum at c<=0.8. For all the cases eta lies in the 4-300 range. Reasons which can bring about a low value of the isotope effect and its dependence on the composition and temperature are discussed. It is concluded that attaining suitable packing of the radical nearest environment, but not the reactions, is the limiting stage of H atom detachment in the given system