| Résumé | Rate parameters are reported for hydrogen and deuterium abstraction of methyl radicals embedded in glassy mixtures of CH3OH and CD 3OD. The mole fraction of CH3OH in these isotopomeric mixtures is 0, 0.05, 0.075, 0.10, 0.15, or 1. The nonexponential time dependence of the radical concentration is analyzed in terms of distributions of first-order rate constants. For the isotopomerically pure matrices, lognormal distributions describe the decay satisfactorily. The large difference between characteristic H and D transfer rate constants indicates tunneling. In the mixtures, there is competition between H and D abstraction processes which depends on the local structure about a radical, so that the corresponding rate parameters contain information about this structure. On the basis of earlier work [J. Chem. Phys. 86, 6622 (1987)], the analysis begins with the assumption that the structure about a radical resembles one of the crystalline phases of methanol. The entire set of decay curves is described by a (disordered) β-phase structure in which the radical replaces a methanol molecule and is located near the position associated with a methyl group. However, this static picture is inadequate because the radical can diffuse through the glass on the time scale of the kinetic measurements. Diffusion allows the radical to encounter more CH3OH molecules than would be expected for the static structure on a statistical basis - the effective mole fraction of CH 3OH in the mixtures is higher than the analytical concentration. For the xH=0.05 mixture, we estimate that on the average the radical encounters approximately 26 methanol molecules before abstraction occurs. This corresponds to diffusion over roughly 1100 pm through the lattice. © 1993 American Institute of Physics. |
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