A reaction mechanism for the fourhyphen;center exchange reaction H2plus;D2rarr;2HD is described in detail. The principal idea is that H2and D2react with higher probability when the energy present originates in vibrational rather than translational energy. Detailed calculations are presented which show that the mechanism agrees with available results of shock tube experiments, when experimental rate constants are used for the vibrational relaxation. It is shown that the energy necessary for reaction to occur is about 38 kcal, in addition to the vibrational zerohyphen;point energy, and that the main part of the experimental HD yield is formed in elementary reactions which involve either D2(3)or H2(2), where the superscripts are the vibrational quantum numbers. The concentrations of these species are depleted below the equilibrium values, and in this way the mechanism explains the surprising experimental dependence of the reaction rate on the concentration of Ar, which constitutes about 90percnt; of the reaction mixture. It is very plausible that the important elementary reactions are D2(3)plus;H2(1), D2(2)plus;H2(2), and possibly also D2(1)plus;H2(2), where in the latter reaction the translational energy must be at least 8 kcal. H2, D2pairs involving more vibrational energy are too rare to contribute significantly to the reaction. The calculations show that a stationary distribution of the molecules over the vibrational levels should be obtained experimentally within 4percnt; of the time of reaction. The mechanism implies an overhyphen;all rate expression of complex form.
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