Isotope Exchange Rates. III. The Homogeneous Four-Center Reaction H2+D2

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Abstract
The homogeneous isotopic exchange reaction between hydrogen and deuterium in an excess of argon was studied in a 1-in.-diam single-pulse shock tube. Reaction times behind the reflected shock were close to 1 msec and the temperature range was 1060°—1420°K. The hydrogen and deuterium were present in concentrations from 1%—10%, at total reaction densities of 1.18 to 5.31×10−2 mole/liter. Conversions were kept low in order to minimize the reverse reaction. From the average rates the reaction was found to be second order (total) and empirically represented by Δ[HD]/Δt=kp[Ar]0.98[D2]0.66[H2]0.38,kp=1012.84T12exp(−42 260/RT),cm3mole·sec−1. This power-rate law, with the surprising asymmetry in the H2—D2 dependence, can be rationalized by assuming that exchange occurs with high probability only during encounters between H2—D2 pairs, one of which is vibrationally excited to approximately 30 kcal/mole; that the probability for metathesis is low between molecules in their low-lying vibrational states even when their relative kinetic energy along the line of centers plus vibrational excitation exceeds the activation energy. Support for this mechanism is provided by an approximate calculation of the pre-exponential term of the exchange rate constant from vibrational relaxation data.