A paradox: The thermal rate coefficient for the H+DCl → HCl+D exchange reaction

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Abstract
Previously reported photolysis experiments indicate that the frequency factors associated with the hydrogen‐exchange reactions H+DCl μ HCl+D and D+HCl → DCl+H are on the order of 1010 cm3/mol⋅sec. This result indicates that the above processes are associated with very small steric factors, in contrast to what one might be led to expect from recent crossed beam experiments. A series of unadjusted, quasiclassical trajectory calculations have been carried out to compute the thermal rate coefficients and activation parameters for a series of 13 thermal processes of the type A+BC → AB+C, where A=H, D, or Cl and BC=H2, D2, HCl, DCl, or Cl2. In addition, hot‐atom yield ratios have been computed from the IRP equation for the reactions D*+DCl → D2+Cl, D*+Cl2 → DCl + Cl as a function of the initial D* laboratory energy. Previously formulated, valence‐bond representations were employed for the potential‐energy surfaces in all the calculations. The computations yield (1) hot‐atom [DCl]/[D2] yield ratios within a factor of 2 of the experimental values; (2) thermal activation energies in satisfactory agreement with experiment for all processes investigated; and (3) frequency factors in reasonable accord with experiment for all the reactions except the hydrogen exchange reactions, where the computed values are a factor of about 103–104 larger than indicated by the photolysis experiments. An investigation of a network of elementary thermal reactions that are apparently occurring in the photolysis experiments indicates that the presently computed thermal rate coefficients for the exchange reactions are too large to be compatible with observed HD and D2 quantum yields. These results make it clear that a serious error exists in either the potential‐energy surface, the theoretical scattering calculations, the measured photolysis data, or the interpretation of that data. The agreement of the computed hot‐atom yields, rate coefficients, and thermal activation parameters for all reactions except the hydrogen exchange reactions coupled with the results of recent crossed‐beam experiments strongly indicate that the error lies in the interpretation of the photolysis data.

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