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

Previously reported photolysis experiments indicate that the frequency factors associated with the hydrogen-exchange reactions H+DCl → HCl+D and D+HCl → DCl+H are on the order of 1010 cm3/molcenter-dotsec. A series of unadjusted, quasiclassical trajectory calculations were 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. 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