The intramolecular kinetic isotope effect for the reaction O(3P)+HD

The branching ratio for the reaction O(³P)+HD to produce OH and OD was measured over the temperature range 339-500 K using a discharge-flow reactor. The OH and OD products were detected using laser-induced fluorescence under steady-state conditions. In order to determine the relative concentrations of OH and OD produced in this reaction, calibration runs were performed using the reactions of O(³P)+H₂ and D₂ to produce known relative amounts of the same species. Kinetic modeling showed that secondary reactions and the production of vibrationally excited OD did not affect the measured branching ratio. We found that the OH/OD ratio increased rapidly with decreasing temperature, in qualitative agreement with theory, showing that the reaction is dominated by tunneling below 400 K. At the lowest temperatures, the observed branching ratio is larger than predicted. The reason for this discrepancy is not known.