Hydrogen abstraction reactions by atomic fluorine. III. Temperature dependence of the intermolecular kinetic isotope effect for the thermal F+H2 reaction

Abstract

The intermolecular kinetic isotope effect for the title reaction has been investigated using a moderated nuclear recoil technique. The following Arrhenius temperature dependence was obtained: kH2/kD2 = (1.04±0.06)exp[(382±35)/RT]. This result is in quantitative agreement with an independent study based upon a discharge flow reactor method. The advantages and limitations of the nuclear recoil procedure are considered, and relevant literature results are critically reviewed.

The kinetics of the reaction F + H2 ? HF + H. A critical review of literature data

“…It was noted that the results from the two research groups who studied this isotope effect over a wide temperature range [15,16] are in excellent agreement with each other. The differences between them are rather insignificant, being around 2% in the overlapping temperature range of the experiments.…”

“…It should be noted that values of the kinetic isotope effect k FϩH 2 /k FϩD 2 , calculated from the average values of k FϩH 2 and k FϩD 2 from these two groups [5,6], are in good agreement with those obtained by the competitive methods [15,16], with differences of less than 10% over the whole temperature range.…”

“…Their measurements gave a kinetic isotope effect smaller than unity at 195 K (0.88, inverse isotope effect) which increased with temperature (1.16 at 295 K). This is in contrast to the normal behavior of kinetic isotope effects and to the results obtained by the competitive methods [15,16] [14]. He estimates the rate constants and activation energies reported by them to be uncertain by a factor of at least two.…”

“…The Arrhenius expression obtained for the temperature range 163 -417 K is (3) This isotope effect has also been determined by Grant and Root [16] using an entirely different technique, also based on relative rate measurements. They obtained the Arrhenius expression k FϩH 2 /k FϩD 2 ϭ (1.04 Ϯ 0.06) exp [(192 Ϯ 18)/T ] for the temperature range 273 -475 K, which is in excellent agreement with our expression.…”

“…The differences between them are rather insignificant, being around 2% in the overlapping temperature range of the experiments. Since the results from [15] extend to much lower temperatures (163 -417) than the results from [16] (273 -475 K) and their error limits are much smaller, we chose eq. (3), which represents the results of ref.…”