The rate constants for the reactions of fluorine atoms with H2O and D2O have been determined over the temperature range 240-373 K by using a discharge flow system at 1-2 Torr of total pressure. F atoms were detected by chemical conversion with deuterium. The resulting D atoms were detected by atomic resonance scattering. The rate constants are k1 = (1.6 ± 0.3) × 10-11 exp[(-28 ± 42)/T] cm3 molecule-1 s-1 for F + H2O → HF + OH and k2 = (8.4 ± 1.2) × 10-12 exp[(-260 ± 110)/T] cm3 molecule-1 s-1 for F + D2O → DF + OD. The reported error limits are at the 95% confidence level. The reactions F + H2 → HF + H (k3 = (1.2 ± 0.1) × 10-10 exp[(-470 ± 30)/T] cm3 molecule-1 s-1) and F + D2 → DF + D (k4 = (9.3 ± 1.1) × 10-11 exp[(-680 ± 50)/T] cm3 molecule-1 s-1) were also studied over the same temperature range. The rate constants for the latter reactions are in excellent agreement with previous studies. The low activation energy and A factor for the F + H2O/D2O reactions, as well as the slightly enhanced kinetic isotope effect (3.9 at 298 K), suggest a tunneling mechanism for these reactions, and thus the mechanistic interpretation of this system requires some knowledge of the potential energy surface at the microscopic level. Ab initio calculations on this system predict a classical barrier height of approximately 10 kcal/mol, while a semiempirical BEBO calculation predicts a barrier height of <1 kcal/mol. Neither of these models is able to accurately reproduce the observed reaction rate parameters when used in transition-state theory and a one-dimensional tunneling model. With use of the theoretical implications in conjunction with the experimental evidence, a potential energy surface has been calculated for this reaction that leads to better agreement with the experimental results. The best-fit prediction of the observed activation energy and kinetic isotope effect suggest that the mechanism for this reaction involves tunneling through a classical barrier height of approximately 4 kcal/mol.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry