Variational transition state theory calculations of the reaction rates of F with H₂, D₂, and HD and the intermolecular and intramolecular kinetic isotope effects

We use variational transition state theory to calculate rate constants and kinetic isotope effects for the reactions F+H₂→HF+H (with rate constant k₁), F+D₂→DF+D(k₂), and two other isotopic analogs as functions of temperature. The calculations are performed using a recently proposed partly empirical, partly ab initio potential energy surface, called surface No. 5, and also using a new surface, called surface No. 5A, introduced here to test the effect of a higher classical saddle point on the reaction rates, kinetic isotope effects, and reaction thresholds. The various theoretical results are compared to the available experiments to test the validity of these potential energy surfaces. For those rate constants and kinetic isotope effects for which there is more than one experimental value at a given temperature, the theoretical results for reactions on surface No. 5 agree with experiment about as well as the individual experiments agree with each other. At T > 373 K where there is only one experimental measurement for k₁ and k₂, the theoretical rate constants for surface No. 5 are up to 44% lower than experiment, and at T < 190 K where there is only one experimental measurement of k₁/k₂, the theoretical rate constant ratio is as much as 43% low. These discrepancies could be due to experimental error or they could indicate that the theoretical activation energy is low, but by no more than 0.5 kcal/mol, which is considerably less than the 2.5 kcal/mol discrepancy from the most recently published large-scale calculation. If, however, the potential energy surface were adjusted to raise the activation energy by even 0.5 kcal/mol above the value for surface No. 5, the threshold energies would no longer be in such good agreement with experiment, and several other aspects of the results presented here would be in much worse agreement with experiment. Taken as a whole then, we interpret the present calculations as providing further confirmation of the methods used to obtain surface No. 5. With this interpretation, the most important implication about the experiments is that Persky's low-temperature measurements of k ₁/k₂, especially those for T≥200 K, are systematically high.