DFT and ab initio dual-level direct dynamics studies on the reactions of fluorine atom with HOCl and HOBr.
The multichannel reactions (1) HOCl + F --> products and (2) HOBr + F --> products have been investigated using the dual-level direct dynamics method. The minimum energy paths (MEPs) are calculated at both the MPW1K/6-311G(d,p) and QCISD/6-311G(d,p) levels, then the single-point energies are further corrected at the QCISD(T)/6-311++G(3df,3pd) level of theory. There are hydrogen-bonded complexes with the energies less than those of the reactants or products located at the entrance or exit channel of both hydrogen abstraction reactions; while for the halogen abstraction channels only one complex exists at the reactant side in the bromine abstraction channel. The rate constants are evaluated by the improved canonical variational transition-state theory (ICVT). The agreement of the rate constants with available experimental values for two reactions at room temperature is good. Theoretical results indicate that for the reaction HOCl + F, hydrogen abstraction channel leading to the formation of HF + ClO will predominate the reaction over the whole temperature range, and the reaction of HOBr + F may proceed mainly through the bromine abstraction channel at the lower temperature while the contribution of hydrogen abstraction channel will become significant as the temperature increases. Because of lack of the kinetic data of these reactions, the present theoretical results are expected to be useful and reasonable to estimate the dynamical properties of these reactions over a wide temperature range where no experimental value is available.