Direct Dynamic Studies on Tropospheric Reactivity of Fluorinated Ethanes:  Scope and Limitations of the General Reaction Parameter Method

A direct dynamics study on the gas-phase reactions of OH radical with polyfluorinated ethanes has been carried out. Their thermal rate constants were calculated using canonical variational transition state theory augmented by multidimensional semiclassical small and large curvature tunneling approximations. The potential energy surface for the 1,1- and 1,2-difluoroethane reaction with hydroxyl radical was investigated with ab initio methods and a semiempirical PM3 Hamiltonian using specific reaction parameters (SRP). The reaction proceeds via hydrogen atom abstraction from both α and β carbon atoms with respect to fluorine substitution. In total, 26 stationary points were found, corresponding to the three and four reaction channels for 1,1- and 1,2-difluoroethane, respectively. Reactant molecules and product radicals, transition state structures, and pre-reactive complexes were characterized. Pre-reactive complexes are formed on both sides of the reaction path, directing the reaction to the different reaction channels. The main interactions between reactant and product molecules in the pre-reactive complexes are weak hydrogen bonds between hydrogen atoms from the OH radical or water, and fluorine atoms from the hydrocarbon moiety. Data obtained from the electronic structure calculations were further used to calculate the reaction rate coefficients. Variational transition state theory was used for that purpose in terms of the interpolated and direct versions. Good agreement is obtained with experimental data, and measured rate coefficients are reproduced within a factor of 2. Reaction rate constants for tri-, tetra-, and penta-fluorinated ethanes were calculated in terms of direct dynamics using SRP derived for the ethane reaction with the OH radical to explore the scope and limitations of SRP as a general reaction parameter set.