A Theoretical Study on Hydrogen Abstraction by Fluorine Atoms in Interaction with Methane

The stationary points of potential energy surfaces for hydrogen abstraction and substitution reactions in methane have been calculated. The computational procedure included nonrelativistic calculations with the hybrid density functional M08-HX, followed by scalar relativistic corrections and spin–orbit coupling. Error in the enthalpy of the hydrogen abstraction reaction is about 1 kcal/mol, and that for the substitution reaction is 1.8 kcal/mol. The error is reduced to 0.75 and 0.05 kcal/mol, respectively, by using the coupled-cluster approach instead of the density functional theory. An analysis of the cross sections of the potential energy surfaces of electronic states correlating with the ground 2P state of fluorine has led to the conclusion that both reactions are realized through the ground relativistic electronic state correlating with one of the components of the F(2P3/2) multiplet. For the hydrogen abstraction reaction, this conclusion is supported by independent studies. The results of electronic structure calculations were used to evaluate the hydrogen abstraction rate constant in terms of the canonical variational transition state theory. For the interval 200−500 K, the calculation error was about 10−15%. It can be concluded that the M08-HX functional is an adequate tool to calculate the thermal properties and rate constants for reactions of organic compounds with atomic fluorine.