Theoretical Modeling of the Hydrogen Abstraction Reaction of Fluoromethane by the Hydroxyl Radical

Abstract

We have performed theoretical modeling of the reaction rate constants of the hydrogen abstraction reaction of fluoromethane (CH3F) by the hydroxyl radical (OH) using dual-level variational transition state theory calculation including multidimensional tunneling corrections from 200 to 1000 K. Correlated electronic structure theory with extended basis set calculation was applied for both the low-level reaction-path and the high-level stationary-point calculation. An improved interpolated correction scheme was used for better estimating the width of energy barrier by performing an intermediate-level electronic structure calculation. The calculated rate constants are in good agreement with available experimental values at most temperatures. The hydrogen kinetic isotope effects (KIEs) were also evaluated and analyzed. The current study suggested that the reaction has a relatively wide barrier, and the tunneling effects are thus not very important even at low temperatures. The current study also showed that the variational effects, which lowered the rate constants by over an order of magnitude at room temperature, are very important for the reaction. The best estimate of the classical barrier height is between 2.8 and 3.1 kcal/mol. The calculation also suggested that the calculated KIEs are sensitive to the theories employed for obtaining the low-level potential energy surfaces. It is demonstrated that the dynamical behaviors predicted by the current calculation can also be deduced from future KIE experiments on the current reaction.