Theoretical Investigation of the Hydrogen Abstraction Reaction of the OH Radical with CH3CHF2 (HFC152-a):  A Dual Level Direct Density Functional Theory Dynamics Study

The hydrogen abstraction reaction of the OH radical with CH3CHF2 (HFC152-a) has been studied theoretically over a wide temperature range, 200−3000 K. Two different reactive sites of the molecule, CH3 and CHF2 groups have been investigated precisely, and results confirm that CHF2 position of the molecule is a highly reactive site. In this study, three recently developed hybrid density functional theories, namely, MPWB1K, MPW1B95, and MPW1K, are used. The MPWB1K/6-31+G(d,p) method gives the best result for kinetic calculations, including barrier heights, reaction path information and geometry of transition state structures and other stationary points. To refine the barrier height of each channel, a single point energy calculation was performed in MPWB1K/MG3S method. The obtained rate constants by dual level direct dynamics with the interpolated single point energy method (VTST-ISPE) using DFT quantum computational methods, are consistent with available experimental data. The canonical variational transition state theory (CVT) with the zero-curvature and also the small-curvature tunneling correction methods is used to calculate the rate constants. Over the temperature range 200−3000 K, the variation effect, tunneling contribution, branching ratio of each channel are calculated. The rate constants and their temperature dependency in the form of a fitted three-parameter Arrhenius expression are k 1(T) = 2.00 × 10-19(T)2.24 exp(−1273/T), k 2(T) = 1.95 × 10-19(T)2.46 exp(−2374/T), and k(T) = 3.13 × 10-19(T)2.47 exp(− 1694/T) cm3 molecule-1 s-1. For the H abstraction from the CHF2 group, a nonclassical reflection effect is detected as a dominant quantum effect.