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

Abstract

The hydrogen abstraction reaction of the OH radical with CH 3 CHF 2 (HFC152-a) has been studied theoretically over a wide temperature range, 200-3000 K. Two different reactive sites of the molecule, CH 3 and CHF 2 groups have been investigated precisely, and results confirm that CHF 2 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 threeparameter 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) cm 3 molecule -1 s -1 . For the H abstraction from the CHF 2 group, a nonclassical reflection effect is detected as a dominant quantum effect.

Theoretical study of a reaction mechanism of tropospheric interest: CH3CH2F + OH

“…This maximum exothermicity is reasonable, since the fluorine atom can stabilise the product radical CH 3 CHF to which it is directly connected due to the electronegativity of the F atom. Such a stability is attributed to hyperconjugation effect that comes from coupling a p-orbital of fluorine with carbon radical atomic orbitals [7]. In previous reports, Sekušak et al [19] have located only one reactant complex for H abstraction reactions at MP2/6-31+G(d,p) level, which is similar to our RC1/ RC3.…”

Direct Ab Initio Dynamics Study on the Reaction of CH3CHF2 (HFC‐152a) with the Cl Atom

“…2.60 10 The conclusions drawn from calculations for reactions CH 3 CHF 2 + O( 3 P) [40,41] and CH 3 CHF 2 + OH [42] are similar: a-H abstraction reactions dominate the total reaction over the lower temperature range [27,28] or the whole temperature range, [29] and k 1 /k values decrease gradually as the temperature increases.…”

Theoretical Dynamic Studies on the Reaction of CH3C(O)CH3−nFn with the Hydroxyl Radical and the Chlorine Atom

“…As pointed out by Taghikhani and Parsafar, [33] the quantum effect may reduce the rate constants astonishingly. This is an unusual case of dynamics calculations, in which quantum effects preclude particles (supermolecules) to pass over the classical barrier, and a special case, in which the nonclassical reflection effect-at energies above the barrier-overcomes tunneling through the classically forbidden region of the barrier.…”