Multiconformer transition state theory rate constant and branching ratios for the OH‐initiated reaction of CH₃OCF₂CHF₂ and its primary product, HC(O)OCF₂CHF₂

Abstract

Obtaining rate constants and branching ratios for the atmospheric degradation of organic compounds is crucial for their environmental assessment and for a deeper understanding of their atmospheric chemistry and reactivity. The theoretical prediction of such quantities is a challenging endeavor that grows with conformational complexity. In this work, we have theoretically calculated the rate constants and branching ratios for the tropospheric OH‐initiated oxidation reactions of the CH3OCF2CHF2 hydrofluoroether and its primary product, the HC(O)OCF2CHF2 fluorinated ester. The calculations were performed with a cost‐effective multiconformer transition state theory protocol employing the recently developed constrained transition state randomization method. The calculated kfalse(HO⋅+CH3OCF2CHF2false)=1.6×10−140.1emcm30.1emmolecule−10.1emnormals−1 rate constant lies within the error bars of the recommended experimental value, while the calculated rate coefficient of kfalse(HO⋅+HC(O)OCF2CHF2false)=2.6×10−140.1emcm30.1emmolecule−10.1emnormals−1 represents, to our knowledge, the first determination of this reaction's rate constant. The results provide new insight and elucidate the main mechanistic issues discussed in recent experimental work while they also encourage the use of this protocol in investigations of the tropospheric reactivity of other fluorinated organic compounds.

A computer-based solution to the oxidation kinetics of fluorinated and oxygenated volatile organic compounds

“…30,31 Naturally, one way to evaluate the impact of these approximations is to verify the quality of the rate constants calculated by the MC-TST/CTSR protocol, which can be considered an improved version of our initial protocol. 30,31 The CTSR approach is still relatively new 37 and only two MC-TST/CTSR calculated rate constants have been compared with experimental results, both agreeing with the experimental rate coefficient 36,37 within a factor of two. In addition, it is interesting to note, since error cancellation is an important and sometimes unpredictable factor in TST calculations, 40,42,[82][83][84] that the MC-TST/CTSR rate constant of the reaction between cOH and 2-butanethiol 37 considering a full description of the reactant conformers is actually farther from the recommended experimental value than the MC-TST/CTSR rate constant based on the CONFAB approach.…”

“…Fortunately, for reactions involving the OH radical and fluorine-containing saturated oxygenated VOCs such as the ones explored in this work, the use of VTST is not necessary, since the transition states are well approximated by following the conventional TST approach of placing the dividing surfaces that separate reactants from products at the saddle-points of the potential energy surfaces (PESs), instead of having to variationally optimize them. Within such a framework, we have recently developed, 30,31 tested 32–36 and improved 37 a computational protocol for a cost-effective calculation of bimolecular rate constants for the reaction between ˙OH and VOCs of moderate/large size. This protocol is based on multiconformer transition state theory (MC-TST) 38–44 and a method for performing transition state sampling called constrained transition state randomization (CTSR).…”

Cost-effective approach for atmospheric accretion reactions: a case of peroxy radical addition to isoprene

“…The reaction rate coefficients were calculated with the multiconformational transition state theory (MC-TST) approach for the bimolecular reaction following the expression: 43–47 where the quantum-mechanical tunneling coefficient κ t is 1 for the reaction between heavy atoms, T is the room temperature (= 298.15 K), h is the Planck's constant, P ref is the reference pressure (= 2.45 × 10 19 molecules cm −3 ) and k B is the Boltzmann's constant. Q R, j and Q TS, i are the partition functions of the reactant (peroxy radical) and transition state conformers j and i , respectively, Q isop is a partition function for the isoprene conformer and Δ E j and Δ E i correspond to the zero-point corrected electronic energies of the reactant and transition state conformers relative to the lowest energy conformers, respectively.…”