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

Abstract

The OH radical is the most powerful atmospheric oxidant, being responsible for the chemical breakdown of many pollutants released into the troposphere, including saturated volatile organic compounds (VOCs). Numerous of...

A Multiconformational Transition State Theory Approach to OH Tropospheric Degradation of Fluorotelomer Aldehydes

“…Although the CTSR procedure does not guarantee that the lowest energy TS is found, it has shown to be a considerable improvement [22,23,31] to the oversimplified method that was used in previous versions of the MC‐TST protocol: [16,17] the twelve MC‐TST/CTSR rate constants that we have so far calculated have been compared with experimental results, agreeing with the recommended experimental rate coefficient [22,23,31] within a factor of two.…”

“…(2) and (3) is the total rate constant for the reaction between C x F 2 x +1 CH 2 CHO and ⋅OH. Previous work concerning the CF 3 CH 2 CHO+⋅OH reaction ($${x=1}$$ ) has shown that the branching ratios calculated by structure‐activity relationships (SAR) [15,32] and by our own MC‐TST/CTSR protocol [31] yielded identical results: $${{\rm{\Gamma }}_{{\rm{CH}}_{\rm{2}} } = }$$ 0 % and $${{\rm{\Gamma }}_{{\rm{CHO}}} = }$$ 100 %.…”

“…Recently we have included the approach used by Petit and Harvey [26] in our MC‐TST/CTSR protocol, [17,21,23,31] which consists of manipulating Eq. (4) by multiplying its numerator and denominator by K eq , thus yielding: $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr k_{{\rm{OH}}} = {{k_1 k_{{\rm{OH}}}^{{\rm{MC - TST}}} } \over {k_1 + k_{{\rm{OH}}}^{{\rm{MC - TST}}} }} \equiv k_{{\rm{OH}}} ({\rm{calc}})\hfill\cr}}$$$ …”

“…Based on the derivation given in the supplementary information of Ref. [28], the rate coefficient which we use in our MC‐TST/CTSR protocol is calculated by using the following expression: [16,17,23,31] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr k_{{\rm{OH}}}^{{\rm{MC - TST}}} = k(T){{k_{\rm{B}} T} \over {n_0 hQ_{{\rm{OH}}} }}{{\sum _i^M {{\alpha _i \omega _{{\rm{TS}}_i } Q_{{\rm{TS}}_i } } \over {\sigma _{{\rm{rot}}{\rm{,TS}}_i } }}} \over {\sum _j^N {{\alpha _j \omega _{{\rm{OVOC}}_j } Q_{{\rm{OVOC}}_j } } \over {\sigma _{{\rm{rot}}{\rm{,OVOC}}_j } }}}}e^{ - V^{\rm{\ddag }} /k_{\rm{B}} T} \hfill\cr}}$$$ …”

“…As for the TS sampling, and to maintain coherence with the work performed for the $${x=1}$$ FTAL reaction, [31] we have kept the CTSR procedure exactly the same: for each optimized reactant molecule, the CTSR code first randomly generated ⋅OH‐molecule structures which were then subjected to constraints targeting an optimization of TSs at the PM3 level of theory. The three constraints were: 1) the distance between the ⋅OH oxygen atom and the hydrogen atom under attack lies in the [1.2, 1.4] Å interval; 2) the distance between the ⋅OH oxygen atom and all reactants’ non‐hydrogen atoms is greater than 1.70 Å and 3) the distance between the ⋅OH hydrogen atom and all reactants’ hydrogen atoms is greater than 0.70 Å.…”