Theoretical and Ftir Study of the Atmospheric Reaction of Methyl Dichloroacetate with •Oh and Cl• Radicals: Kinetics, Products and Mechanism
Date
2024-04-17Author
Straccia C., Vianni G.
Cardona, Alejandro L.
Blanco, María B.
Rivela, Cynthia B.
Teruel, Mariano A.
Ventura, Oscar N.
ORCID
https://orcid.org/0000-0003-2656-1597https://orcid.org/0000-0003-4790-4579
https://orcid.org/0000-0002-5001-3200
https://orcid.org/0000-0001-5474-0061
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The atmospheric degradation of methyl dichloroacetate initiated by •OH and Cl• radicals can occur via H-atom abstraction from the alkyl groups, (Cl2HC– or –CH3). The product yields for the gas phase reaction with •OH were determined experimentally in a 480 L Pyrex glass atmospheric-simulation reactor coupled to an in-situ Fourier transform infrared (FTIR) spectrometer. In addition to those results, we present in this paper a complete degradation mechanism, based on thermodynamic data obtained by identifying all critical points on the potential-energy surface for said reactions, employing density functional calculations with the M06-2X and MN15 hybrid exchange-correlation functionals and the aug-cc-pVTZ basis sets. A conformational search for reactants and transition states was performed. The energies of these conformers were later corrected at the CCSD(T,Full)-F12/CBS level using the SVECV-f12 composite method. The corrected energies were then used to obtain the theoretical rate coefficients in a multi-conformer approach. The products and their yields for the reaction with •OH were Cl2CHCOOH (44±3%), COCl2 (43±3%), and CO (41±6%). The analysis of the mechanism suggests that formation of P1 (Cl2CO, phosgene) occurs mainly by abstraction from the Cl2HC– group, since the formation of P4 (Cl2CHC(O)OH, dichloroacetic acid) and P5 (CO, carbon monoxide) are more favorable in the path for abstraction from the –OCH3 group. The multi-conformer calculated rate constants values were compared with the values obtained employing only the low-lying TS’s and with our own previous experimental studies. Branching ratios for the reaction with •Cl were compared with the experimental yields of products.
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Cardona, Alejandro L. and Blanco, María B. and Rivela, Cynthia B. and Teruel, Mariano and Ventura, Oscar N., Theoretical and Ftir Study of the Atmospheric Reaction of Methyl Dichloroacetate with •Oh and Cl• Radicals: Kinetics, Products and Mechanism. Available at SSRN: https://ssrn.com/abstract=4796935 or http://dx.doi.org/10.2139/ssrn.4796935
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