Atmospheric Oxidation of Monochloroacetic Acid (CH2ClCOOH) Initiated by Cl Atom: An Insight from DFT Study

The degradation mechanism of monochloroacetic acid, (CH2ClCOOH) initiated by Cl atom, has been investigated using at M06-2X/ 6-311+G(d, p) level of theory and further single point energy calculation followed by the couple-cluster CCSD(T) method along with cc-pVTZ basis set. We have considered three distinct reaction pathways for titled reaction viz two H-atom abstractions and one Cl-atom abstraction of CH2ClCOOH molecule by the Cl-atom. The overall rate coefficient determined by canonical transition state theory within the temperature range of 250-600 K. Kinetic and thermo-chemical results shows H-abstraction from the −CH2Cl site is favorable than others. The percentage branching ratio calculations also showed that the H-abstraction from the -CH2Cl contributes 99.97%. Atmospheric lifetime of the CH2ClCOOH molecule is found to be 23 days and 4.4 hours. The estimated Global warming potentials (GWPs) of monochloroacetic acid are 0.03, 0.01 and 0.003 for 20 years, 100 years and 500 years respectively. Moreover, the aerial degradation of more dominant product radical (•CHClCOOH) was performed in presence of NO• radical, yielding formyl chloride (CHClO), carbon dioxide (CO2) and hydroxyl radical (•OH) as end degradation products.