Hydroxyl radical (OH) reactivity with XC(O)H and XC(O)OH (X = F, Cl, Br, and I): mechanism, kinetics, and atmospheric implications.

The mechanisms and kinetics of atmospheric degradation for halogenated formyl and carboxylic acid compounds, denoted as HC(O)X and XC(O)OH (where X represents F, Cl, Br, and I), were thoroughly investigated through quantum chemical calculations and kinetic modeling, initiated by hydroxyl (OH) radicals. The potential energy surfaces of the hydrogen abstraction reaction pathways were analyzed at the CCSD(T)-MP2/CBS//M06-2X theoretical level, using the aug-cc-pVXZ (D, T, and Q) basis sets for systems with F, Cl, and Br substitutions. For iodine-containing compounds, the aug-cc-pVXZ-PP (D, T, and Q) basis sets were used. Our findings demonstrate that the OH attack on HC(O)X (X = F, Cl, Br, and I) is more favorable than on XC(O)OH, clearly indicating that hydrogen abstraction from the carbonyl position is the preferred pathway. Reaction enthalpies and Gibbs free energies were calculated at the M06-2X/aug-cc-pVTZ and aug-cc-pVTZ-PP levels, while kinetic calculations employed canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) corrections over a range of 250-500 K temperatures. The calculated rate coefficients were then used to evaluate the atmospheric implications of these systems by estimating their tropospheric lifetimes with respect to oxidation by the OH radical.