First-principle Study On The Evolution Of Halogenated Aldehyde In The Atmosphere

Based on the first-principle method and transition state theory, the following two questions on the evolution of typical halogenated aldehydes in the atmosphere are theoretically investigated:The atmospheric reactions of HO2 with FCHO and Cl CHO have been theoretically investigated using at the CCSD?T?/aug-cc-pvtz//M06-2X/6-311++G?d,p? level of theory. Our studies predict that there are three reaction channels that are the hydrogen atom transfer plus oxygen addition, radical addition, and hydrogen abstraction. The calculated results show that the proton transfer plus the radical addition reaction is a dominant pathway because of the lower energy barriers of 3.7, 3.2 kcal/mol relative to the free reactants HO2 and XCHO?X=F, Cl?, respectively. The evaluated barriers reflect that the HO₂ + ClCHO reaction is more feasible than the HO₂ + FCHO. Additionally, the reaction barriers of other pathways are too high to occur in the atmosphere. Furthermore, the rate constants of the two dominant reaction channels are computed to be 1.85 × 10-17 and 3.80 × 10-17 cm3 molecule-1 s-1 at 298 K, respectively, which demonstrates that the HO2 + XCHO?X=F, Cl? reactions are of minor importance in the atmosphere. The findings in the present work could have potential applications in evaluating the atmospheric fate of XCHO?X=F, Cl?.Additionally, the mechanisms and kinetics of the hydrogen atom in FCHO abstracted by OH in the presence of water, formic acid, or sulfuric acid are theoretically investigated at the CCSD?T?/6-311++G?3df,3pd?//M06-2X/6-311++G?3df,3pd? level of theory. The calculated results show that the barriers of the transition states involving catalysts are lowered to-2.89,-6.25 and-7.76 kcal/mol from 3.64 kcal/cal with respect to the free reactants, respectively, which reflects that those catalysts play an important role in the hydrogen abstraction reaction of FCHO with OH. Additionally, using conventional transition state theory with Eckart tunneling correction, the kinetic data demonstrates that the entrance channel X…FCHO + OH?where X= water, formic acid, or sulfuric acid? is significantly more favorable than the pathway X…OH + FCHO. Moreover, the rate constants of the reactions of FCHO with OH radical with water, formic acid, or sulfuric acid introduced are computed to be smaller than that of the naked OH + FCHO reaction, which indicates the addition of those molecules cannot promote the reaction of the FCHO with the OH radical in the atmosphere.