Theoretical investigation of reaction mechanisms for carboxylic acid formation in the atmosphere

Theoretical calculations have been carried out to investigate the mechanism of several chemical reactions that may explain the formation of formic acid in the atmosphere. All the envisaged processes involve the so-called Criegee intermediate, H2COO, which is generated in the course of the ozonolysis reaction. We focus on isomerization of carbonyl oxide through bimolecular reactions with H2CO, H2O, SO2, and CO2. The results are compared with those obtained for unimolecular isomerization mechanisms previously reported in the literature. In the bimolecular processes, there is always formation of an intermediate adduct, the stability of which increases in the order CO2 (-24.5 kcal/mol) < SO2 (-43.1 kcal/mol) < H2O (-45 kcal/mol) < H2CO (-49 kcal/mol) (values at the CCSD(T) level with zero-point energy correction at the B3LYP level). Note that the formation of this adduct may or may not be preceded by the formation of a stable complex. Afterward, the adduct decomposes to form the final products according to a one-step (H2O, SO2, CO2) or a stepwise mechanisms (H2CO). The whole H2COO + M --> HCOOH + M reaction energy is -118.3 kcal/mol at the CCSD(T) level. The computed results for activation energies suggest that the reactions with H2O, H2CO, and SO2 are likely to occur, whereas that with CO2 is unfavorable. Because of the high concentration of H2O in atmospheric conditions, the reaction with this molecule should play a major role.