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 aci d in the atmosphere. All the envisaged processes involve the so-called Crie gee 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 kca l/mol) (values at the CCSD(T) level with zero-point energy correction at th e B3LYP level). Note that the formation of this adduct may or may not be pr eceded by the formation of a stable complex. Afterward, the adduct decompos es 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 ener gy is -118.3 kcal/mol at the CCSD(T) level. The computed results for activa tion energies suggest that the reactions with H2O, H2CO, and SO2 are likely to occur, whereas that with CO2 is unfavorable. Because of the high concen tration of H2O in atmospheric conditions, the reaction with this molecule s hould play a major role.