Theoretical study of formic acid anhydride formation from carbonyl oxide in the atmosphere

We report the first theoretical study on the formation mechanism of tropospheric formic acid anhydride (FAA). Experimental studies on this subject have raised controversy, and the reaction mechanisms proposed are examined here with the help of theoretical calculations at the density functional theory and various correlated ab initio levels (MP4, CCSD, CASSCF, CASPT2) using extended basis sets. The investigated processes are initiated by the reaction of carbonyl oxide with either formaldehyde or formic acid. In the first case, a secondary ozonide is formed that then isomerizes to hydroxymethylformate (HMF). Stepwise and concerted mechanisms have previously been proposed for the isomerization process on the basis of experimental results. Our calculations confirm the existence of both mechanisms, but the stepwise one appears to be more favorable. HMF decomposition into FAA and H-2 is shown to be unlikely (activation barrier about 90 kcal/mol). Conversely, reaction of HMF with molecular oxygen in the singlet state leads to FAA and H2O2 through a small barrier close to 9 kcal/mol at the B3LYP level. In the case of the carbonyl oxide + formic acid pathway, the transitory product is hydroperoxymethylformate (HPMF). Decomposition of HPMF into FAA and H2O proceeds through a large activation barrier (about 50 kcal/mol). The process may be assisted by a formic acid molecule, lowering the activation barrier for FAA formation to 29.8 kcal/mol at the B3LYP level. Reactions energies are -113.7 kcal/mol for H2COO + H2CO --> FAA + H-2, -174.6 kcal/mol for H2COO + H2CO + O-2 --> FAA + H2O2, and -101.7 kcal/mol for H2COO + HCOOH --> FAA +/- H2O (values at the B3LYP level with ZPE corrections). Therefore, the mechanism involving singlet O-2 appears to be the most favorable one in atmospheric conditions, both kinetically and thermodynamically.