No abstract

The formation of OH radicals in the atmosphere as a result of the reaction of substituted carbonyl oxides with water was investigated by means of high‐level theoretical methods. The reaction between carbonyl oxide with water produces mainly α‐hydroxy hydroperoxide but up to 5 % of OH radicals may be formed when the carbonyl oxide has a hydrogen atom in the β position. Vibrationally excited α‐hydroxy hydroperoxide may decompose to produce OH radicals or become collisionally stabilized and react with a water molecule to produce H2O2 (see Scheme).

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 H 2 O 2 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 H 2 O 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 H 2 COO + H 2 CO f FAA + H 2 , -174.6 kcal/mol for H 2 COO + H 2 CO + O 2 f FAA + H 2 O 2 , and -101.7 kcal/mol for H 2 COO + HCOOH f FAA + H 2 O (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.