Detailed mechanism and kinetics of the reaction of Criegee intermediate CH₂OO with HCOOH investigated: Via infrared identification of conformers of hydroperoxymethyl formate and formic acid anhydride

The reaction of Criegee intermediate CH₂OO with HCOOH is important in atmospheric chemistry, but its mechanism and kinetics are little investigated. We recorded time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH₂I₂/O₂/N₂/HCOOH at 298 K. Bands of CH₂OO were observed initially upon irradiation; their decrease was accompanied with the appearance of several bands near 887, 925, 1052, 1115, 1170, 1342, 1391, and 1760 cm^-1, assigned to the absorption of hydroperoxymethyl formate [HC(O)OCH₂OOH, HPMF], that decreased in intensity at a later period with the appearance of absorption bands of the anti-conformer of formic acid anhydride [anti-(HCO)₂O, FAN] near 998, 1101, 1767, and 1821 cm^-1. The main contributions of the infrared absorption of HPMF are from an open-form conformer, but small contributions from the intramolecular hydrogen-bonded conformer that absorbs near 1070, 1170, and 1732 cm^-1 were identified. Observed infrared spectra of both conformers of HPMF and anti-FAN agree satisfactorily with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. We derived a rate coefficient for CH₂OO + HCOOH to be k = (1.4 ± 0.3) × 10^-10 cm³ molecule^-1 s^-1 from formation of HPMF. We found also that anti-FAN was produced mainly from the open-form conformer with rate coefficient k = (1460 ± 30) s^-1; the intramolecular hydrogen-bonded conformer of HPMF is stable.