Reactions of the Criegee Intermediate Methyl Vinyl Ketone Oxide with HC(O)OH: Infrared Spectra of the Hydrogen-Transfer Adduct 2-Hydroperoxybut-3-en-2-yl Formate

Reactions between the Criegee intermediate and HC(O)OH exhibit large rate coefficients, contributing significantly to secondary organic aerosol formation in the atmosphere. This study examines the reaction of methyl vinyl ketone oxide [MVKO, (C₂H₃)C(CH₃)OO], a Criegee intermediate generated via isoprene ozonolysis in the atmosphere, with HC(O)OH. Product analysis was performed using step-scan Fourier-transform infrared spectroscopy, capturing time-resolved infrared absorption spectra following irradiation at 248 nm of a flowing mixture of (<i>Z</i>)-(CH₂I)HC═C(CH₃)I/HC(O)OH and O₂ at 298 K and 10-40 Torr. Ten absorption bands near 1726, 1425, 1378, 1310, 1247, 1215, 1170, 1068, 984, and 952 cm^-1 were assigned to 2-hydroperoxybut-3-en-2-yl formate [HPBF, C₂H₃C(CH₃)(OCHO)OOH], the hydrogen-transfer adduct of MVKO and HC(O)OH. Additional weak bands near 1675, 1600, 1432, 1387, 1330, and 1254 cm^-1 were tentatively attributed to 2-hydroperoxybuta-1,3-diene [HPBD, (C₂H₃)C(═CH₂)OOH], a hydrogen-transfer isomer of MVKO formed via HC(O)OH-catalyzed rearrangement. Two further bands near 1733 and 1200 cm^-1 were tentatively assigned to a complex of HPBD and HC(O)OH, with additional features overlapping those of HPBD. Spectral assignments were supported by B3LYP + D3/aug-cc-pVTZ calculations of vibrational wavenumbers and IR intensities. The identification of HPBF and HPBD is consistent with the reaction pathway scheme predicted by the CCSD(T)/aug-cc-pVTZ//B3LYP + D3/aug-cc-pVTZ method. In contrast to reactions of CH₂OO or CH₃CHOO with HC(O)OH, no dehydrated end product of the adduct was observed, which was attributed to the lack of an abstractable hydrogen atom in HPBF.