Detailed mechanism and kinetics of reactions of anti- and syn-CH3CHOO with HC(O)OH: infrared spectra of conformers of hydroperoxyethyl formate.

The reaction of CH3CHOO with HC(O)OH has a large rate coefficient so that it might play a significant role in the formation of secondary organic aerosols (SOA) in the atmosphere. We investigated the detailed mechanism and kinetics of the reactions of Criegee intermediate anti- and syn-CH3CHOO with HC(O)OH with a step-scan Fourier-transform infrared spectrometer by recording time-resolved absorption spectra of transient species and end products produced upon irradiation at 308 nm of a flowing mixture of CH3CHI2/O2/HC(O)OH at 298 K and 60 Torr. Thirteen bands of hydroperoxyethyl formate [HC(O)OCH(CH3)OOH, HPEF], the hydrogen-transferred adduct of CH3CHOO and HC(O)OH, were observed. Careful analysis deconvoluted these bands into absorption of three conformers of HPEF: a transient HPEF (P2*/P3*), a more stable open-form HPEF (mainly P2), and a stable intramolecularly hydrogen-bonded HPEF (mainly P1). At a later period, the end-product formic acetic anhydride [CH3C(O)OC(O)H, FAA], a dehydrated product of HPEF, was observed; this end-product is the same as that observed in CH2OO + CH3C(O)OH. Theoretical calculations on the reaction pathway scheme were performed to elucidate these reaction paths. Syn-CH3CHOO + HC(O)OH produced conformers P2*/P3* initially, followed by conversion to conformers P2, whereas anti-CH3CHOO + HC(O)OH produced conformers P2 and P1 directly. We derived a rate coefficient for the reaction CH3CHOO + HC(O)OH to be k = (2.1 ± 0.7) × 10-10 cm3 molecule-1 s-1 at 298 K and 40-80 Torr; the rate coefficient appeared to show insignificant conformation-specificity. We also found that FAA was produced mainly from the dehydration of the open-form HPEF (P2) with a rate coefficient k = (1420 ± 70) s-1; the intramolecularly hydrogen-bonded HPEF (P1) is stable.

A spectroscopic method for monitoring photochemical reactions in the gas phase.

We have developed an infrared spectroscopic method for monitoring photochemical reactions in the gas phase. This method is based on the major components such as repetitive scan FT-IR spectrometer, multi-pass long-path gas cell, and Nd:YAG laser (ns). The FT-IR spectrometer was used as it is. The gas cell was further modified for the photolysis of the precursor. The vacuum line was designed and constructed solely in our laboratory. We make optical arrangements for both separation of fourth-harmonics (266 nm) from the fundamental (1064 nm) of Nd:YAG laser as well as to guide the UV light to the gas cell housed in the sample compartment of FT-IR. A special arrangement was done in order to get a multi-pass of UV light across the gas cell so that photolysis efficiency will increase significantly. We estimate the photolysis efficiency based on laser power, optical path-length of the laser light, vapor pressure of the precursor, and its absorption cross-section. Furthermore, we have done quantitative analysis for the precursor and photo-products using infrared absorbance and optical path length. This method is tested and validated by monitoring the photodegradation pathways of halobenzenes in the UV light.•Repetitive scan FT-IR spectrometer coupled with a multi-pass long-path gas cell and Nd:YAG laser.•Estimate photolysis efficiency of precursor and concentration of photoproducts.•Monitoring gaseous phase photochemical reactions up-to 100 of ms with spectral resolution 2 cm-1 is possible with this system.

Mechanism and kinetics of the reaction of the Criegee intermediate CH2OO with acetic acid studied using a step-scan Fourier-transform IR spectrometer.

Acetic acid, CH3C(O)OH, plays an important role in the acidity of the troposphere. The reactions of Criegee intermediates with CH3C(O)OH have been proposed to be a potential source of secondary organic aerosol in the atmosphere. We investigated the detailed mechanism and kinetics of the reaction of the Criegee intermediate CH2OO with CH3C(O)OH. The time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH2I2/O2/CH3C(O)OH at 298 K were recorded using a step-scan Fourier-transform infrared spectrometer. The decrease in the intensity of the bands of CH2OO was accompanied by the appearance of bands near 886, 971, 1021, 1078, 1160, 1225, 1377, 1402, 1434, and 1777 cm-1, assigned to the absorption of hydroperoxymethyl acetate [CH3C(O)OCH2OOH, HPMA], the hydrogen-transferred adduct of CH2OO and CH3C(O)OH. Two types of conformers of HPMA, an open form and an intramolecularly hydrogen-bonded form, were identified. At a later reaction period, bands of the open-form HPMA became diminished, and new bands appeared at 930, 1045, 1200, 1378, 1792, and 1810 cm-1, assigned to formic acetic anhydride [CH3C(O)OC(O)H, FAA], a dehydrated product of HPMA. The intramolecularly hydrogen-bonded HPMA is more stable. From the temporal profiles of HPMA and FAA, we derived a rate coefficient k = (1.3 ± 0.3) × 10-10 cm3 molecule-1 s-1 for the reaction CH2OO + CH3C(O)OH to form HPMA and a rate coefficient k = 980 ± 40 s-1 for the dehydration of the open-form HPMA to form FAA. Theoretical calculations were performed to elucidate the CH2OO + CH3C(O)OH reaction pathway and to understand the distinct reactivity of these two forms of HPMA.