An Experimental and Master Equation Investigation of Kinetics of the CH2OO+RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the CH2OO + RCN reactions (R = H, CH3, C2H5) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH2ICl), whose photolysis at 193 nm in the presence of O2 produces CH2OO. Observed bimolecular rate coefficients for CH2OO + HCN, CH2OO + CH3CN, and CH2OO + C2H5CN reactions at 296 K are (2.22 +/- 0.65) x 10-14 cm3 molecule-1 s-1, (1.02 +/- 0.10) x 10-14 cm3 molecule-1 s-1, and (2.55 +/- 0.13) x 10-14 cm3 molecule-1 s-1, respectively, suggesting that reaction with CH2OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (omega B97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH2OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH2OO + HCN and CH2OO + CH3CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH2O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH2O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.

Automated summary

Direct kinetic measurements were performed on the reactions of CH2OO + RCN (R = H, CH3, C2H5) in the temperature and pressure ranges of 233-360 K and 10-250 Torr, respectively, using time-resolved UV-absorption spectroscopy. Chloroiodomethane (CH2ICl) was used as a photolytic precursor to produce CH2OO through its photolysis in the presence of O2 at 193 nm. The observed bimolecular rate coefficients for the reactions of CH2OO with HCN, CH3CN, and C2H5CN at 296 K were determined to be (2.22 +/- 0.65) x 10-14 cm3 molecule-1 s-1, (1.02 +/- 0.10) x 10-14 cm3 molecule-1 s-1, and (2.55 +/- 0.13) x 10-14 cm3 molecule-1 s-1, respectively, indicating that the reaction with CH2OO can be a significant degradation pathway for nitriles in the atmosphere. Both the temperature and pressure dependencies of all the reactions were found to be negligible in the studied conditions. Quantum chemical calculations and master equation modeling results suggested that the lowest-energy reaction path of CH2OO + RCN led to the formation of a 1,2,4-dioxazole compound through ring closure. Subsequent decomposition of 1,2,4-dioxazole could generate important atmospheric end products such as CH2O + RNCO and CO + RC(NH)OH.