An Experimental and Master Equation Investigation of Kinetics of the CHOO + RCN Reactions (R = H, CH, CH) and Their Atmospheric Relevance.

We have performed direct kinetic measurements of the CHOO + RCN reactions (R = H, CH, CH) 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 (CHICl), whose photolysis at 193 nm in the presence of O produces CHOO. Observed bimolecular rate coefficients for CHOO + HCN, CHOO + CHCN, and CHOO + CHCN reactions at 296 K are (2.22 ± 0.65) × 10 cm molecule s, (1.02 ± 0.10) × 10 cm molecule s, and (2.55 ± 0.13) × 10 cm molecule s, respectively, suggesting that reaction with CHOO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CHOO + 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 CHOO + HCN and CHOO + CHCN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CHO + 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 CHO + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.