High-level theoretical characterization of the vinoxy radical (•CH2CHO) + O2 reaction

Abstract

Numerous processes in atmospheric and combustion chemistry produce the vinoxy radical (CHCHO). To understand the fate of this radical and to provide reliable energies needed for kinetic modeling of such processes, we have examined its reaction with O using highly reliable theoretical methods. Utilizing the focal point approach, the energetics of this reaction and subsequent reactions were obtained using coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] extrapolated to the complete basis set limit. These extrapolated energies were appended with several corrections including a treatment of full triples and connected quadruple excitations, i.e., CCSDT(Q). In addition, this study models the initial vinoxy radical + O reaction for the first time with multireference methods. We predict a barrier for this reaction of approximately 0.4 kcal mol. This result agrees with experimental findings but is in disagreement with previous theoretical studies. The vinoxy radical + O reaction produces a 2-oxoethylperoxy radical which can undergo a number of unimolecular reactions. Abstraction of a β-hydrogen (a 1,4-hydrogen shift) and dissociation back to reactants are predicted to be competitive to each other due to their similar barriers of 21.2 and 22.3 kcal mol, respectively. The minimum-energy β-hydrogen abstraction pathway produces a hydroperoxy radical (QOOH) that eventually decomposes to formaldehyde, CO, and OH. Two other unimolecular reactions of the peroxy radical are α-hydrogen abstraction (38.7 kcal mol barrier) and HO elimination (43.5 kcal mol barrier). These pathways lead to glyoxal + OH and ketene + HO formation, respectively, but they are expected to be uncompetitive due to their high barriers.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates

“…[8][9] They are 5 formation of an OH or HO 2 radical co-product. [27][28][29][30] The stable products are identified using multiplexed photoionization mass spectrometry (MPIMS) (Section IV). The origin of the identified products is further interrogated via experiments using formic acid as a Criegee intermediate scavenger.…”

“…28,[31][32][33] ROO intermediates are predicted to decay rapidly to closed-shell species along with an 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 OH or HO 2 radical co-product via a hydroperoxyalkyl radical (QOOH) intermediate. [28][29][30] Rapid unimolecular decay of ROO is facilitated by internal excitation of the radical products formed in the dioxole pathway, along with exothermic O 2 addition and submerged subsequent barriers that lead to stable products.…”

“…28,[35][36][37][39][40] Thus, formaldehyde (along with OH and CO) is expected to be the primary product channel under low pressure conditions. 28 The unpaired electron of vinoxy radicals in the ground state is primarily localized on the carbon, [29][30][44][45][46] which facilitates rapid reaction with O 2 to form ROO. [47][48] The vinoxy radical + O 2 reaction generates the 2-oxoethylperoxy radical as shown in Scheme 6.…”

“…A recent high-level theoretical study of the vinoxy + O 2 reaction predicts formaldehyde + OH + CO to be the dominant product channel, noting other pathways may be minor. 29 The proposed mechanism is shown in Scheme 6 and proceeds by O 2 addition to the vinoxy radical via a low barrier (0.4 kcal mol -1 ) to form the 2-oxoethylperoxy radical in an exothermic reaction (-21.9 kcal mol -1 ). Scheme 6.…”

“…Reaction of vinoxy radical with O 2 and subsequent decay to formaldehyde + OH + CO or ketene + HO 2 via the QOOH-2 intermediate. Energies (kcal mol -1 ) are reported by Weidman et al 29 (CCSD(T)/CBS).…”