Experimental and theoretical study on the impact of a nitrate group on the chemistry of alkoxy radicals

Abstract

The chemistry of nitrated alkoxy radicals, and its impact on RO2 measurements using the laser induced fluorescence (LIF) technique, is examined by a combined theoretical and experimental study.

Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene

“…H-Migration of an aliphatic or an a-ONO 2 -H-atom across the epoxide functionality is an order of magnitude faster than the predictions by the SAR by Vereecken and Nozière for corresponding H-migrations in non-cyclic RO 2 radicals. This contrasts with the results for epoxidized alkoxy radicals by Novelli et al, 49 who found little Table 5 Theoretical rate predictions for ring opening in nitrate-epoxy-alkyl radicals derived from isoprene + NO 3 , at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ with MC-TST level of theory. Indicated are the ZPE-corrected barrier height (E b , kcal mol À1 ), the 298 K rate coefficient (k(298 K), s À1 ), and the parameters for a Kooij expression k(T) = A  (T/K) n  exp(ÀE a /T) (A in s À1 , E a in K).…”

“…Their fate as governed by the competition between unimolecular and bimolecular reactions is important not only for the atmospheric chemistry of isoprene, but also plays a critical role in the experimental detection of the nitrate-RO 2 radicals by a ROxLIF instrument. For the latter, we extend the recent work by Novelli et al 49 on nitrate-substituted alkoxy radicals (called nitrate-RO hereafter) to the nitrate-RO derived from isoprene. The chemistry of these unsaturated alkoxy radicals can proceed by reversible epoxidation and subsequent O 2 capture, which affects both the detection of RO 2 in the ROxLIF instrument, and the yield of MVK and MACR in the SAPHIR chamber and the atmosphere.…”

“…58,59 At the chosen level of theory used here, the rate coefficients are expected to have an accuracy of a factor of 2 to 4. We also compare the rate coefficient predictions against available structureactivity relationships (SARs) for alkoxy decomposition 38,49 and H-migration, 60 and RO 2 radical H-migration. 33 The analysis of nitrate-RO 2 thermal equilibrium population was likewise based on the ratios of the multi-conformer RO 2 partition functions.…”

“…The final signal also includes the contribution from HO 2 radicals present in the air sampled from the chamber, and the RO 2 concentration is derived by subtracting this contribution from the ROxLIF measurement based on the HO 2 LIF measurement described higher. A recent paper by Novelli et al 49 highlights how the LIF technique is likely to be blind to specific nitrated RO 2 radicals due to the necessity of an OH or an HO 2 radical to be formed for an RO 2 radical to be detected. Specifically, it was found that the LIF technique cannot measure nitrated RO 2 radicals formed from cis-2butene and 2,3-dimethyl-2-butene, as the corresponding alkoxy radicals decompose to NO 2 instead of forming HO x radicals.…”

“…It is interesting to note that unsaturated, hydroxylated RO radicals are less likely to form epoxy-RO 2 radicals than unsaturated nitrate-RO radicals. Compared to a nitrate group, an -OH substituent tends to lead to faster isomerisation and decomposition reactions, 38,49,60 which often dominate over the O 2 addition in the epoxy-alkyl intermediate. Examples include the fast elimination of C H 2 OH radicals from the b-OH-alkoxy radical in Fig.…”