Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory

Abstract

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1‑hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Unimolecular and water reactions of oxygenated and unsaturated Criegee intermediates under atmospheric conditions

“…The reaction rates are affected by the polarity of the oxygenated groups and strong H-bonding, as well as by the difference in mechanism between cycloadduct formation for R19 and R21 on one hand, and CO 2 elimination for R20 on the other hand; the reactivity thus depends strongly on the position of the carboxy group. The reactivity is distinct from bimolecular reactions of SCI with acids, 12,[44][45][46][47] where the lack of geometric constraints on the transition state tends to favor barrierless 1,4-insertion across the carboxylic acid group, whereas the unimolecular reactions discussed here occur by 1,2-insertion. Syn-a and synb-COOH-substituted SCI react very fast by intramolecular insertion, with rate coefficients exceeding 10 5 s À1 .…”

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation

“…As with carbonyl compounds, CIs possess π-orbitals that manifest in strongly absorbing π* ← π transitions, resulting in population of ππ* excited states. ,,,− The π-conjugation extends over the COO moiety, which leads to a bathochromic shift of the π* ← π absorption profile; cf. that of the corresponding carbonyl compound. − These π* ← π absorptions have been used to probe the bimolecular chemistry of CIs and, particularly in the case of larger and more functionalized CIs such as methyl vinyl ketone oxide and methacrolein oxide, may contribute to their removal by solar photolysis. − …”

Dramatic Conformer-Dependent Reactivity of the Acetaldehyde Oxide Criegee Intermediate with Dimethylamine Via a 1,2-Insertion Mechanism

“…These photoionization spectra are in accord with formation of the HDEA insertion product (parent ion observed at m / z 105) and the corresponding HO 2 -loss fragment ion ( m / z 72), consistent with the proposed 1,2-insertion mechanism and previously observed dissociative photoionization of functionalized hydroperoxides (Scheme ). − ,, A representative mass spectrum (integrated over the full kinetic time window 0–40 ms) measured at a photoionization energy of 10.5 eV is shown in Figure S2 (left). Kinetic time profiles of m / z 105 and m / z 72 (Figure ) exhibit rapid formation of a stable product between CH 3 CHOO and DMA.…”

“…Carbonyl oxides (known as Criegee intermediates) are one possible ozonolysis intermediate that could play a role in SOA formation. Indeed, bimolecular reactions of thermalized Criegee intermediates [known as stabilized Criegee intermediates (sCIs)] with co-reactants such as SO 2 , organic acids, and alcohols have been implicated in the formation of aerosol precursors in the troposphere: − the former via the production of SO 3 and the latter two via the generation of lower-volatility functionalized hydroperoxide species.…”