Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions

Isoprene is the most abundant unsaturated hydrocarbon in the atmosphere, and its ozonolysis forms several Criegee intermediates (CIs), including CH₂OO, MVKO, and MACRO. CIs can oxidize SO₂ to SO₃, leading to H₂SO₄ and secondary aerosol formation, but direct atmospheric measurement of CIs is impractical, so kinetics are used to estimate their concentrations and impacts. CI reactivity is highly structure dependent: H-syn CIs (e.g., CH₂OO, anti-CH₃CHOO) are removed rapidly by reactions with water vapor, while alkyl-syn CIs (e.g., syn-CH₃CHOO, (CH₃)₂COO) undergo significant unimolecular decay via 1,4-H transfer. Resonance-stabilized C4 CIs from isoprene (MVKO, MACRO) have distinct behaviors; syn-MVKO was shown to be long-lived and impactful for SO₂ oxidation due to slow unimolecular and water reactions. For MACRO, theory previously predicted fast water reactions for anti-MACRO and fast unimolecular decay for syn-MACRO, implying limited atmospheric impact of anti-MACRO. This study uses UV–visible spectroscopy to identify MACRO under thermal conditions and quantifies its bimolecular reactions with SO₂ and H₂O and its effective atmospheric lifetime, testing the hypothesis that resonance stabilization alters its decay pathways and enables a longer lifetime than expected, thereby affecting SO₂ oxidation.

Prior laboratory and theoretical work established that CI atmospheric fate depends on structure: CH₂OO and anti-CH₃CHOO decay predominantly via rapid reactions with H₂O monomer and dimer; syn-CH₃CHOO and (CH₃)₂COO exhibit significant unimolecular decay via 1,4-H transfer, generating OH. For isoprene-derived, resonance-stabilized CIs, MVKO and MACRO feature extended conjugation. Experiments and theory on MVKO identified non-interconverting syn/anti families and fast interconversion of cis/trans conformers, with anti-MVKO predicted to undergo rapid ring-closure and thus not observed, while syn-MVKO is comparatively long-lived and reacts rapidly with SO₂ and formic acid, significantly influencing atmospheric SO₂ oxidation in model studies. For MACRO, theory predicted syn-MACRO undergoes fast unimolecular decay (kuni ≈ 2500 s⁻¹), while anti-MACRO unimolecular decay is slow (~10 s⁻¹), but reactions with water vapor were predicted to be fast (kwater-eff 6.3 × 10⁻¹⁶ to 7.2 × 10⁻¹⁵ cm³ s⁻¹ at RH 70% and 298 K), which would limit its steady-state concentration. Newland et al. (2015) inferred an effective water reactivity for isoprene-derived non-CH₂OO CIs of ~1.1 × 10⁻¹⁵ cm³ s⁻¹ from ozonolysis experiments, though later insights suggest that mixture complexities and dominant CH₂OO loss to water could bias such inference. Theoretical analyses (Vereecken et al.) used structure-activity scaling to correct barrier underestimation for CI + H₂O reactions, but lacked anchor data for conjugated CIs. These studies motivated direct spectroscopic and kinetic measurements of MACRO under thermal conditions.

Experimental: MACRO was generated by UV photolysis (248 nm) of E-1,3-diiodo-2-methylprop-1-ene (ICH₂C(CH₃)CHI) in O₂, forming CH₂=C(CH₃)CHOO (MACRO) and I atoms. Experiments were conducted at 298 K and total pressures 150–500 Torr (N₂ balance) in a flow reactor. Time-resolved UV–visible absorption spectra were recorded using a broadband source, grating spectrometer (Andor SR303i), and fast CMOS camera (Andor Marana-4BU11) with exposure times of 0.21 or 0.43 ms, accumulating multiple laser shots to improve S/N. Difference spectra (relative to pre-photolysis) were background-corrected. Spectral components included a broad band centered near 397 nm (MACRO), IO with structured bands (400–460 nm), and I₂ extending to ~520 nm. To isolate MACRO, high [SO₂] experiments (up to 2.93 × 10¹⁴ cm⁻³) were used to scavenge CIs; subtracting spectra with high SO₂ from those without SO₂ at each delay removed most IO and I₂ contributions (SO₂ scavenge method). Spectra were decomposed via least-squares into MACRO, IO, and I₂ using literature cross sections for IO and I₂ and MACRO peak cross section σ = 3 × 10⁻¹⁸ cm². Time-dependent MACRO absorbance at 397 nm was converted to concentration via ΔAbs = σ L [MACRO](t). Kinetics: MACRO decays were fitted to single exponential to obtain pseudo–first-order rates kobs; plots of kobs vs [SO₂] yielded bimolecular kSO2 (slope) and intercept k0 for other losses. For water kinetics, kobs was plotted vs [H₂O] (0–~6 × 10¹⁷ cm⁻³; up to ~18 Torr partial pressure). Caution was noted at 150 Torr due to bath gas composition change with high H₂O. A weighted average across six datasets (300 and 500 Torr) provided kwater-eff. Unimolecular decay was estimated by extrapolating kobs to zero initial [MACRO] to remove bimolecular contributions. Spectroscopic assignment of the long-lived band to anti-MACRO was based on observed millisecond lifetimes inconsistent with predicted sub-millisecond syn-MACRO lifetime. Theoretical: Reactant and transition state geometries were optimized at B3LYP/6-311+G(2d,2p). Electronic energies were computed at QCISD(T)/CBS; for MACRO + (H₂O)₂ transition states, a correction scheme was applied. Conventional transition state theory with RRHO and tunneling corrections provided rate coefficients. Basis set effects (CBS vs AVTZ) were analyzed and scaled using analogous (CH₂=CH)CHOO systems to refine predicted barriers and rates for MACRO reactions with H₂O monomer and dimer and for unimolecular decay.

- UV–visible spectrum of MACRO: a broad, Gaussian-like band centered at 397 nm with FWHM ~77 nm; absence of oscillatory structure observed under thermal conditions. - Identity: The observed long-lived species is assigned to anti-MACRO based on millisecond lifetimes incompatible with syn-MACRO's predicted kuni ≈ 2500 s⁻¹. - Reaction with SO₂: kSO2 = (1.5 ± 0.4) × 10⁻¹⁰ cm³ s⁻¹ at 298 K and 150–500 Torr; no significant pressure dependence within uncertainties. This is about 4 times larger than syn-MVKO + SO₂ (4.0–4.2 × 10⁻¹¹ cm³ s⁻¹). - Reaction with H₂O (effective): kwater-eff = (9 ± 5) × 10⁻¹⁷ cm³ s⁻¹ at 298 K (weighted average of data at 300–500 Torr). Slopes of kobs vs [H₂O] are small and sometimes negative at the measurement limit; overall indicates much slower water reaction than previous predictions by 1–2 orders of magnitude. - Unimolecular decay (anti-MACRO): Experimental extrapolation suggests kuni < 50 s⁻¹ (lifetime > 20 ms). Best theoretical estimate for kuni ≈ 7 s⁻¹ at 298 K (uncertainty factor ~3); theoretical upper limit ~25 s⁻¹. - Atmospheric effective loss rate and lifetime (RH = 70%, 298 K): katm = kuni + kwater-eff[H₂O] ≈ 56 s⁻¹ (best estimate; < 74 s⁻¹ using kuni upper limit 25 s⁻¹), corresponding to τ ≈ 18 ms (best estimate; > 7.8 ms using 2σ upper bounds). - Theory vs experiment for H₂O reactions: Updated high-level calculations and basis-set corrections predict slower MACRO + H₂O rates than earlier theory, narrowing but not eliminating the gap with experiment; extended conjugation correlates with reduced reactivity toward water while retaining high reactivity toward SO₂. - Atmospheric impact: Given similar yields from isoprene ozonolysis (anti-MACRO ~15%, syn-MVKO ~14%), comparable atmospheric loss rates, and a larger SO₂ reaction rate, anti-MACRO is predicted to contribute at least as much, likely more, to SO₂ oxidation than syn-MVKO. CH₂OO, despite higher yield, is too short-lived under humid conditions to contribute substantially.

The study directly probes MACRO under thermal conditions, demonstrating that the anti conformer has a surprisingly long lifetime in humid air due to a much slower-than-expected reaction with water vapor and slow unimolecular decay. This overturns prior assumptions that anti-MACRO would be rapidly removed by water, thereby limiting its atmospheric relevance. Resonance stabilization from the conjugated C=C–C=O system appears to suppress both unimolecular decay and reactions with water, but does not diminish reactivity toward SO₂; indeed, anti-MACRO reacts with SO₂ extremely rapidly. Consequently, anti-MACRO can achieve significant steady-state concentrations in isoprene-rich, humid environments, enabling substantial SO₂ oxidation. Comparison with syn-MVKO indicates that, given similar production yields and comparable atmospheric loss rates, anti-MACRO's larger kSO2 confers an equal or greater role in SO₂ oxidation. Theoretical analysis highlights the need for high-level treatments (QCISD(T)/CBS and basis-set corrections) to accurately predict barriers for conjugated CI + H₂O reactions; earlier methods likely overestimated water reactivity. These findings reconcile discrepancies with ozonolysis-derived effective water rate coefficients by recognizing the complexity of mixed-CI systems and the dominant, rapid water loss of CH₂OO masking slower C4 CI losses.

This work synthesizes and spectroscopically identifies methacrolein oxide (MACRO) and quantifies its key atmospheric reactions under thermal conditions. The anti conformer exhibits an exceptionally fast reaction with SO₂, kSO2 = (1.5 ± 0.4) × 10⁻¹⁰ cm³ s⁻¹ at 298 K, while its reaction with water vapor is much slower than predicted, with kwater-eff = (9 ± 5) × 10⁻¹⁷ cm³ s⁻¹. Together with a slow unimolecular decay (best estimate kuni ≈ 7 s⁻¹), anti-MACRO attains an atmospheric lifetime of order 10⁻² s (≈18 ms at RH 70%, 298 K), far longer than previously thought. Given similar formation yields to syn-MVKO and a larger kSO2, anti-MACRO likely plays a substantial, potentially dominant, role in atmospheric SO₂ oxidation in isoprene-influenced regions. Future research should integrate updated anti-MACRO kinetics into global models, refine unimolecular rate determinations experimentally, and further develop high-accuracy theoretical treatments for conjugated CI reactions with water to reduce remaining uncertainties.

- The determination of kwater-eff is near the experimental detection limit; some kobs vs [H₂O] slopes are small or negative, and a strict lower bound could not be established. High [H₂O] at low total pressure (150 Torr) alters bath gas composition, complicating interpretation; thus, kwater-eff was derived from 300–500 Torr datasets. - The unimolecular decay rate kuni for anti-MACRO could not be directly measured due to concurrent bimolecular processes; the experimental estimate relies on extrapolation of kobs to zero [MACRO] and carries uncertainty. Theoretical kuni relies on high-level calculations and known underestimation biases, yielding an informed but approximate best estimate. - Assignment to anti-MACRO is based on lifetime considerations rather than direct conformer-specific spectroscopy under thermal conditions. - Spectral differences from jet-cooled measurements (absence of oscillatory structure) indicate sensitivity to temperature and conformer distributions. - Theoretical predictions for CI + H₂O reactions in conjugated systems remain sensitive to barrier heights and basis-set effects; despite improvements, discrepancies with experiment persist within an order of magnitude.