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.
Highlights
Isoprene (2-methyl-1,3-butadiene), a five-carbon doubly unsaturated hydrocarbon, is the most abundant non-methane species emitted into Earth’s atmosphere
Products from the reaction of MACR-oxide with formic acid are investigated using the Sandia Multiplexed Photoionization Mass Spectrometer (MPIMS) apparatus interfaced with the tunable vacuum ultraviolet (VUV) radiation of the Chemical Dynamics Beamline
The stationary points for the reaction of the four conformers of MACR-oxide with formic acid are determined at the CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ (CCSD(T)/TZF) level of theory including zero-point energy (ZPE) corrections
Summary
Isoprene (2-methyl-1,3-butadiene), a five-carbon doubly unsaturated hydrocarbon, is the most abundant non-methane species emitted into Earth’s atmosphere. The primary source of isoprene is foliar emissions from the southern (48%) and northern (38%) tropics, with total emissions approaching 600 Tg year−1 [1]. Ozonolysis is an important sink of tropospheric isoprene (~10%) and proceeds via 1,3-cycloaddition of ozone to either of the two C=C double bonds to yield a highly internally excited primary ozonide (POZ) as shown in Scheme 1 [2,3]. 1. Reaction scheme illustrating generation of anti-MACR-oxide
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