Electronic spectroscopy of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

Abstract

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the atmosphere, proceeds through methyl vinyl ketone oxide (MVK-oxide), methacrolein oxide, and formaldehyde oxide (CH 2 OO) Criegee intermediates. The present study focuses on MVK-oxide, a four-carbon unsaturated carbonyl oxide intermediate, using vacuum ultraviolet photoionization at 118 nm and UV-visible induced depletion of the m/z = 86 mass channel to characterize its first π * ← π electronic transition. The electronic spectrum is broad and unstructured with its peak at 388 nm (3.2 eV). The MVK-oxide spectrum is shifted to a significantly longer wavelength than CH 2 OO and alkylsubstituted Criegee intermediates studied previously due to extended conjugation across the vinyl and carbonyl oxide groups. Electronic excitation results in rapid dissociation at λ ≤ 430 nm to methyl vinyl ketone and O 1 D products, the latter detected by 2 + 1 resonance enhanced multiphoton ionization using velocity map imaging. Complementary electronic structure calculations (CASPT2(12,10)/AVDZ) predict two π * ← π transitions with significant oscillator strength for each of the four conformers of MVK-oxide with vertical excitation energies (and corresponding wavelengths) in the 3.1-3.6 eV (350-400 nm) and 4.5-5.5 eV (220-280 nm) regions. The computed electronic absorption profile of MVK-oxide, based on a Wigner distribution of ground state configurations and summed over the four conformers, is predicted to peak at 397 nm. UV-visible spectroscopy on the first π * ← π transition is shown by a combination of experiment and theory to provide a sensitive method for detection of the MVK-oxide Criegee intermediate that will enable further studies of its photochemistry and unimolecular and bimolecular reaction dynamics.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

“…Barber et al (30) recently reported a scheme for the selective production of MVK-oxide and employed this method to record an IR-action spectrum. The same scheme was used by Vansco et al (31) to obtain a UV-Vis depletion spectrum, which displayed broad absorption features from 300 to 450 nm, consistent with both syn-and anti-conformers. Both studies generated MVKoxide via 248 nm photolysis of 1,3-diiodobut-2-ene in O 2 /Ar carrier gas that was sampled following supersonic expansion (T∼10 K) under collision-free conditions.…”

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

“…Both have extended conjugation involving six π electrons across the vinyl (CH 2 =C) and carbonyl oxide (C=O + -O -) functional groups. [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).…”

“…distinctly different than simple saturated carbonyl oxides, such as formaldehyde oxide (CH 2 OO) and alkyl-substituted Criegee intermediates, which have four π electron systems (C=O + O -). [8][9][10][11][12][13][14][15][16] In addition, MVK-oxide and MACR-oxide each have four conformational forms with similar predicted ground state energies (within ca. 3 kcal mol -1 ).…”

Surprisingly Long Lifetime of Methacrolein Oxide, an Isoprene Derived Criegee Intermediate, Under Humid Conditions

“…[32][33] Via this new synthesis scheme, recent studies have shown that the resonancestabilization would affect the reactivity and thus the atmospheric fate of MVKO. [32][33][34][35][36] For MVKO, there are four possible isomers/conformers. 2,32,[34][35] Similar to simpler CIs, the barrier of rotating the carbonyl C = O bond is high, resulting in non-interconverting syn and anti isomers (following the nomenclature of Barbar et al).…”

“…[32][33][34][35][36] For MVKO, there are four possible isomers/conformers. 2,32,[34][35] Similar to simpler CIs, the barrier of rotating the carbonyl C = O bond is high, resulting in non-interconverting syn and anti isomers (following the nomenclature of Barbar et al). 32,37 However, the barrier of rotating the C─C single bond between the C = C and C = O bonds is low, giving essentially an equilibrium mixture of cis and trans conformers.…”