Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production

Abstract

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CHOO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH, CH═CH) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

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

“…This is followed by barrierless C-C bond fission to acetyl radical and vinoxy radical products. 15 An analogous isomerization pathway to dioxole is predicted for syn-MACR-oxide, which generates 4methyl-3H-1,2-dioxole (dioxole-2) as shown in Scheme 4. 19 In Scheme 4, energies (kcal mol -1 ) are reported by Kuwata et al 19 relative to the lowest energy anti-trans-MACR-oxide conformer (CBS-QB3// B3LYP/6-311G(d,p)).…”

“…Experiments are performed using the Sandia Multiplexed Photoionization Mass Spectrometer (MPIMS) apparatus interfaced with the tunable-VUV radiation of the Chemical Dynamics Beamline 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 11 2-ene or 1,3-diiodo-2-methylprop-1-ene precursor, respectively, as described by Barber et al 15 and Vansco et al 9 .…”

“…Pulsed UV-photolysis of the precursors generates a resonance-stabilized monoiodoalkenyl radical that subsequently reacts with O 2 to produce the Criegee intermediates. 9,15 The gas mixture is continuously sampled through an orifice on the side of the reactor tube and the resultant molecular beam is intercepted orthogonally with the tunable VUV radiation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60…”

“…Here, is the thermal decay rate for anti-MVK-oxide (2140 s -1 ) 15 and is the effective rate of reaction between anti-MVK-oxide and formic acid at a specific concentration. The model assumes that 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 24 m/z 30 arises solely from unimolecular decay of anti-MVK-oxide to radical products and subsequent peroxy radical chemistry, the thermal decay rate (298 K) of anti-MVK-oxide is 2140 s -1 , and the rate constant for the reaction of anti-MVK-oxide with formic acid is the same as that for syn-MVK-oxide (3.0  10 -10 cm 3 s -1 ).…”

“…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 ).…”