Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via "Criegee intermediates"--carbonyl oxides--that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH(2)OO) as a product of the reaction of CH(2)I with O(2). This reaction enabled direct laboratory determinations of CH(2)OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H(2)O. The CH(2)OO reactions with SO(2) and NO(2) proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.

Although carbonyl oxides, "Criegee intermediates," have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10(-14) ± 0.4 × 10(-14) centimeter(3) second(-1).

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed ''roaming,'' has been described in the photodissociation of H 2CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH 3CHO to CH4 ؉ CO. The CH 4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at Ϸ95% of the total available energy. Quasiclassical trajectory calculations on fulldimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH 4 ؉ CO products is via a roaming pathway where a CH 3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH 4 ؉ CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH 3 roaming mechanism is attributed to the fact that the CH3 ؉ HCO radical asymptote and the TS saddle-point barrier to CH 4 ؉ CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H 2CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.reaction dynamics ͉ roaming mechanisms ͉ photochemistry ͉ quasiclassical trajectories ͉ transition state S ince its introduction by Eyring in 1935 (1), the concept of the ''transition state'' (TS) has been central to chemistry because the products, rates, and dynamics of a reaction are often determined by this special molecular configuration (2). For reactions with potential barriers, the TS is a transient molecular structure at the highest point along the minimum energy path connecting reactants to products and thus is a central construct in reaction rate theory and the classification of reaction types. In transition state theory (TST), the reaction rate coefficient is obtained from the ''one-way'' flux through a dividing surface containing the TS. In the more general variational version of TST, denoted VTST, the TS dividing surface is chosen to minimize the reactive flux. This theory is widely used in mathematical modeling of reaction rates in combustion, atmospheric, and biological chemistry, impacting fields as diverse as energy production (3), climate change (4), and enzyme function (5).Although the conventional transition state paradigm will remain essential to chemists, several reaction pathways have been reported in the last 8 years (6-13) in which it is not obvious how to use present implementations of TST or VTST. If such mechanisms are common, they may represent a significant challenge for reaction rate theories. In this report, we show that the ''roaming atom mechanism'' in formaldehyde (H 2 CO) dissociation (9) is not unique to H 2 CO but also occurs in acetaldehyde (CH 3 CHO) dissociation. Moreover, in CH 3 CHO, we find that the roa...

Criegee biradicals, i.e., carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF(3)COCF(3)), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH(2)OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH(2)I radicals, which react with O(2) to yield CH(2)OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH(2)OO and the formation of reaction products are monitored. The rate coefficient for CH(2)OO + hexafluoroacetone is k(1) = (3.0 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1), supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k(2) = (9.5 ± 0.7) × 10(-13) cm(3) molecule(-1) s(-1), and with acetone, k(3) = (2.3 ± 0.3) × 10(-13) cm(3) molecule(-1) s(-1), are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH(2)OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck-Condon-factor calculations. No secondary ozonide was observable in the reaction of CH(2)OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).

Rate coefficients are directly determined for the reactions of the Criegee intermediates (CI) CH2OO and CH3CHOO with the two simplest carboxylic acids, formic acid (HCOOH) and acetic acid (CH3COOH), employing two complementary techniques: multiplexed photoionization mass spectrometry and cavity-enhanced broadband ultraviolet absorption spectroscopy. The measured rate coefficients are in excess of 1×10−10 cm3 s−1, several orders of magnitude larger than those suggested from many previous alkene ozonolysis experiments and assumed in atmospheric modeling studies. These results suggest that the reaction with carboxylic acids is a substantially more important loss process for CIs than is presently assumed. Implementing these rate coefficients in global atmospheric models shows that reactions between CI and organic acids make a substantial contribution to removal of these acids in terrestrial equatorial areas and in other regions where high CI concentrations occur such as high northern latitudes, and implies that sources of acids in these areas are larger than previously recognized.

Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Carbonyl oxides, also known as Criegee intermediates, are key intermediates in both gas phase ozonolysis of unsaturated hydrocarbons in the troposphere and solution phase organic synthesis via ozonolysis. Although the study of Criegee intermediates in both arenas has a long history, direct studies in the gas phase have only recently become possible through new methods of generating stabilised Criegee intermediates in sufficient quantities. This advance has catalysed a large number of new experimental and theoretical investigations of Criegee intermediate chemistry. In this article we review the physical chemistry of Criegee intermediates, focusing on their molecular structure, spectroscopy, unimolecular and bimolecular reactions. These recent results have overturned conclusions from some previous studies, while confirming others, and have clarified areas of investigation that will be critical targets for future studies. In addition to expanding our fundamental understanding of Criegee intermediates, the rapidly expanding knowledge base will support increasingly predictive models of their impacts on society.

Carbonyl oxide species play a key role in tropospheric oxidation of organic molecules and in low-temperature combustion processes. In the late 1940s, Criegee first postulated the participation of carbonyl oxides, now often called "Criegee intermediates," in ozonolysis of alkenes. However, despite decades of effort, no gas phase Criegee intermediate has before been observed. As a result, knowledge of gas phase carbonyl oxide reactions has heretofore been inferred by indirect means, with derived rate coefficients spanning orders of magnitude. We have directly detected the primary Criegee intermediate, formaldehyde oxide (CH2OO), in the chlorine-initiated gas-phase oxidation of dimethyl sulfoxide (DMSO). This work not only establishes that the Criegee intermediate is formed in DMSO oxidation also but opens the possibility for explicit kinetics studies on this critical atmospheric species.