Intramolecular Hydrogen-shift Reactions of Peroxy Radicals

Straight chain alkanes with more than five carbons, emitted in cities due to incomplete combustion and fuel evaporation, undergo atmospheric gas-phase oxidation with the hydroxyl radical to produce alkyl radicals. These alkyl radicals subsequently add O2, leading to the formation of peroxy radicals. Following further reaction of these radicals in urban areas, hydroxy-substituted peroxy radicals are formed. Previously, the fate of these peroxy radicals was assumed to be dominated by reaction with nitric oxide, a common air pollutant. Computational and experimental investigations of the oxidation mechanism of 2-hexanol, however, demonstrate that hydrogens α to the hydroxy group exhibit a significantly reduced energetic barrier toward intramolecular hydrogen shifts to the peroxy radical. The barrier reduction for these hydrogen shift reactions results in rate constants that are orders of magnitude larger than for alkyl hydrogens that lack α substitution. Due to significant reductions of nitric oxide emissions in North America, these rate constants are sufficiently large to suggest that this chemistry is competitive even in large cities, particularly during warm summer days. Gas-phase alkyl hydroperoxides, a class of compounds previously expected to exist in negligible quantities in cities, are major products of this chemistry. Further oxidation of alkyl hydroperoxides leads to the formation of hydroperoxy-substituted peroxy radicals. The chemistry of such peroxy radicals is evaluated through the oxidation of 2-hydroperoxy-2-methylpentane. Experimental observations confirm the previously reported computational result that these peroxy radicals rapidly isomerize by intramolecular hydrogen shift of the hydroperoxide hydrogen. This isomerization occurs on timescales that are much faster than those of bimolecular reaction in essentially all regions of the troposphere. As a consequence of the isomerization, one peroxy radical isomer produced in the oxidation of 2-hydroperoxy-2-methylpentane exhibits an α hydroperoxide hydrogen shift. This reaction rate constant is similar to that reported for the α hydroxy hydrogen shift in the 2-hexanol system. Alkoxy radicals produced in the oxidation of 2-hydroperoxy-2-methylpentane are similarly shown to undergo a very rapid hydrogen shift of the hydroperoxide hydrogen. One of these shifts results in a peroxy radical that exhibits an α hydroxy hydrogen shift. Thus, the rapid scrambling of hydroperoxy-subsituted alkoxy and peroxy radicals is a key process that can enable additional unimolecular pathways that are otherwise inaccessible. This chemistry has the potential to introduce significant mechanistic complexity and, due to the rapid nature of the reactions, cannot be neglected even under typical "high NO" conditions employed in chamber studies.</p

Stereoselectivity in Atmospheric Autoxidation

We show that the diastereomers of hydroxy peroxy radicals formed from OH and O_2 addition to C2 and C3, respectively, of crotonaldehyde (CH_3CHCHCHO) undergo gas-phase unimolecular aldehydic hydrogen shift (H-shift) chemistry with rate coefficients that differ by an order of magnitude. The stereospecificity observed here for crotonaldehyde is general and will lead to a significant diastereomeric-specific chemistry in the atmosphere. This enhancement of specific stereoisomers by stereoselective gas-phase reactions could have widespread implications given the ubiquity of chirality in nature. The H-shift rate coefficients calculated using multiconformer transition state theory (MC-TST) agree with those determined experimentally using stereoisomer-specific gas-chromatography chemical ionization mass spectroscopy (GC–CIMS) measurements

Atmospheric autoxidation is increasingly important in urban and suburban North America

Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we combine computational and experimental approaches to investigate the rate of autoxidation for organic peroxy radicals (RO_2) produced in the oxidation of a prototypical atmospheric pollutant, n-hexane. We find that the reaction rate depends critically on the molecular configuration of the RO_2 radical undergoing hydrogen transfer (H-shift). RO_2 H-shift rate coefficients via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, respectively) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1 s^(−1) at 296 K, while the 1,4 H-shift is calculated to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coefficients of these reactions increase rapidly with temperature and exhibit a large, primary, kinetic isotope effect. The observed H-shift rate coefficients are sufficiently fast that, as a result of ongoing NO_x emission reductions, autoxidation is now competing with bimolecular chemistry even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temperatures are elevated

Stereoselectivity in Atmospheric Autoxidation

We show that the diastereomers of hydroxy peroxy radicals formed from OH and O_2 addition to C2 and C3, respectively, of crotonaldehyde (CH_3CHCHCHO) undergo gas-phase unimolecular aldehydic hydrogen shift (H-shift) chemistry with rate coefficients that differ by an order of magnitude. The stereospecificity observed here for crotonaldehyde is general and will lead to a significant diastereomeric-specific chemistry in the atmosphere. This enhancement of specific stereoisomers by stereoselective gas-phase reactions could have widespread implications given the ubiquity of chirality in nature. The H-shift rate coefficients calculated using multiconformer transition state theory (MC-TST) agree with those determined experimentally using stereoisomer-specific gas-chromatography chemical ionization mass spectroscopy (GC–CIMS) measurements

Atmospheric Fate of Methyl Vinyl Ketone: Peroxy Radical Reactions with NO and HO_2

First generation product yields from the OH-initiated oxidation of methyl vinyl ketone (3-buten-2-one, MVK) under both low and high NO conditions are reported. In the low NO chemistry, three distinct reaction channels are identified leading to the formation of (1) OH, glycolaldehyde, and acetyl peroxy R2a, (2) a hydroperoxide R2b, and (3) an α-diketone R2c. The α-diketone likely results from HO_x-neutral chemistry previously only known to occur in reactions of HO_2 with halogenated peroxy radicals. Quantum chemical calculations demonstrate that all channels are kinetically accessible at 298 K. In the high NO chemistry, glycolaldehyde is produced with a yield of 74 ± 6.0%. Two alkyl nitrates are formed with a combined yield of 4.0 ± 0.6%. We revise a three-dimensional chemical transport model to assess what impact these modifications in the MVK mechanism have on simulations of atmospheric oxidative chemistry. The calculated OH mixing ratio over the Amazon increases by 6%, suggesting that the low NO chemistry makes a non-negligible contribution toward sustaining the atmospheric radical pool

Atmospheric autoxidation is increasingly important in urban and suburban North America

Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we combine computational and experimental approaches to investigate the rate of autoxidation for organic peroxy radicals (RO_2) produced in the oxidation of a prototypical atmospheric pollutant, n-hexane. We find that the reaction rate depends critically on the molecular configuration of the RO_2 radical undergoing hydrogen transfer (H-shift). RO_2 H-shift rate coefficients via transition states involving six- and seven-membered rings (1,5 and 1,6 H-shifts, respectively) of α-OH hydrogens (HOC-H) formed in this system are of order 0.1 s^(−1) at 296 K, while the 1,4 H-shift is calculated to be orders of magnitude slower. Consistent with H-shift reactions over a substantial energetic barrier, we find that the rate coefficients of these reactions increase rapidly with temperature and exhibit a large, primary, kinetic isotope effect. The observed H-shift rate coefficients are sufficiently fast that, as a result of ongoing NO_x emission reductions, autoxidation is now competing with bimolecular chemistry even in the most polluted North American cities, particularly during summer afternoons when NO levels are low and temperatures are elevated

Low-pressure gas chromatography with chemical ionization mass spectrometry for quantification of multifunctional organic compounds in the atmosphere

Oxygenated volatile organic compounds (OVOCs) are formed during the oxidation of gas-phase hydrocarbons in the atmosphere. However, analytical challenges have hampered ambient measurements for many of these species, leaving unanswered questions regarding their atmospheric fate. We present the development of an in situ gas chromatography (GC) technique that, when combined with the sensitive and specific detection of chemical ionization mass spectrometry (CIMS), is capable of the isomer-resolved detection of a wide range of OVOCs. The instrument addresses many of the issues typically associated with chromatographic separation of such compounds (e.g., analyte degradation). The performance of the instrumentation is assessed through data obtained in the laboratory and during two field studies. We show that this instrument is able to successfully measure otherwise difficult-to-quantify compounds (e.g., organic hydroperoxides and organic nitrates) and observe the diurnal variations in a number of their isomers

Intramolecular Hydrogen Shift Chemistry of Hydroperoxy-Substituted Peroxy Radicals

Gas-phase autoxidation – the sequential regeneration of peroxy radicals (RO_2) via intramolecular hydrogen shifts (H-shifts) followed by oxygen addition – leads to the formation of organic hydroperoxides. The atmospheric fate of these peroxides remains unclear, including the potential for further H-shift chemistry. Here, we report H-shift rate coefficients for a system of RO_2 with hydroperoxide functionality produced in the OH-initiated oxidation of 2-hydroperoxy-2-methylpentane. The initial RO_2 formed in this chemistry are unable to undergo α-OOH H-shift (HOOC–H) reactions. However, these RO_2 rapidly isomerize (>100 s^(–1) at 296 K) by H-shift of the hydroperoxy hydrogen (ROO–H) to produce a hydroperoxy-substituted RO_2 with an accessible α-OOH hydrogen. First order rate coefficients for the 1,5 H-shift of the α-OOH hydrogen are measured to be ∼0.04 s^(–1) (296 K) and ∼0.1 s^(–1) (318 K), within 50% of the rate coefficients calculated using multiconformer transition state theory. Reaction of the RO_2 with NO produces alkoxy radicals which also undergo rapid isomerization via 1,6 and 1,5 H-shift of the hydroperoxy hydrogen (ROO–H) to produce RO_2 with alcohol functionality. One of these hydroxy-substituted RO_2 exhibits a 1,5 α-OH (HOC–H) H-shift, measured to be ∼0.2 s^(–1) (296 K) and ∼0.6 s^(–1) (318 K), again in agreement with the calculated rates. Thus, the rapid shift of hydroperoxy hydrogens in alkoxy and peroxy radicals enables intramolecular reactions that would otherwise be inaccessible

Investigation of a potential HCHO measurement artifact from ISOPOOH

Recent laboratory experiments have shown that a first generation isoprene oxidation product, ISOPOOH, can decompose to methyl vinyl ketone (MVK) and methacrolein (MACR) on instrument surfaces, leading to overestimates of MVK and MACR concentrations. Formaldehyde (HCHO) was suggested as a decomposition co-product, raising concern that in situ HCHO measurements may also be affected by an ISOPOOH interference. The HCHO measurement artifact from ISOPOOH for the NASA In Situ Airborne Formaldehyde instrument (ISAF) was investigated for the two major ISOPOOH isomers, (1,2)-ISOPOOH and (4,3)-ISOPOOH, under dry and humid conditions. The dry conversion of ISOPOOH to HCHO was 3 ± 2 % and 6 ± 4 % for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. Under humid (relative humidity of 40–60 %) conditions, conversion to HCHO was 6 ± 4 % for (1,2)-ISOPOOH and 10 ± 5 % for (4,3)-ISOPOOH. The measurement artifact caused by conversion of ISOPOOH to HCHO in the ISAF instrument was estimated for data obtained on the 6 September 2013 flight of the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. Prompt ISOPOOH conversion to HCHO was the source of < 4 % of the observed HCHO, including in the high-isoprene boundary layer. Time-delayed conversion, where previous exposure to ISOPOOH affects measured HCHO later in the flight, was conservatively estimated to be < 10 % of observed HCHO, and is significant only when high ISOPOOH sampling periods immediately precede periods of low HCHO