Hydroperoxides as hydrogen bond donors

\begin{wrapfigure}{r}{0pt} \centering \includegraphics[scale=0.32]{BEs.eps} \end{wrapfigure} Hydroperoxides are formed in the atmosphere following autooxidation of a wide variety of volatile organics emitted from both natural and anthropogenic sources. This raises the question of whether they can form hydrogen bonds that facilitate aerosol formation and growth. Using a combination of Fourier transform infrared spectroscopy, FT-IR, and {\it ab initio} calculations, we have compared the gas phase hydrogen bonding ability of {\it tert}-butylhydroperoxide (tBuOOH) to that of {\it tert}-butanol (tBuOH) for a series of bimolecular complexes with different acceptors. The hydrogen bond acceptor atoms studied are nitrogen, oxygen, phosphorus and sulphur. Both in terms of calculated redshifts and binding energies (BE), our results suggest that hydroperoxides are better hydrogen bond donors than the corresponding alcohols. In terms of hydrogen bond acceptor ability, we find that nitrogen is a significantly better acceptor than the other three atoms, which are of similar strength. We observe a similar trend in hydrogen bond acceptor ability with other hydrogen bond donors including methanol and dimethylamine

Unimolecular Reactions Following Indoor and Outdoor Limonene Ozonolysis

Limonene is one of the monoterpenes with the largest biogenic emissions and is also widely used as an additive in cleaning products, leading to significant indoor emissions. Studies have found that the formation of secondary organic aerosols (SOAs) from limonene oxidation has important implications for indoor air quality. Although ozonolysis is considered the major limonene oxidation pathway under most indoor conditions, little is known about the mechanisms for SOA formation from limonene ozonolysis. Here, we calculate the rate coefficients of the possible unimolecular reactions of the first-generation peroxy radicals formed by limonene ozonolysis using a high-level multiconformer transition state theory approach. We find that all of the peroxy radicals formed initially in the ozonolysis of limonene react unimolecularly with rates that are competitive both indoors and outdoors, except under highly polluted conditions. Differences in reactivity between the peroxy radicals from ozonolysis and those formed by OH, NO₃, and Cl oxidation are discussed. Finally, we sketch possible oxidation mechanisms for the different peroxy radicals under both indoor and pristine atmospheric conditions and in more polluted environments. In environments with low concentrations of HO₂ and NO, efficient autoxidation will lead to the formation of highly oxygenated organic compounds and thus likely aid in the growth of SOA

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

Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-pinene and β-pinene by Hydroxyl Radicals

Atmospheric oxidation of monoterpenes (emitted primarily by evergreen trees) is known to contribute to the formation and growth of aerosol particles. While recent research has tied the formation of organic aerosol to unimolecular chemistry of the organic peroxy radicals (RO_2) formed in the oxidation of monoterpenes, the fundamental physical chemistry of these RO_2 remains obscure. Here we use isomer-specific measurements and ab initio calculations to determine the unimolecular reaction rates and products of RO_2 derived from the hydroxyl radical (OH) oxidation of α-pinene and β-pinene. Among all of the structural isomers of the first-generation RO_2 from both monoterpenes, we find that the first-generation RO_2 produced following opening of the four-membered ring undergo fast unimolecular reactions (4 ± 2 and 16 ± 5 s^(–1) for α-pinene and β-pinene, respectively) at 296 K, in agreement with high-level ab initio calculations. The presence of the hydroxy group and carbon–carbon double bond in the ring-opened RO_2 enhances the rates of these unimolecular reactions, including endo-cyclization and H-shift via transition states involving six- and seven-membered rings. These reaction rate coefficients are sufficiently large that unimolecular chemistry is the dominant fate of these monoterpene-derived RO_2 in the atmosphere. In addition, the overall yields of first-generation α-pinene and β-pinene hydroxy nitrates, C_(10)H_(17)NO_4, at 296 K and 745 Torr are measured to be 3.3 ± 1.5% and 6.4 ± 2.1%, respectively, for conditions where all RO_2 are expected to react with NO ([NO] > 1000 ppbv). These yields are lower than anticipated

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

Unimolecular Reactions of Peroxy Radicals Formed in the Oxidation of α-pinene and β-pinene by Hydroxyl Radicals

Atmospheric oxidation of monoterpenes (emitted primarily by evergreen trees) is known to contribute to the formation and growth of aerosol particles. While recent research has tied the formation of organic aerosol to unimolecular chemistry of the organic peroxy radicals (RO_2) formed in the oxidation of monoterpenes, the fundamental physical chemistry of these RO_2 remains obscure. Here we use isomer-specific measurements and ab initio calculations to determine the unimolecular reaction rates and products of RO_2 derived from the hydroxyl radical (OH) oxidation of α-pinene and β-pinene. Among all of the structural isomers of the first-generation RO_2 from both monoterpenes, we find that the first-generation RO_2 produced following opening of the four-membered ring undergo fast unimolecular reactions (4 ± 2 and 16 ± 5 s^(–1) for α-pinene and β-pinene, respectively) at 296 K, in agreement with high-level ab initio calculations. The presence of the hydroxy group and carbon–carbon double bond in the ring-opened RO_2 enhances the rates of these unimolecular reactions, including endo-cyclization and H-shift via transition states involving six- and seven-membered rings. These reaction rate coefficients are sufficiently large that unimolecular chemistry is the dominant fate of these monoterpene-derived RO_2 in the atmosphere. In addition, the overall yields of first-generation α-pinene and β-pinene hydroxy nitrates, C_(10)H_(17)NO_4, at 296 K and 745 Torr are measured to be 3.3 ± 1.5% and 6.4 ± 2.1%, respectively, for conditions where all RO_2 are expected to react with NO ([NO] > 1000 ppbv). These yields are lower than anticipated

Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH₃C(O)CH₂O₂) self-reaction and its reaction with hydro peroxy (HO₂) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO₂ and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH₃C(O)CH₂O₂ concentrations. The overall rate constant for the reaction between CH₃C(O)CH₂O₂ and HO₂ was found to be (5.5 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH₃C(O)CH₂O₂ self-reaction rate constant was measured to be (4.8 ± 0.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO₂ self-reaction was also observed as a function of acetone (CH₃C(O)CH₃) concentration which is interpreted as a chaperone effect, resulting from hydrogen-bond complexation between HO₂ and CH₃C(O)CH₃. The chaperone enhancement coefficient for CH₃C(O)CH₃ was determined to be k_A″ = (4.0 ± 0.2) × 10⁻²⁹ cm⁶ molecule⁻² s⁻¹, and the equilibrium constant for HO₂·CH₃C(O)CH₃ complex formation was found to be K_c(R14) = (2.0 ± 0.89) × 10⁻¹⁸ cm³ molecule⁻¹; from these values, the rate constant for the HO₂ + HO₂·CH₃C(O)CH₃ reaction was estimated to be (2 ± 1) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. Results from UV absorption cross-section measurements of CH₃C(O)CH₂O₂ and prompt OH radical yields arising from possible oxidation of the CH₃C(O)CH₃-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and the prompt OH radical yields thus remain unexplained