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Organic hydrotrioxides (ROOOH) are known to be strong oxidants used in organic synthesis. Previously, it has been speculated that they are formed in the atmosphere through the gas-phase reaction of organic peroxy radicals (RO 2 ) with hydroxyl radicals (OH). Here, we report direct observation of ROOOH formation from several atmospherically relevant RO 2 radicals. Kinetic analysis confirmed rapid RO 2 + OH reactions forming ROOOH, with rate coefficients close to the collision limit. For the OH-initiated degradation of isoprene, global modeling predicts molar hydrotrioxide formation yields of up to 1%, which represents an annual ROOOH formation of about 10 million metric tons. The atmospheric lifetime of ROOOH is estimated to be minutes to hours. Hydrotrioxides represent a previously omitted substance class in the atmosphere, the impact of which needs to be examined.

The vibrational spectroscopy of the water dimer provides an understanding of basic hydrogen bonding in water clusters, and with about one water dimer for every 1,000 water molecules, it plays a critical role in atmospheric science. Here, we review how the experimental and theoretical progress of the past decades has improved our understanding of water dimer vibrational spectroscopy under both cold and warm conditions. We focus on the intramolecular OH-stretching transitions of the donor unit, because these are the ones mostly affected by dimer formation and because their assignment has proven a challenge. We review cold experimental results from early matrix isolation to recent mass-selected jet expansion techniques and, in parallel, the improvements in the theoretical anharmonic models. We discuss and illustrate changes in the vibrational spectra of complexes upon increasing temperature, and the difficulties in recording and calculating these spectra. In the atmosphere, water dimer spectra at ambient temperature are crucial.

In this study we revisit one of the simplest reactions: the self-reaction of the ethene-derived hydroxyperoxy radical formed via sequential addition of ˙OH and O2 to ethene. Previous studies of this reaction suggested that the branching to ‘accretion products’, compounds containing the carbon backbone of both reactants, was minimal. Here, CF3O− GC-CIMS is used to quantify the yields of ethylene glycol, glycolaldehyde, a hydroxy hydroperoxide produced from , and a C4O4H10 accretion product. These experiments were performed in an environmental chamber at 993 hPa and 294 K. We provide evidence that the accretion product is likely dihydroxy diethyl peroxide (HOC2H4OOC2H4OH 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 ROOR) and forms in the gas-phase with a branching fraction of 23 ± 5%. We suggest a new channel in the chemistry leading directly to the formation of (together with glycolaldehyde and an alkoxy radical). Finally, by varying the ratio of the formation rate of and in our chamber, we constrain the ratio of the rate coefficient for the reaction of to that of and find that this ratio is 0.22 ± 0.07, consistent with previous flash photolysis studies.

Oxidation of the monoterpene Δ3-carene (C10H16) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Δ3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C7-C10 species. We then use computational methods to develop the first stages of a Δ3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the α-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Δ3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH3S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH3S will predominantly react with O2 to form the peroxy radical CH3SOO. The formed CH3SOO has two competing unimolecular reaction pathways: isomerization to CH3SO2, which further decomposes into CH3 and SO2, or a hydrogen shift followed by HO2 loss, leading to CH2S. Previous theoretical calculations have suggested that CH2S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH3 and SO2 is the dominant route and the formation of CH2S and HO2 can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH3SH reaction for CH3S formation under low NOx conditions and find a SO2 yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO2 yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.

Autoxidation in the atmosphere has been realized in the last decade as an important process that forms highly oxidized products relevant for the formation of secondary organic aerosol and likely with detrimental human health effects. It is experimentally shown that the OH radical-initiated oxidation of trimethylamine, the most highly emitted amine in the atmosphere, proceeds via rapid autoxidation steps dominating its atmospheric oxidation process. All three methyl groups are functionalized within a timescale of 10 s following the reaction with OH radicals leading to highly oxidized products. The exceptionally large density of functional groups in the oxidized products is expected to define their chemical properties. A detailed reaction mechanism based on theoretical calculations is able to describe the experimental findings. The comparison with results of the reinvestigated OH radical- and ozone-initiated autoxidation of a series of terpenes and aromatics reveals the trimethylamine process as the most efficient one discovered up to now for atmospheric conditions.

n-Pentanol has been considered as a promising alternative fuel for compression-ignition engines due to its potential to reduce greenhouse gases and pollutant emissions. Engine performance is strongly dominated by fuel oxidation chemistry, and thus a more accurate determination of the coefficients of the reactions ruling its oxidation is essential for the utilization of n-pentanol in combustion engines. The reactions involving 1‑hydroxy‑1-pentyl and molecular oxygen were found to play an important role in controlling the low temperature oxidation chemistry, but have not been investigated experimentally or theoretically; this is also the case for the reactions of the 1‑hydroxy‑1-peroxypentyl radical, which is formed by the addition of oxygen to the radical center of 1‑hydroxy‑1-pentyl. This work presents a theoretical study with high level ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level of theory to shed light on the fate of the 1‑hydroxy‑1-peroxypentyl radical. The rate coefficients of all the possible intra-molecular hydrogen shift reactions of that radical were computed using variational transition state theory with small curvature tunneling corrections. For certain reactions, tunneling and variational effects are very pronounced, proving the need for robust methodologies to account for these effects. The hydrogen shift reaction leading to a concerted HO2 elimination and formation of n-pentanal is the dominant pathway and governs the reactivity of 1‑hydroxy‑1-peroxypentyl radical at any temperature. The reverse of this reaction was thereby investigated as well. For this prominent pathway, the effects of multistructural (multiple conformers) torsional anharmonicity of the stationary points were taken into account in order to refine the forward and reverse rate coefficients. The rate coefficients calculated at room temperature are compared to those calculated using a previously developed cost-effective multi-conformer transition state theory approach. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory was used to compute the pressure-dependent rate coefficients, which indicate significant pressure dependence at intermediate and high temperatures. Implementation of the calculated reaction rate coefficients in chemical kinetics models of n-pentanol revealed that our computed rate coefficients enable better insights into the chemistry of n-pentanol, and help to understand how n-pentanal is formed.

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.

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene, monoterpenes, and very recently, dimethyl sulfide. Here, we present a high-level theoretical multiconformer transition-state theory study of the atmospheric autoxidation in amines exemplified by the atmospherically important trimethylamine (TMA) and dimethylamine and generalized by the study of the larger diethylamine. Overall, we find that the initial hydrogen shift reactions have rate coefficients greater than 0.1 s

The spectroscopy of cis-OSSO and trans-OSSO is explored and put into the context of the Venusian atmosphere, along with other sulfur compounds potentially present there, namely, S2O, C1-S2O2, trigonal-S2O2, and S3. UV-vis spectra were calculated using the nuclear ensemble approach. The calculated OSSO spectra are shown to match well with the 320-400 nm near-UV absorption previously measured on Venus, and we discuss the challenges of assigning OSSO as the Venusian near-UV absorber. The largest source of uncertainty is getting accurate concentrations of sulfur monoxide (3SO) in the upper cloud layer of Venus (60-70 km altitude) since the 3SO self-reaction is what causes cis- and trans-OSSO to form. Additionally, we employed the matrix-isolation technique to trap OSSO formed by microwave discharging a gas mixture of argon and SO2 and then depositing the mixture onto a cold window (6-12 K). Anharmonic vibrational transition frequencies and intensities were calculated at the coupled cluster level to corroborate the matrix-isolation FTIR spectra. The computationally calculated UV-vis and experimentally recorded IR spectra presented in this work aid future attempts at detecting these sulfur compounds in the Venusian atmosphere.

We have observed nine bimolecular hydrogen- or deuterium-bound complexes at room temperature using Fourier transform infrared (FTIR) spectroscopy. The complexes were formed using methanol or ethanol as hydrogen bond donors, as well as deuterated isotopologues of these, in order to study isotopic effects on hydrogen bonds. The complexes were formed using either a dimethylether- (O) or trimethylamine (N) acceptor, to facilitate comparison of two different types of hydrogen or deuterium bonds, OH(D)·O and OH(D)·N. For each complex, the characteristic OH- or OD-stretching fundamental band in the bimolecular complex was observed. The Gibbs energy of complex formation was determined at room temperature for each complex to compare the relative stability of hydrogen- and deuterium-bound bimolecular complexes. It is well known that deuterium-bound complexes are more stable at low temperatures because of the lower frequency of its intermolecular modes and thus a lower zero point vibrational energy. However, at room temperature, entropic contributions to the stability should also be considered. At room temperature, we find the Gibbs energy of complex formation for each pair of corresponding hydrogen- and deuterium-bound complex to be similar. The similar values of the Gibbs energies at room temperature is explained from a difference in the entropy, upon complexation, which favors the formation of the hydrogen-bound complex more than the deuterium-bound complex at higher temperatures.

Fourier transform infrared spectroscopy has been used to follow the reaction of CH

While action spectroscopy of cold molecular ions is a well-established technique to provide vibrationally resolved absorption features, fluorescence experiments are still challenging. Here we report the fluorescence spectra of pyronin-Y and resorufin ions at 100 K using a newly constructed setup. Spectra narrow upon cooling, and the emission maxima blueshift. Temperature effects are attributed to the population of vibrational excited levels in S1, and that frequencies are lower in S1 than in S0. This picture is supported by calculated spectra based on a Franck-Condon model that not only predicts the observed change in maximum, but also assigns Franck-Condon active vibrations. In-plane vibrational modes that preserve the mirror plane present in both S0 and S1 of resorufin and pyronin Y account for most of the observed vibrational bands. Finally, at low temperatures, it is important to pick an excitation wavelength as far to the red as possible to not reheat the ions.

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH

The infrared (IR) spectrum of tert-butyl hydroperoxide (TBHP) in the region of the first OH-stretching overtone has been observed under jet-cooled and thermal (300 K, 3 Torr) conditions at ∼7017 cm−1. The jet-cooled spectrum is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of OH radical products, while direct IR absorption is utilized under thermal conditions. Prior spectroscopic studies of TBHP and other hydroperoxides have shown that the OH-stretch and XOOH (X = H or C) torsion vibrations are strongly coupled, resulting in a double well potential associated with the torsional motion about the OO bond that is different for each of the OH-stretching vibrational states. A low barrier between the wells on the torsional potential results in tunneling split energy levels, which leads to four distinct transitions associated with excitation of the coupled OH-stretch-torsion states. In order to interpret the experimental results, two theoretical models are used that include the OH-stretch-torsion coupling in TBHP. Both methods are utilized to compute the vibrational transitions associated with the coupled OH-stretch-torsion states of TBHP, revealing the underlying transitions that compose the experimentally observed features. A comparison between theory and experiment illustrates the necessity for treatments that include OH-stretch and COOH torsion in order to unravel the spectral features observed in the first OH-stretching overtone region of TBHP.

The vibrational spectra of gas phase tert-butyl hydroperoxide have been recorded in the OH-stretching fundamental and overtone regions (ΔvOH = 1–5) at room temperature using conventional Fourier transform infrared (ΔvOH = 1–3) and cavity ring-down (ΔvOH = 4–5) spectroscopy. In hydroperoxides, the OH-stretching and COOH torsion vibrations are strongly coupled. The double-well nature of the COOH torsion potential leads to tunneling splitting of the energy levels and, combined with the low frequency of the torsional vibration, results in spectra in the OH-stretching regions with multiple vibrational transitions. In each of the OH-stretching regions, both an OH-stretching and a stretch–torsion combination feature are observed, and we show direct evidence for the tunneling splitting in the OH-stretching fundamental region. We have developed two complementary vibrational models to describe the spectra of the OH-stretching regions, a reaction path model and a reduced dimensional local mode model, both of which describe the features of the vibrational spectra well. We also explore the torsional dependence of the OH-stretching transition dipole moment and show that a Franck–Condon treatment fails to capture the intensity in the region of the stretch–torsion combination features. The accuracy of the Franck–Condon treatment of these features improves with increasing ΔvOH.

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.

Absolute intensities of the ΔvOH = 1 − 2 and ΔvOD = 1 − 3 transitions were determined for a range of alcohols (methanol, ethanol, 2-propanol, 1-propanol and tert-butanol) using conventional Fourier transform infrared (FTIR) spectroscopy. The intensities of the OH stretching transitions are stronger than the corresponding OD stretching transitions and become increasingly stronger with higher overtone transitions as expected from the reduced masses of the oscillators. Furthermore, accurate absolute intensities of the third and fourth OH stretching overtone transitions were determined using our newly constructed integrated cavity ring down (CRD) and FTIR spectrometer with experimental uncertainties generally less than 10%. The experiments were complemented by local mode calculations, with the potential energy surfaces and the dipole moment functions determined at the CCSD(T)/aug-cc-pVTZ level of theory. The calculated oscillator strengths of the ΔvOH = 4 − 5 transitions are within 25% of the experimental results.

With an annual emission of about 500 Tg, isoprene is an important molecule in the atmosphere. While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen-shift (H-shift) reactions remain speculative. Using a high-level multiconformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation mechanism. We find that most of the (1,4, 1,5, and 1,6) aldehydic and (1,5 and 1,6) α-hydroxy H-shifts have rate constants at 298.15 K in the range 10–2 to 1 s–1, which make them competitive with bimolecular reactions in the atmosphere under typical atmospheric conditions. In addition, we find that the rate coefficients of different diastereomers can differ by up to 3 orders of magnitude, illustrating the importance of chirality. Implementation of our calculated reaction rate coefficients into the most recent GEOS-Che...

Formation and growth of atmospheric aerosols are governed by the Gibbs energy of complex formation (ΔG⦵). A number of hydrogen bound bimolecular complexes in the gas phase at room temperature have ...

Here, we present a new instrument named LUNA2 (LUminescence iNstrument in Aarhus 2), which is purpose-built to measure dispersed fluorescence spectra of gaseous ions produced by electrospray ionization and cooled to low temperatures (

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.

Emissions of aromatic compounds cause air pollution and detrimental health effects. Here, we explore the reaction kinetics and products of key radicals in benzene photo-oxidation. After initial OH addition and reaction with O₂, the effective production rates of phenol and bicyclic peroxy radical (BCP-peroxy) are experimentally constrained at 295 K to be 420 ± 80 and 370 ± 70 s⁻¹, respectively. These rates lead to approximately 53% yield for phenol and 47% yield for BCP-peroxy under atmospheric conditions. The reaction of BCP-peroxy with NO produces bicyclic hydroxy nitrate with a branching ratio

The atmospheric reaction of OH radicals with dimethyl disulfide, CH3SSCH3, proceeds primarily via OH addition forming CH3S and CH3SOH as reactive intermediates, and to a lesser extent via H-abstraction resulting in the peroxy radical CH3SSCH2OO in the presence of O2. The latter undergoes a fast two-step isomerization process leading to HOOCH2SSCHO. CH3S and CH3SOH are both converted to SO2 and CH3O2 with near unity yields under atmospheric conditions.

We have detected the H2O·DMA and H2O·TMA (DMA, dimethylamine; TMA, trimethylamine) bimolecular complexes at room temperature in the gas phase using Fourier transform infrared spectroscopy. For both complexes, five vibrational bands associated with the H2O molecule are observed and assigned. Within a reduced dimensional local mode framework, we set up a six-dimensional model, including the three H2O vibrational modes and three of the six intermolecular modes, all described with internal curvilinear coordinates. The single points on the potential energy surface and Eckart corrected dipole moment surface are calculated with the CCSD­(T)-F12a/cc-pVDZ-F12 method. Combining the measured and calculated transition intensities, we determine the Gibbs energy of complex formation of both complexes from each of the observed bands. The multiple determinations give similar Gibbs energies, for each complex, and increase the confidence in the combined experimental and theoretical approach, and improve the accuracy of the determined Gibbs energies. The average Gibbs energies of complex formation are found to be 5.0 ± 0.2 and 3.8 ± 0.2 kJ/mol for H2O·DMA and H2O·TMA, respectively. In addition to the experimental uncertainty, there is a potential error on the calculated intensities corresponding to 0.4 kJ/mol. However, the small spread among the four determinations suggests that this error is even less. The Gibbs energies of these complexes serve as accurate benchmarks for theoretical approaches that are prevalent in hydrogen bonding and nucleation studies.

Monoterpenes are a group of volatile organic compounds that are emitted to the atmosphere in large amounts by natural sources. Some monoterpenes such as limonene and Δ3-carene are also widely used as additives in detergents and perfumes, and thus have a potential impact on indoor air quality and human health.The volatile organic compounds like monoterpenes may undergo a series of autoxidation processes in the atmosphere to form highly oxygenated compounds, which have been linked to the formation of secondary organic aerosols. For this process to occur, the unimolecular reactions of the peroxy radicals formed during oxidation must have rate coefficients comparable to or greater than those of the competing bimolecular reactions with HO2, NO or other RO2 radicals.We studied the hydrogen shift (H-shift) and the cyclization reactions of all 45 hydroxy peroxy radicals formed by hydroxyl radical (OH) and O2 addition to six monoterpenes (α-pinene, β-pinene, Δ3-carene, camphene, limonene and terpinolene). The reaction rate coefficients of the possible unimolecular reaction were initially studied at a lower level of theory. Those deemed likely to be atmospherically competitive were then calculated using the multi-conformer transition states theory approach developed by Møller et al. (J. Phys. Chem. A, 120, 51, 10072-10087, 2016). This approach has been shown to agree with the experimental values to within a factor of 4 for other systems.It was found that double bonds are key to fast unimolecular reactions in the first-generation monoterpene hydroxy peroxy radicals. The H-shift reactions abstracting a hydrogen from a carbon adjacent to a double bond are found to typically be fast enough to compete with the bimolecular reactions, likely due to the resonance stability of the nascent allylic radical. The reactivity of the cyclization reaction between the carbon-carbon double bonds and the peroxy group, which forms an endoperoxide ring, is high as well. The H-shifts abstracting the hydrogen from the hydroxy group may be competitive in some cases but the reaction rate coefficients for these reactions are more uncertain. Generally, the cyclization reaction and the allylic H-shift reactions are the dominant reaction paths for the studied peroxyl radicals. Since the OH radical addition consumes one double bond, we suggest that the monoterpenes with more than one double bond in their structure are likely to have unimolecular reactions that can be important for the first-generation monoterpene peroxy radicals. On the other hand, the ones with only one double bond initially are not likely to have fast unimolecular reactions that can compete with the bimolecular reactions under the atmospheric condition, unless a double bond can be formed during their oxidation process as found for α-pinene and β-pinene. This result greatly limits the amount of potentially important unimolecular reaction paths in atmospheric monoterpene oxidation.

The importance of peroxy radical hydrogen shift reactions in the atmosphere has gained acceptance in recent years. Recent theoretical calculations have suggested that these can be stereoselective i.e. that different stereoisomers react with significantly different rate coefficients. Combining experiments (GC-CIMS) with high-level calculations (MC-TST), we show that two hydroxy peroxy radical diastereomers formed in the oxidation of crotonaldehyde have rate coefficients for their peroxy radical hydrogen shift reactions that differ by more than a factor of 10. The difference is large enough that under urban atmospheric conditions, one diastereomer would react primarily by the unimolecular H-shift, while the other would react mainly by bimolecular reactions leading to diastreomeric enhancement of the products.For a large set of peroxy radical hydrogen shift reactions in the oxidation of isoprene, the stereospecific rate coefficients are calculated to assess the global importance of this phenomenon in the atmosphere. These calculated rate coefficients are implemented into the global chemistry-transport model GEOS-Chem to model the effect. Results show that more than 30 % of all isoprene molecules emitted undergo a minimum of one peroxy radical hydrogen shift reaction during its oxidation. Furthermore, the results show that the different diastereomers may react with rate coefficients differing by up to almost a factor of 1000, highlighting how important it is to account for this phenomenon.

Monoterpenes are a group of volatile organic compounds (VOCs) emitted to the atmosphere in large amounts. Studies have linked the autoxidation of monoterpenes to the formation of secondary organic aerosols, which impact Earth's climate and human health. Here, we study the hydroxy peroxy radicals formed by OH- and O2-addition to the six atmospherically relevant monoterpenes α-pinene, β-pinene, Δ3-carene, camphene, limonene, and terpinolene. The six monoterpenes all have a six-membered ring but differ in the binding pattern of the four remaining carbon atoms and the position of the double bond(s). We use a multiconformer transition state theory approach to calculate the rate coefficients of the peroxy radical hydrogen-shift (H-shift) and endoperoxide formation reactions of these peroxy radicals. Our results suggest that primarily the isomers with a carbon-carbon double bond remaining after OH- and O2-addition undergo unimolecular reactions with rate coefficients large enough to be of atmospheric importance. This greatly limits the number of potentially important unimolecular pathways. Specifically, we find that the ring-opened α- and β-pinene isomers as well as isomers of limonene and terpinolene have unimolecular reactions that are fast enough to likely dominate their reactivity under most atmospheric conditions.

Absolute OH- and OD-stretching transition intensities have been calculated for a series of alcohols (methanol, ethanol, 2-propanol, 1-propanol, and tert-butanol) with one-dimensional (1D) and three-dimensional (3D) local mode models. We compare the calculated intensities for the ΔvOH = 1-5 and ΔvOD = 1-3 transitions with experimental values. Potential energy and dipole moment surfaces are calculated at the CCSD(T)-F12a/VDZ-F12 level of theory. The 1D local mode model includes only the OH(D)-stretching mode, whereas the 3D local mode model also includes the CO-stretching and COH(D)-bending modes. We analyze the effect on vibrational intensities of using either a molecule-fixed Eckart frame or a space-fixed Cartesian frame. We find that both Eckart embedding and inclusion of the CO-stretching and COH(D)-bending modes in the local mode model are important for the OH/OD-stretching fundamental transition intensities, but have a minor effect on overtone intensities. The 3D reduced-dimensional local model, when combined with coupled cluster surfaces, accurately predicts OH/OD-stretching transition intensities and wavenumbers, for all alcohols included in this work.

The atmospheric reaction of OH radicals with dimethyl disulfide, CH3SSCH3, proceeds primarily via OH addition forming CH3S and CH3SOH as reactive intermediates, and to a lesser extent via H-abstraction resulting in the peroxy radical CH3SSCH2OO in the presence of O2. The latter undergoes a fast two-step isomerization process leading to HOOCH2SSCHO. CH3S and CH3SOH are both converted to SO2 and CH3O2 with near unity yields under atmospheric conditions.

The conformer-specific reactivity of gas-phase pyruvic acid following the S1(nπ*) ← S0 excitation at λmax = 350 nm (290-380 nm) and the effect of water are investigated for the two lowest energy conformers. Conformer-specific gas-phase pyruvic acid photolysis rate constants and their respective populations are measured by monitoring their distinct vibrational OH-stretching frequencies. The geometry, relative energies, fundamental vibrational frequencies, and electronic transitions of the pyruvic acid conformers and their monohydrated complexes are calculated with density functional theory and ab initio methods. Results from experiment and theory show that the more stable conformer with an intramolecular hydrogen bond dominates the gas-phase photolysis of pyruvic acid. Water greatly affects the gas-phase pyruvic acid conformer population and photochemistry through hydrogen bonding interactions. The addition of water decreases the gas-phase relative population of the more stable conformer and decreases the molecule's gas-phase photolysis rate constants. The theoretical results show that even a single water molecule interrupts the intramolecular hydrogen bond, which is essential for the efficient photodissociation of gas-phase pyruvic acid. Results of this study suggest that the aqueous-phase photochemistry of pyruvic acid proceeds through hydrogen-bonded conformers lacking an intramolecular hydrogen bond.

6 pags., 5 figs., Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate., Additional National Oceanic and Atmospheric Administration (NOAA) support for ATom was provided by NASA funding via Inter-Agency Transfer NNH15AB12l and by funding from the NOAA Climate Program Office and the NOAA Atmospheric Chemistry, Carbon Cycle, and Climate program. K.H.M. and H.G.K. acknowledge the financial support of the Independent Research Fund Denmark, the University of Copenhagen, and the Danish Ministry for Higher Education and Science’s Elite Research travel grant. J.L.J.’s group acknowledges NASA grants NHX15AH33A and 80NSSC19K0124. A.K. was supported by the Austrian Science Fund’s Erwin Schrodinger Fellowship. A.S.-L., Q.L., and C.A.C. are supported by the European Research Council (ERC) Executive Agency under the European Union’s Horizon 2020 Research and Innovation programme (Project ERC-2016-COG 726349 CLIMAHAL). The National Center for Atmospheric Research is sponsored by the National Science Foundation. E.A. and J.B. were funded in part by NASA Atmospheric Composition Program. T.H.B. and C.M.J. acknowledge support from the National Science Foundation Center for Aerosol Impacts on Chemistry of the Environment under grant CHE 1801971. B.W. and M.D. have received funding from the ERC under the European Union’s Horizon 2020 research and innovation framework program under grant 640458 (A-LIFE) and from the University of Vienna.

We studied the reaction of electronically excited sulfur dioxide in the triplet state (3SO2) with a variety of alkane species, including propane, n-butane, isobutane, n-pentane, n-hexane, cyclohexa...

Epoxide formation was established a decade ago as a possible reaction pathway for beta-hydroperoxy alkyl radicals in the atmosphere. This epoxide-forming pathway required excess energy to compete with O-2 addition, as the thermal reaction rate coefficient is many orders of magnitude too slow. However, recently, a thermal epoxide forming reaction was discovered in the ISOPOOH + OH oxidation pathway. Here, we computationally investigate the effect of substituents on the epoxide formation rate coefficient of a series of substituted beta-hydroperoxy alkyl radicals. We find that the thermal reaction is likely to be competitive with O-2 addition when the alkyl radical carbon has a OH group, which is able to form a hydrogen bond to a substituent on the other carbon atom in the epoxide ring being formed. Reactants fulfilling these requirements can be formed in the OH-initiated oxidation of many biogenic hydrocarbons. Further, we find that beta-OOR alkyl radicals react similarly to beta-OOH alkyl radicals, making epoxide formation a possible decomposition pathway in the oxidation of ROOR peroxides. GEOS-Chem modeling shows that the total annual production of isoprene dihydroxy hydroperoxy epoxide is 23 Tg, making it by far the most abundant C-5-tetrafunctional species from isoprene oxidation.

Hydroxymethyl hydroperoxide (HMHP), formed in the reaction of the C1 Criegee intermediate with water, is among the most abundant organic peroxides in the atmosphere. Although reaction with OH is thought to represent one of the most important atmospheric removal processes for HMHP, this reaction has been largely unstudied in the laboratory. Here, we present measurements of the kinetics and products formed in the reaction of HMHP with OH. HMHP was oxidized by OH in an environmental chamber; the decay of the hydroperoxide and the formation of formic acid and formaldehyde were monitored over time using CF_3O– chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF). The loss of HMHP by reaction with OH is measured relative to the loss of 1,2-butanediol [k_(1,2-butanediol+OH) = (27.0 ± 5.6) × 10^(–12) cm^3 molecule^(–1)s^(–1)]. We find that HMHP reacts with OH at 295 K with a rate coefficient of (7.1 ± 1.5) × 10^(–12) cm^3molecule^(–1)s^(–1), with the formic acid to formaldehyde yield in a ratio of 0.88 ± 0.21 and independent of NO concentration (3 × 10^(10) – 1.5 × 10^(13) molecules cm^(–3)). We suggest that, exclusively, abstraction of the methyl hydrogen of HMHP results in formic acid, while abstraction of the hydroperoxy hydrogen results in formaldehyde. We further evaluate the relative importance of HMHP sinks and use global simulations from GEOS-Chem to estimate that HMHP oxidation by OH contributes 1.7 Tg yr^(–1) (1–3%) of global annual formic acid production.

The reaction of electronically excited triplet state sulfur dioxide (3SO2) with water was investigated both theoretically and experimentally. The quantum chemical calculations find that the reaction leads to the formation of hydroxyl radical (OH) and hydroxysulfinyl radical (HOSO) via a low energy barrier pathway. Experimentally the formation of OH was monitored via its reaction with methane, which itself is relatively unreactive with 3SO2, making it a suitable probe of OH production from the reaction of 3SO2 and water. This reaction has implications for the formation of OH in environments that are assumed to be depleted in OH, such as volcanic plumes. This reaction also provides a mechanism for the formation of OH in planetary atmospheres with little or no oxygen (O2) or ozone (O3) present.

Vibrational transition frequencies and oscillator strengths of the FH-stretching transitions in the FH⋯CO, FH⋯FH, FH⋯CO 2 and FH⋯N 2 complexes were calculated with different vibrational models. The calculated vibrational frequencies were found to agree well with experimental values. The experimental oscillator strengths of the FH-stretches in the FH monomer and FH dimer were also predicted well by the different vibrational models. However, with our best theoretical methods there is still a factor of 2–4 between the experimental and calculated oscillator strengths for the FH-stretches in the FH⋯CO, FH⋯CO 2 and FH⋯N 2 complexes.

Dimethyl sulfoxide (DMSO) is an important intermediate in the atmospheric oxidation of dimethyl sulfide (DMS). DMS, emitted from the ocean, is the main natural sulfur source, and its oxidation products are essential in the formation of sulfate aerosols. The high atmospheric concentration of water makes hydrogen bonding with DMS and its oxidation products likely. Through hydrogen bonding, water can potentially catalyze and affect the steps of the oxidation. We investigate binary hydrogen-bound complexes involving DMSO. Both water·DMSO and methanol·DMSO complexes are identified in an Ar matrix, and at room temperature, a Gibbs free energy of 0.7 kJ/mol for the formation of the methanol·DMSO complex is determined. Assuming a similar Gibbs free energy of the hydrate, it would suggest a relatively high abundance of the DMSO hydrate relative to the monomer in the atmosphere. The effect of changing the atom divalently bound to the hydrogen bond accepting oxygen, from S to C (DMSO to acetone), is found to significantly decrease the equilibrium constant of complex formation.

The gas phase ultraviolet spectrum and stability of 1,1-dihydroxyacetone (the geminal diol of methylglyoxal) are investigated using spectroscopic and computational methods. Experimental gas phase electronic spectra recorded in the ultraviolet–visible range were used to follow the hydration of methylglyoxal. We show that upon the addition of water, methylglyoxal hydrates to form the geminal diol. The electronic spectra of methylglyoxal and its geminal diol conformers were calculated using TD-DFT and LR-CC methods to facilitate experimental peak assignments. The lowest energy electronic transition of the diol is experimentally measured and compared with the results of the theoretical calculations. We find that upon formation of the diol, the lowest-energy transition of methylglyoxal (centered around ∼430 nm) vanishes, and the remaining transitions, including the methylglyoxal diol absorptions (λmax = 240 nm), fall outside the available solar actinic flux in the troposphere. This likely has significant impac...

This work considers the nature of the intermolecular hydrogen bond in a series of 15 different complexes with OH donor groups and N, O, P, or S acceptor atoms. To complement the existing literature, room-temperature gas-phase vibrational spectra of the methanol–pyridine, ethanol–pyridine, and 2,2,2-trifluoroethanol–pyridine complexes were recorded. These complexes were chosen, as they exhibit hydrogen bonds of intermediate strength as compared to previous investigations that involved strong or weak hydrogen bonds. Non Covalent Interactions (NCI) theory was used to calculate various properties of the intermolecular hydrogen bonds, which were compared to the experimental OH-stretching vibrational red shifts. We find that the experimental OH-stretching red shifts correlate strongly with the kinetic energy density integrated within the reduced density gradient volume that describes a hydrogen bond [G(s0.5)]. Given that vibrational red shifts are commonly used as a metric of the strength of a hydrogen bond, th...

Hydroperoxides are formed in significant amounts in the atmosphere by oxidation of volatile organic compounds and are key in aerosol formation. In a room-temperature experiment, we detected the formation of bimolecular complexes of tert-butyl hydroperoxide (t-BuOOH) and the corresponding alcohol tert-butanol (t-BuOH), with dimethyl ether (DME) as the hydrogen-bond acceptor. Using a combination of Fourier-transform infrared spectroscopy and quantum chemical calculations, we compare the strength of the OH-O hydrogen bond and the total strength of complexation. We find that, both in terms of observed red shifts and determined equilibrium constants, t-BuOOH is a significantly better hydrogen-bond donor than t-BuOH, a result that is backed by a number of calculated parameters and can be explained by a weaker OH bond in the hydroperoxide. On the basis of combined experimental and theoretical results, we find that the hydroperoxide complex is stabilized by ∼4 kJ/mol (Gibbs free energy) more than the alcohol complex. Measured red shifts show the same trend in hydrogen-bond strength with trimethylamine (N acceptor atom) and dimethyl sulfide (S acceptor atom) as the hydrogen-bond acceptors.