Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH₃ OC(O)H, were elucidated to be CH₃ OH + CO, 2 CH₂ O and CH₄  + CO₂ as in part distinct from the dissociation of the radical cation (CH₃ OH^+•  + CO and CH₂ OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂ O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.

Atmospheric insight into the reaction mechanism and kinetics of isopropenyl methyl ether (i-PME) initiated by OH radicals and subsequent oxidation of product radicals

Studies on primary gas-phase reactions of emitted saturated and unsaturated ethers with oxidants and subsequent secondary reactions of product radicals with O₂ in the presence of NO are important in their atmospheric chemical processes. To accomplish these findings, we have examined the chemistry of OH-initiated oxidation of isopropenyl methyl ether (i-PME) CH₃C(CH₂)OCH₃ by electronic structure ca using density functional theory. Our energetic calculations show that OH additions to carbon-carbon double bonds of i-PME are more favorable reaction pathways than H-abstraction reactions from the various CH sites of the titled molecule. The rate constant values which are obtained from the transition state theory also signify that OH-addition reactions have faster reaction rates than H-abstraction reactions. Our calculated total rate constant of the reaction is found 9.90 × 10^-11 cm³ molecule^-1 s^-1. The percentage branching ratio calculations imply that OH-addition reactions have 98.09% contribution in the total rate constant. The atmospheric lifetime of i-PME is found to be 2.8 h. Further, we have identified 2-hydroxy-2-methoxypropanol, methyl acetate, methy-1,2-hydroxyacetate and 1-hydroxypropane-2-one, 1,2-dihydroxypropan-2-yl format, 2-hydroxyacetic acid, acetic acid, and formaldehyde from the secondary oxidation of product radicals.

Material balance in the O2 electrode of Li-O2 cells with a porous carbon electrode and TEGDME-based electrolytes

This work figures out the material balance of the reactions occurring in the O₂ electrode of a Li–O₂ cell, where a Ketjenblack-based porous carbon electrode comes into contact with a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity. The ratio of electrolyte weight to cell capacity (E/C, g A h⁻¹) is a good parameter to correlate with cycle life. Only 5 cycles were obtained at an areal capacity of 4 mA h cm⁻² (E/C = 10) and a discharge/charge current density of 0.4 mA cm⁻², which corresponds to the energy density of 170 W h kg⁻¹ at a complete cell level. When the areal capacity was decreased to half (E/C = 20) by setting a current density at 0.2 mA cm⁻², the cycle life was extended to 18 cycles. However, the total electric charge consumed for parasitic reactions was 35 and 59% at the first and the third cycle, respectively. This surprisingly large amount of parasitic reactions was suppressed by half using redox mediators at 0.4 mA cm⁻² while keeping a similar cycle life. Based on by-product distribution, we will propose possible mechanisms of TEGDME decomposition and report a water breathing behavior, where H₂O is produced during charge and consumed during discharge.

The material balance in the O₂ electrode of a Li–O₂ cell with a Ketjenblack-based porous carbon electrode and a tetraethylene glycol dimethyl ether-based electrolyte under more practical conditions of less electrolyte amount and high areal capacity.

Atmospheric Chemistry of Enols: The Formation Mechanisms of Formic and Peroxyformic Acids in Ozonolysis of Vinyl Alcohol

Vinyl alcohol (VA), for a long time, is thought to be a missing source of formic acid (FA) in the atmospheric models. However, a recent study has shown that FA is just a byproduct in the OH-initiated oxidation of VA, which stimulates investigation on the other sinks of VA in the atmosphere. In this study, the detailed ozonolysis mechanism of VA was investigated theoretically for the first time. The results show that two primary ozonides (syn- and anti-POZ) can be formed in the ozonolysis of VA and that FA coupled with the simplest Criegee intermediate (CH₂OO) can be produced as the main nascent products. Thus, the ozonolysis of VA is predicted to be a more efficient process to produce FA in the atmosphere compared with its OH-initiated oxidation. Moreover, it is found that the syn-POZ can directly decompose to peroxyformic acid plus formaldehyde, breaking the known "Criegee mechanism" to form carbonyl oxide with carbonyl compound. This special mechanism by providing a new source of peroxy acids in the atmosphere enriches the atmospheric chemistry of enols. The atmospheric lifetime of VA by ozonolysis is predicted to be 30 h, comparable with its prevalent reaction with the OH radical. Therefore, the obtained theoretical results can be usefully incorporated into a future modeling study of enols.

Impacts of SO2, Relative Humidity, and Seed Acidity on Secondary Organic Aerosol Formation in the Ozonolysis of Butyl Vinyl Ether

Alkyl vinyl ethers are widely used as fuel additives. Despite this, their atmospheric chemistry and secondary organic aerosol (SOA) formation potentials are still not well-known under complex pollution conditions. In this work, we examined the impact of SO₂, relative humidity (RH), and particle acidity on the formation and oxidation state (OSc) of SOA from butyl vinyl ether (BVE) ozonolysis. Increasing SO₂ concentration produced a notable promotion of SOA formation and OSc due to the significant increase in H₂SO₄ particles and formation of more highly oxidized components. Increased RH in the presence of SO₂ appeared to promote, suppress, and dominate the formation and OSc of SOA in the dry range (1-10%), low RH range (10-42%), and moderate RH range (42-64%), respectively. This highlights the importance of competition between H₂O and SO₂ in reacting with the stabilized Criegee intermediate in BVE ozonolysis at ambient RH. Increased particle acidity mainly contributed to the change in chemical composition of BVE-dominated SOA but not to SOA formation. The results presented here extend previous analysis of BVE-derived SOA and further aid our understanding of SOA formation potential of BVE ozonolysis under highly complex pollution conditions.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O₃, OH, Cl, and NO₃) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm³ molecule^-1 s^-1) are (1.8 ± 0.3) × 10^-18, (3.1 ± 0.7) × 10^-11, (2.5 ± 0.5) × 10^-10, and (1.1 ± 0.4) × 10^-14, for reactions with O₃, OH, Cl, and NO_3, respectively. While results for reactions with O₃, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO₃ is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τ_O3 = 9 days, τ_OH = 5 h, τ_Cl = 5 days, τ_NO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

Absolute and relative-rate measurement of the rate coefficient for reaction of perfluoro ethyl vinyl ether (C2F5OCF[double bond, length as m-dash]CF2) with OH

The rate coefficient (k₁) for the reaction of OH radicals with perfluoro ethyl vinyl ether (PEVE, C₂F₅OCF[double bond, length as m-dash]CF₂) has been measured as a function of temperature (T = 207-300 K) using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence (PLP-LIF) at pressures of 50 or 100 Torr N₂ bath gas. In addition, the rate coefficient was measured at 298 K and in one atmosphere of air by the relative-rate technique with loss of PEVE and reference reactant monitored in situ by IR absorption spectroscopy. The rate coefficient has a negative temperature dependence which can be parameterized as: k₁(T) = 6.0 × 10^-13 exp[(480 ± 38/T)] cm³ molecule^-1 s^-1 and a room temperature value of k₁ (298 K) = (3.0 ± 0.3) × 10^-12 cm³ molecule^-1 s^-1. Highly accurate rate coefficients from the PLP-LIF experiments were achieved by optical on-line measurements of PEVE and by performing the measurements at two different apparatuses. The large rate coefficient and the temperature dependence indicate that the reaction proceeds via OH addition to the C[double bond, length as m-dash]C double bond, the high pressure limit already being reached at 50 Torr N₂. Based on the rate coefficient and average OH levels, the atmospheric lifetime of PEVE was estimated to be a few days.

Atmospheric degradation of 2-chloroethyl vinyl ether, allyl ether and allyl ethyl ether: Kinetics with OH radicals and UV photochemistry

Unsaturated ethers are oxygenated volatile organic compounds (OVOCs) emitted by anthropogenic sources. Potential removal processes in the troposphere are initiated by hydroxyl (OH) radicals and photochemistry. In this work, we report for the first time the rate coefficients of the gas-phase reaction with OH radicals (k_OH) of 2-chloroethyl vinyl ether (2ClEVE), allyl ether (AE), and allyl ethyl ether (AEE) as a function of temperature in the 263-358 K range, measured by the pulsed laser photolysis-laser induced fluorescence technique. No pressure dependence of k_OH was observed in the 50-500 Torr range in He as bath gas, while a slightly negative T-dependence was observed. The temperature dependent expressions for the rate coefficients determined in this work are: The estimated atmospheric lifetimes (τ_OH) assuming k_OH at 288 K were 3, 2, and 4 h for 2ClEVE, AE and AEE, respectively. The kinetic results are discussed in terms of the chemical structure of the unsaturated ethers by comparison with similar compounds. We also report ultraviolet (UV) and infrared (IR) absorption cross sections (σ_λ and σ(ν˜), respectively). We estimate the photolysis rate coefficients in the solar UV actinic region to be less than 10^-7 s^-1, implying that these compounds are not removed from the atmosphere by this process. In addition, from σ(ν˜) and τ_OH, the global warming potential of each unsaturated ether was calculated to be almost zero. A discussion on the atmospheric implications of the titled compounds is presented.

Matrix isolation study of the early intermediates in the ozonolysis of selected vinyl ethers

The Criegee mechanism of the ozonolysis reaction of vinyl ethers has been observed by matrix isolation FTIR spectroscopy.

Computational study on the mechanisms and rate constants of the OH-initiated oxidation of ethyl vinyl ether in atmosphere

The hydroxylation reactions of ethyl vinyl ether (EVE) in the present of O2 and NO are analyzed by using MPWB1K/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) level of theory. According to the calculated thermodynamic data, the detailed reaction mechanisms of EVE and OH are proposed. All of the ten possible reaction pathways are discussed. The major products of the title reaction are ethyl formate and formaldehyde, which is in accordance with experimental detection. The rate constants of the primary reactions over the temperature of 250-400K and the pressure range of 100-2000Torr are computed by employing MESMER program. At 298K and 760Torr, OH-addition channels are predominate and the total rate constant is ktot=4.53×10(-11)cm(3)molecule(-1)s(-1). The Arrhenius equation is obtained as ktot=6.27×10(-12)exp(611.5/T), according to the rate constants given at different temperatures. Finally, the atmospheric half life of EVE with respect to OH is estimated to be 2.13h.

Atmospheric chemistry of enols: a theoretical study of the vinyl alcohol + OH + O(2) reaction mechanism

Enols are emerging as trace atmospheric components that may play a significant role in the formation of organic acids in the atmosphere. We have investigated the hydroxyl radical ((•)OH) initiated oxidation chemistry of the simplest enol, vinyl alcohol (ethenol, CH2═CHOH), using quantum chemical calculations and energy-grained master equation simulations. A lifetime of around 4 h was determined for vinyl alcohol in the presence of tropospheric levels of (•)OH. The reaction proceeds by (•)OH addition at both the α (66%) and β (33%) carbons of the π-system, yielding the C-centered radicals (•)CH2CH(OH)2, and HOCH2C(•)HOH, respectively. Subsequent trapping by O2 leads to the respective peroxyl radicals. About 90% of the chemically activated population of the major peroxyl radical adduct (•)O2CH2CH(OH)2 is predicted to undergo fragmentation to produce formic acid and formaldehyde, with regeneration of (•)OH. The minor peroxyl radical HOCH2C(OO(•))HOH is even less stable and undergoes almost exclusive HO2(•) elimination to form glycolaldehyde (HOCH2CHO). Formation of the latter has not been proposed before in the oxidation of vinyl alcohol. A kinetic mechanism for use in atmospheric modeling is provided, featuring phenomenological rate coefficients for formation of the three main product channels ((•)O2CH2CH(OH)2 [8%]; HC(O)OH + HCHO + (•)OH [56%]; HOCH2CHO + HO2(•) [37%]). Our study supports previous findings that vinyl alcohol should be rapidly removed from the atmosphere by reaction with (•)OH and O2 with glycolaldehyde being identified as a previously unconsidered product. Most importantly, it is shown that direct chemically activated reactions can lead to (•)OH and HO2(•) (HOx) recycling.

Mechanistic and kinetic study on the ozonolysis of n-butyl vinyl ether, i-butyl vinyl ether and t-butyl vinyl ether

Density functional theory (DFT) and ab initio method are employed to elucidate the mechanisms for O(3)-initiated oxidation of n-butyl vinyl ether (n-BVE) and its isomers (i-BVE and t-BVE). For each BVE, the reactions proceed via O(3) cycloaddition resulting in the formation of primary ozonides (POZs) and then two self-decomposition pathways of POZs are followed. Major products are identified to be formaldehyde and butyl formates (CH(3)CH(2)CH(2)CH(2)OCHO for n-BVE, (CH(3))(2)CHCH(2)OCHO for i-BVE and (CH(3))(3)COCHO for t-BVE). The total and individual rate constants for main product channels have been calculated using the modified multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) approach. At 298 K and 101 kPa, the calculated total rate constants are 2.50×10(-16), 3.41×10(-16) and 4.17×10(-16) cm(3) molecule(-1) s(-1) for n-BVE+O(3), i-BVE+O(3) and t-BVE+O(3), respectively, which are in perfect agreement with experimental results. The total rate coefficients are almost pressure independent in the range of 0.001-101 kPa but obviously positive temperature dependent over the whole study temperature range (200-400 K). Also, the favorable reaction pathways have been determined through the estimation of branching ratios. Moreover, the influence of alkoxy group structure on the reactivity of vinyl ethers was examined.

Thermal dissociation of ethylene glycol vinyl ether

The pyrolysis of ethylene glycol vinyl ether (EGVE), an initial product of 1,4-dioxane dissociation, was examined in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS) at 57 ± 2 and 122 ± 3 Torr over 1200-1800 K. DFST/time-of-flight mass spectrometry experiments were also performed to identify reaction products. EGVE was found to dissociate via two channels: (1) a molecular H atom transfer/C-O scission to produce C(2)H(3)OH and CH(3)CHO, and (2) a radical channel involving C-O bond fission generating ˙CH(2)CH(2)OH and ˙CH(2)CHO radicals, with the second channel being strongly dominant over the entire experimental range. A reaction mechanism was constructed for the pyrolysis of EGVE which simulates the LS profiles very well over the full experimental range. The decomposition of EGVE is clearly well into the falloff region for these conditions, and a Gorin model RRKM fit was obtained for the dominant radical channel. The results are in good agreement with the experimental data and suggest the following rate coefficient expressions: k(2,∞) = (6.71 ± 2.6) × 10(27) × T(-3.21)exp(-35512/T) s(-1); k(2)(120 Torr) = (1.23 ± 0.5) × 10(92) × T(-22.87)exp(-48 248/T) s(-1); k(2)(60 Torr) = (2.59 ± 1.0) × 10(88) × T(-21.96)exp(-46283/T) s(-1).

THEORETICAL STUDY OF OH-INITIATED ATMOSPHERIC OXIDATION FOR PROPYL VINYL ETHER

This paper reports a theoretical study on the reaction of propyl vinyl ether (PVE, CH3CH2CH2OCH=CH2) with OH radicals in the presence of O 2 and NO x. The reaction pathway has been studied with the density functional theory (DFT/B3LYP) at the 6-31G* level. The total energies of all geometries are corrected at the MP2/6-311+G** level. The profile of the potential energy surface was constructed. The possible channels involved in the reaction were discussed. The results show that six product pathways are energetically feasible for the degradation of PVE initiated by OH radicals in the atmosphere. The main products for this degradation reaction are propyl formate, formaldehyde, and glycolic acid propyl ester in which propyl formate and formaldehyde are mainly from the OH addition to C5 atom.

Kinetic and Mechanistic Study of the Gas-Phase Reactions of a Series of Vinyl Ethers with the Nitrate Radical

NO(3) oxidation of methyl, ethyl, propyl, and butyl vinyl ethers has been studied under tropospheric conditions (atmospheric pressure and T = 293 +/- 3 K) in the LISA indoor simulation chamber. NO(3) was produced inside the reactor by thermal decomposition of N(2)O(5) previously added to the air-VOC mixture, and concentrations were monitored using FTIR spectrometry. All the kinetic experiments were carried out by relative rate technique using isoprene as reference compound, leading to the rate constants k(1) = (7.2 +/- 1.5) x 10(-13), k(2) = (13.1 +/- 2.7) x 10(-13), k(3) = (13.3 +/- 3.0) x 10(-13), and k(4) = (17.0 +/- 3.7) x 10(-13) cm(3) molecule(-1) s(-1) for methyl, ethyl, propyl, and butyl vinyl ethers, respectively. Main oxidation products have been identified like being formaldehyde and respectively methyl, ethyl, propyl, and butyl formates. Production yields of oxidation products were close to 50%. Oxygenated nitrates and peroxynitrates were also detected.

Kinetic Study of the Gas-Phase Reactions of OH and NO3 Radicals and O3 with Selected Vinyl Ethers

Kinetic studies on the gas-phase reactions of OH and NO3 radicals and ozone with ethyl vinyl ether (EVE), propyl vinyl ether (PVE) and butyl vinyl ether (BVE) have been performed in a 405 L borosilicate glass chamber at 298 +/- 3 K in synthetic air using in situ FTIR spectroscopy to monitor the reactants. Using a relative kinetic method rate coefficients (in units of cm3 molecule(-1) s(-1)) of (7.79 +/- 1.71) x 10(-11), (9.73 +/- 1.94) x 10(-11) and (1.13 +/- 0.31) x 10(-10) have been obtained for the reaction of OH with EVE, PVE and BVE, respectively, (1.40 +/- 0.35) x 10(-12), (1.85 +/- 0.53) x 10(-12) and (2.10 +/- 0.54) x 10(-12) for the reaction of NO3 with EVE, PVE and BVE, respectively, and (2.06 +/- 0.42) x 10(-16), (2.34 +/- 0.48) x 10(-16) and (2.59 +/- 0.52) x 10(-16) for the ozonolysis of EVE, PVE and BVE, respectively. Tropospheric lifetimes of EVE, PVE and BVE with respect to the reactions with reactive tropospheric species (OH, NO3 and O3) have been estimated for typical OH and NO3 radical and ozone concentrations.