OH yields from the CH3CO+O2 reaction using an internal standard

OH Kinetics with a Range of Nitrogen-Containing Compounds: N-Methylformamide, t-Butylamine, and N-Methyl-propane Diamine

Emissions of amines and amides to the atmosphere are significant from both anthropogenic and natural sources, and amides can be formed as secondary pollutants. Relatively little kinetic data exist on overall rate coefficients with OH, the most important tropospheric oxidant, and even less on site-specific data which control the product distribution. Structure-activity relationships (SARs) can be used to estimate both quantities. Rate coefficients for the reaction of OH with t-butylamine (k₁), N-methyl-1,3-propanediamine (k₂), and N-methylformamide (k₃) have been measured using laser flash photolysis coupled with laser-induced fluorescence. Proton-transfer-reaction mass spectrometry (PTR-MS) has been used to ensure the reliable introduction of these low-vapor pressure N-containing compounds and to give qualitative information on products. Supporting ab initio calculations are presented for the t-butylamine system. The following rate coefficients have been determined: k_1,298K= (1.66 ± 0.20) × 10^-11 cm³ molecule^-1 s^-1, k(T)₁ = 1.65 × 10^-11 (T/300)^-0.69 cm³ molecule^-1 s^-1, k_2,293K = (7.09 ± 0.22) × 10^-11 cm³ molecule^-1 s^-1, and k_3,298K = (1.03 ± 0.23) × 10^-11 cm³ molecule^-1 s^-1. For OH + t-butylamine, ab initio calculations predict that the fraction of N-H abstraction is 0.87. The dominance of this channel was qualitatively confirmed using end-product analysis. The reaction of OH with N-methyl-1,3-propanediamine also had a negative temperature dependence, but the reduction in the rate coefficient was complicated by reagent loss. The measured rate coefficient for reaction 3 is in good agreement with a recent relative rate study. The results of this work and the literature data are compared with the recent SAR estimates for the reaction of OH with reduced nitrogen compounds. Although the SARs reproduce the overall rate coefficients for reactions, site-specific agreement with this work and other literature studies is less strong.

Unimolecular decomposition of acetyl peroxy radical: a potential source of tropospheric ketene

The unimolecular decomposition of acetyl peroxy radicals followed by subsequent nitration is known to lead to the formation of peroxy acetyl nitrate (PAN) in the troposphere. Using high level quantum chemical calculations, we show that the acetyl peroxy radical is a precursor in the formation of tropospheric ketene. The results show that the presence of a single or double water molecule(s) as a catalyst does not influence the decomposition reaction directly to form ketene and hydroperoxy radicals. The electronic excitation of the reactive and product complexes occurs in the wavelength range of ∼1400 nm, suggesting that the complexes undergo photoexcitation in the near IR region. The results ascertain that the dissociation of acetyl peroxy radicals into ketene and hydroperoxy radicals occurs more likely through the excitation route and the corresponding excitation wavelength reveals that the reactions are red-light driven. Three different product complexes, ketene·HO2, ketene·H2O·HO2 and ketene·(H2O)2·HO2, are formed from the reaction. The direct dynamics simulations show that the product complexes are more stable and possess a long lifetime. The calculated temperature dependent equilibrium constant of the product complexes reveals that their atmospheric abundances decrease with increasing altitudes.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O₂, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O₂ are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O₂ are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO₂ radical coproducts.

Unimolecular decomposition kinetics of the stabilised Criegee intermediates CH2OO and CD2OO

Decomposition kinetics of stabilised CH2OO and CD2OO Criegee intermediates have been investigated as a function of temperature (450-650 K) and pressure (2-350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD2OO was observed to be faster than CH2OO under equivalent conditions. Production of OH radicals following CH2OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH2OO and CD2OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH2OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol-1 to 81.8 kJ mol-1 using a temperature-dependent exponential down model for collisional energy transfer with ΔEdown = 32.6(T/298 K)1.7 cm-1 in He. The low- and high-pressure limit rate coefficients are k1,0 = 3.2 × 10-4(T/298)-5.81exp(-12 770/T) cm3 s-1 and k1,∞ = 1.4 × 1013(T/298)0.06exp(-10 010/T) s-1, with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k1(298 K, 760 Torr) = 1.1+1.5-1.1 × 10-3 s-1. For CD2OO, MESMER calculations result in ΔEdown = 39.6(T/298 K)1.3 cm-1 in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol-1 to 80.1 kJ mol-1. The fitted rate coefficients for CD2OO are k2,0 = 5.2 × 10-5(T/298)-5.28exp(-11 610/T) cm3 s-1 and k2,∞ = 1.2 × 1013(T/298)0.06exp(-9800/T) s-1, with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k2(298 K, 760 Torr) = 5.5+9.2-5.5 × 10-3 s-1. The master equation calculations for CH2OO indicate decomposition yields of 63.7% for H2 + CO2, 36.0% for H2O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr.

The Reaction between CH3O2 and OH Radicals: Product Yields and Atmospheric Implications

The reaction between CH₃O₂ and OH radicals has been shown to be fast and to play an appreciable role for the removal of CH₃O₂ radials in remote environments such as the marine boundary layer. Two different experimental techniques have been used here to determine the products of this reaction. The HO₂ yield has been obtained from simultaneous time-resolved measurements of the absolute concentration of CH₃O₂, OH, and HO₂ radicals by cw-CRDS. The possible formation of a Criegee intermediate has been measured by broadband cavity enhanced UV absorption. A yield of ϕ_HO2 = (0.8 ± 0.2) and an upper limit for ϕ_Criegee = 0.05 has been determined for this reaction, suggesting a minor yield of methanol or stabilized trioxide as a product. The impact of this reaction on the composition of the remote marine boundary layer has been determined by implementing these findings into a box model utilizing the Master Chemical Mechanism v3.2, and constraining the model for conditions found at the Cape Verde Atmospheric Observatory in the remote tropical Atlantic Ocean. Inclusion of the CH₃O₂+OH reaction into the model results in up to 30% decrease in the CH₃O₂ radical concentration while the HO₂ concentration increased by up to 20%. Production and destruction of O₃ are also influenced by these changes, and the model indicates that taking into account the reaction between CH₃O₂ and OH leads to a 6% decrease of O₃.

Observation of a new channel, the production of CH3, in the abstraction reaction of OH radicals with acetaldehyde

Using laser flash photolysis coupled to photo-ionization time-of-flight mass spectrometry (PIMS), methyl radicals (CH₃) have been detected as primary products from the reaction of OH radicals with acetaldehyde (ethanal, CH₃CHO) with a yield of ∼15% at 1-2 Torr of helium bath gas. Supporting measurements based on laser induced fluorescence studies of OH recycling in the OH/CH₃CHO/O₂ system are consistent with the PIMS study. Master equation calculations suggest that the origin of the methyl radicals is from prompt dissociation of chemically activated acetyl products and hence is consistent with previous studies which have shown that abstraction, rather than addition/elimination, is the sole route for the OH + acetaldehyde reaction. However, the observation of a significant methyl product yield suggests that energy partitioning in the reaction is different from the typical early barrier mechanism where reaction exothermicity is channeled preferentially into the newly formed bond. The master equation calculations predict atmospheric yields of methyl radicals of ∼9%. The implications of the observations in atmospheric and combustion chemistry are briefly discussed.

CH3CO + O2 + M (M = He, N2) Reaction Rate Coefficient Measurements and Implications for the OH Radical Product Yield

The gas-phase CH3CO + O2 reaction is known to proceed via a chemical activation mechanism leading to the formation of OH and CH3C(O)OO radicals via bimolecular and termolecular reactive channels, respectively. In this work, rate coefficients, k, for the CH3CO + O2 reaction were measured over a range of temperature (241-373 K) and pressure (0.009-600 Torr) with He and N2 as the bath gas and used to characterize the bi- and ter-molecular reaction channels. Three independent experimental methods (pulsed laser photolysis-laser-induced fluorescence (PLP-LIF), pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS), and a very low-pressure reactor (VLPR)) were used to characterize k(T,M). PLP-LIF was the primary method used to measure k(T,M) in the high-pressure regime under pseudo-first-order conditions. CH3CO was produced by PLP, and LIF was used to monitor the OH radical bimolecular channel reaction product. CRDS, a complementary high-pressure method, measured k(295 K,M) over the pressure range 25-600 Torr (He) by monitoring the temporal CH3CO radical absorption following its production via PLP in the presence of excess O2. The VLPR technique was used in a relative rate mode to measure k(296 K,M) in the low-pressure regime (9-32 mTorr) with CH3CO + Cl2 used as the reference reaction. A kinetic mechanism analysis of the combined kinetic data set yielded a zero pressure limit rate coefficient, kint(T), of (6.4 ± 4) × 10(-14) exp((820 ± 150)/T) cm(3) molecule(-1) s(-1) (with kint(296 K) measured to be (9.94 ± 1.3) × 10(-13) cm(3) molecule(-1) s(-1)), k0(T) = (7.39 ± 0.3) × 10(-30) (T/300)(-2.2±0.3) cm(6) molecule(-2) s(-1), and k∞(T) = (4.88 ± 0.05) × 10(-12) (T/300)(-0.85±0.07) cm(3) molecule(-1) s(-1) with Fc = 0.8 and M = N2. A He/N2 collision efficiency ratio of 0.60 ± 0.05 was determined. The phenomenological kinetic results were used to define the pressure and temperature dependence of the OH radical yield in the CH3CO + O2 reaction. The present results are compared with results from previous studies and the discrepancies are discussed.

HCl yield and chemical kinetics study of the reaction of Cl atoms with CH3I at the 298K temperature using the infra-red tunable diode laser absorption spectroscopy

Pulsed ArF excimer laser (193 nm)-CW infrared (IR) tunable diode laser Herriott type absorption spectroscopic technique has been made for the detection of product hydrochloric acid HCl. Absorption spectroscopic technique is used in the reaction chlorine atoms with methyl iodide (Cl+CH3I) to the study of kinetics on reaction Cl+CH3I and the yield of (HCl). The reaction of Cl+CH3I has been studied with the support of the reaction Cl+C4H10 (100% HCl) at temperature 298 K. In the reaction Cl+CH3I, the total pressure of He between 20 and 125 Torr at the constant concentration of [CH3I] 7.0×10(14) molecule cm(-3). In the present work, we estimated adduct formation is very important in the reaction Cl+CH3I and reversible processes as well and CH3I molecule photo-dissociated in the methyl [CH3] radical. The secondary chemistry has been studied as CH3+CH3ICl = product, and CH3I+CH3ICl = product2. The system has been modeled theoretically for secondary chemistry in the present work. The calculated and experimentally HCl yield nearly 65% at the concentration 1.00×10(14) molecule cm(-3) of [CH3I] and 24% at the concentration 4.0×10(15) molecule cm(-3) of [CH3I], at constant concentration 4.85×10(12) molecule cm(-3) of [CH3], and at 7.3×10(12) molecule cm(-3) of [Cl]. The pressure dependent also studied product of HCl at the constant [CH3], [Cl] and [CH3I]. The experimental results are also very good matching with the modelling work at the reaction CH3+CH3ICl = product (k = (2.75±0.35)×10(-10) s(-1)) and CH3I+CH3ICl = product2 (k = 1.90±0.15)×10(-12) s(-1). The rate coefficients of the reaction CH3+CH3ICl and CH3I+CH3ICl has been made in the present work. The experimental results has been studied by two method (1) phase locked and (2) burst mode.

Developments in Laboratory Studies of Gas-Phase Reactions for Atmospheric Chemistry with Applications to Isoprene Oxidation and Carbonyl Chemistry

Laboratory studies of gas-phase chemical processes are a key tool in understanding the chemistry of our atmosphere and hence tackling issues such as climate change and air quality. Laboratory techniques have improved considerably with greater emphasis on product detection, allowing the measurement of site-specific rate coefficients. Radical chemistry lies at the heart of atmospheric chemistry. In this review we consider issues around radical generation and recycling from the oxidation of isoprene and from the chemical reactions and photolysis of carbonyl species. Isoprene is the most globally significant hydrocarbon, but uncertainties exist about its oxidation in unpolluted environments. Recent experiments and calculations that cast light on radical generation are reviewed. Carbonyl compounds are the dominant first-generation products from hydrocarbon oxidation. Chemical oxidation can recycle radicals, or photolysis can be a net radical source. Studies have demonstrated that high-resolution and temperature-dependent studies are important for some significant species.

Site-specific rate coefficients for reaction of OH with ethanol from 298 to 900 K

The rate coefficients for reactions of OH with ethanol and partially deuterated ethanols have been measured by laser flash photolysis/laser-induced fluorescence over the temperature range 298-523 K and 5-100 Torr of helium bath gas. The rate coefficient, k(1.1), for reaction of OH with C(2)H(5)OH is given by the expression k(1.1) = 1.06 × 10(-22)T(3.58) exp(1126/T) cm(3) molecule(-1) s(-1), and the values are in good agreement with previous literature. Site-specific rate coefficients were determined from the measured kinetic isotope effects. Over the temperature region 298-523 K abstraction from the hydroxyl site is a minor channel. The reaction is dominated by abstraction of the α hydrogens (92 ± 8)% at 298 K decreasing to (76 ± 9)% with the balance being abstraction at the β position where the errors are 2σ. At higher temperatures decomposition of the CH(2)CH(2)OH product from β abstraction complicates the kinetics. From 575 to 650 K, biexponential decays were observed, allowing estimates to be made for k(1.1) and the fractional production of CH(2)CH(2)OH. Above 650 K, decomposition of the CH(2)CH(2)OH product was fast on the time scale of the measured kinetics and removal of OH corresponds to reaction at the α and OH sites. The kinetics agree (within ±20%) with previous measurements. Evidence suggests that reaction at the OH site is significant at our higher temperatures: 47-53% at 865 K.

Transient infrared absorption of t-CH3C(O)OO, c-CH3C(O)OO, and alpha-lactone recorded in gaseous reactions of CH3CO and O2

A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of CH(3)CO and O(2); IR absorption spectra of CH(3)C(O)OO and alpha-lactone were observed. Absorption bands with origins at 1851+/-1, 1372+/-2, 1169+/-6, and 1102+/-3 cm(-1) are attributed to t-CH(3)C(O)OO, and those at 1862+/-3, 1142+/-4, and 1078+/-6 cm(-1) are assigned to c-CH(3)C(O)OO. A weak band near 1960 cm(-1) is assigned to alpha-lactone, cyc-CH(2)C(=O)O, a coproduct of OH. These observed rotational contours agree satisfactorily with simulated bands based on predicted rotational parameters and dipole derivatives, and observed vibrational wavenumbers agree with harmonic vibrational wavenumbers predicted with B3LYP/aug-cc-pVDZ density-functional theory. The observed relative intensities indicate that t-CH(3)C(O)OO is more stable than c-CH(3)C(O)OO by 3+/-2 kJ mol(-1). Based on these observations, the branching ratio for the OH+alpha-lactone channel of the CH(3)CO+O(2) reaction is estimated to be 0.04+/-0.01 under 100 Torr of O(2) at 298 K. A simple kinetic model is employed to account for the decay of CH(3)C(O)OO.