A combined experimental and theoretical study of the reaction between methylglyoxal and OH/OD radical: OH regeneration

Solving the OH + Glyoxal Problem: A Complete Theoretical Description of Post-Transition-State Energy Deposition in Activated Systems

Activated chemistry in coupled reaction systems has broadened our understanding of the chemical kinetics. In the case of intermediates formed in gas phase abstraction reactions (e.g., OH + HC(O)C(O)H (glyoxal) →HC(O)CO + H2O), it is particularly crucial to understand how the reaction energy is partitioned between product species as this determines the propensity for a given product to undergo "prompt" dissociation (e.g., HC(O)CO → HCO + CO) before the excess reaction energy is removed. An example of such an activated system is the OH + glyoxal + O2 coupled reaction system. In this work, we develop a molecular dynamics pipeline, which, combined with a master equation analysis, accurately models previous experimental measurements. This new work resolves previous complexities and discrepancies from earlier master equation modeling for this reaction system. The detailed molecular dynamics approach employed here is a powerful new tool for modeling challenging activated reaction systems.

Impacts of updated reaction kinetics on the global GEOS-Chem simulation of atmospheric chemistry

We updated the chemical mechanism of the GEOS-Chem global 3-D model of atmospheric chemistry to include new recommendations from the NASA Jet Propulsion Laboratory (JPL) chemical kinetics Data Evaluation 19-5 and from the International Union of Pure and Applied Chemistry (IUPAC) and to balance carbon and nitrogen. We examined the impact of these updates on the GEOS-Chem version 14.0.1 simulation. Notable changes include 11 updates to reactions of reactive nitrogen species, resulting in a 7% net increase in the stratospheric NOx (NO + NO2) burden; an updated CO + OH rate formula leading to a 2.7% reduction in total tropospheric CO; adjustments to the rate coefficient and branching ratios of propane + OH, leading to reduced tropospheric propane (-17%) and increased acetone (+3.5%) burdens; a 41% increase in the tropospheric burden of peroxyacetic acid due to a decrease in the rate coefficient for its reaction with OH, further contributing to reductions in peroxyacetyl nitrate (PAN; -3.8%) and acetic acid (-3.4%); and a number of minor adjustments to halogen radical cycling. Changes to the global tropospheric burdens of other species include -0.7% for ozone, +0.3% for OH (-0.4% for methane lifetime against oxidation by tropospheric OH), +0.8% for formaldehyde, and -1.7% for NOx. The updated mechanism reflects the current state of the science, including complex chemical dependencies of key atmospheric species on temperature, pressure, and concentrations of other compounds. The improved conservation of carbon and nitrogen will facilitate future studies of their overall atmospheric budgets.

New Measurements and Calculations on the Kinetics of an Old Reaction: OH + HO2 → H2O + O2

Literature rate coefficients for the prototypical radical-radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO₂ radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H₂O₂ reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k_1,298K ∼1 × 10^-11 cm³ molecule^-1 s^-1, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k_1,H2O, 298K = (2.17 ± 0.09) × 10^-28 cm⁶ molecule^-2 s^-1, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k_1,298K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k_1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO₂ → HCl + O₂. The implications of these results in atmospheric models are discussed.

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.

Direct Trace Fitting of Experimental Data Using the Master Equation: Testing Theory and Experiments on the OH + C2H4 Reaction

Laser flash photolysis coupled with laser-induced fluorescence observation of OH has been used to observe the equilibration of OH + C₂H₄ ↔ HOC₂H₄ over the temperature range 563-723 K and pressures of bath gas (N₂) from 58 to 250 Torr. The time-resolved OH traces have been directly and globally fitted with a master equation in order to extract Δ_RH₀⁰, the binding energy of the HOC₂H₄ adduct, with respect to reagents. The global approach allows the role that OH abstraction plays at higher temperatures to be identified. The resultant value ofΔ_RH₀⁰, 111.8 kJ mol^-1, is determined to be better than 2 kJ mol^-1 and is in agreement with our ab initio calculations (carried out at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level), 111.4 kJ mol^-1, and other state of the art calculations. Parameters for the abstraction channel are also in good agreement with previous experimental studies. To effect this analysis, the MESMER master equation code was extended to directly incorporate secondary chemistry: diffusional loss from the observation region and reaction with the photolytic precursor. These extensions, which, among other things, resolve issues related to the merging of chemically significant and internal energy relaxation eigenvalues, are presented.

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.

Theoretical insight into OH- and Cl-initiated oxidation of CF3OCH(CF3)2 and CF3OCF2CF2H &fate of CF3OC(X•)(CF3)2 and CF3OCF2CF2X• radicals (X=O, O2)

In this study, the mechanistic and kinetic analysis for reactions of CF₃OCH(CF₃)₂ and CF₃OCF₂CF₂H with OH radicals and Cl atoms have been performed at the CCSD(T)//B3LYP/6-311++G(d,p) level. Kinetic isotope effects for reactions CF₃OCH(CF₃)₂/CF₃OCD(CF₃)₂ and CF₃OCF₂CF₂H/CF₃OCF₂CF₂D with OH and Cl were estimated so as to provide the theoretical estimation for future laboratory investigation. All rate constants, computed by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT), are in reasonable agreement with the limited experimental data. Standard enthalpies of formation for the species were also calculated. Atmospheric lifetime and global warming potentials (GWPs) of the reaction species were estimated, the large lifetimes and GWPs show that the environmental impact of them cannot be ignored. The organic nitrates can be produced by the further oxidation of CF₃OC(•)(CF₃)₂ and CF₃OCF₂CF₂• in the presence of O₂ and NO. The subsequent decomposition pathways of CF₃OC(O•)(CF₃)₂ and CF₃OCF₂CF₂O• radicals were studied in detail. The derived Arrhenius expressions for the rate coefficients over 230–350 K are: k_T(1)= 5.00 × 10⁻²⁴T^3.57 exp(−849.73/T), k_T(2)= 1.79 × 10⁻²⁴T^4.84 exp(−4262.65/T), k_T(3)= 1.94 × 10⁻²⁴T^4.18 exp(−884.26/T), and k_T(4) = 9.44 × 10⁻²⁸T^5.25 exp(−913.45/T) cm³ molecule⁻¹ s⁻¹.

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.

Efficient Isoprene Secondary Organic Aerosol Formation from a Non-IEPOX Pathway

With a large global emission rate and high reactivity, isoprene has a profound effect upon atmospheric chemistry and composition. The atmospheric pathways by which isoprene converts to secondary organic aerosol (SOA) and how anthropogenic pollutants such as nitrogen oxides and sulfur affect this process are subjects of intense research because particles affect Earth's climate and local air quality. In the absence of both nitrogen oxides and reactive aqueous seed particles, we measure SOA mass yields from isoprene photochemical oxidation of up to 15%, which are factors of 2 or more higher than those typically used in coupled chemistry climate models. SOA yield is initially constant with the addition of increasing amounts of nitric oxide (NO) but then sharply decreases for input concentrations above 50 ppbv. Online measurements of aerosol molecular composition show that the fate of second-generation RO2 radicals is key to understanding the efficient SOA formation and the NOx-dependent yields described here and in the literature. These insights allow for improved quantitative estimates of SOA formation in the preindustrial atmosphere and in biogenic-rich regions with limited anthropogenic impacts and suggest that a more-complex representation of NOx-dependent SOA yields may be important in models.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

Mechanisms and kinetics of the reaction of CF₃CX₂CX₂CX₂OX with OX (X= H, D) radical are investigated on a sound theoretical basis.

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.

Atmospheric chemistry of 2,3-pentanedione: photolysis and reaction with OH radicals

The kinetics of the overall reaction between OH radicals and 2,3-pentanedione (1) were studied using both direct and relative kinetic methods at laboratory temperature. The low pressure fast discharge flow experiments coupled with resonance fluorescence detection of OH provided the direct rate coefficient of (2.25 ± 0.44) × 10(-12) cm(3) molecule(-1) s(-1). The relative-rate experiments were carried out both in a collapsible Teflon chamber and a Pyrex reactor in two laboratories using different reference reactions to provide the rate coefficients of 1.95 ± 0.27, 1.95 ± 0.34, and 2.06 ± 0.34, all given in 10(-12) cm(3) molecule(-1) s(-1). The recommended value is the nonweighted average of the four determinations: k(1) (300 K) = (2.09 ± 0.38) × 10(-12) cm(3) molecule(-1) s(-1), given with 2σ accuracy. Absorption cross sections for 2,3-pentanedione were determined: the spectrum is characterized by two wide absorption bands between 220 and 450 nm. Pulsed laser photolysis at 351 nm was used and the depletion of 2,3-pentanedione (2) was measured by GC to determine the photolysis quantum yield of Φ(2) = 0.11 ± 0.02(2σ) at 300 K and 1000 mbar synthetic air. An upper limit was estimated for the effective quantum yield of 2,3-pentanedione applying fluorescent lamps with peak wavelength of 312 nm. Relationships between molecular structure and OH reactivity, as well as the atmospheric fate of 2,3-pentanedione, have been discussed.

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.

Quantum chemistry and TST study of the mechanisms and branching ratios for the reactions of OH with unsaturated aldehydes

A theoretical study is presented on the mechanism of OH reactions with three unsaturated aldehydes, relevant to atmospheric chemistry. Using acrolein as test molecule, several methods were tested in conjunction with the 6-311 ++ G(d,p) basis set. Based on the results from this study, the MPWB1K and M05-2X functionals were selected for the further study of acrolein, methacrolein and crotonaldehyde. All possible reaction channels have been modeled. Calculated overall rate coefficients at M05-2X/6-311 ++ G(d,p) are in excellent agreement with experimental data, supporting the proposed mechanisms. The previously proposed global mechanisms were confirmed, and specific mechanisms were identified. The causes of the mechanism for crotonaldehyde being different from the one of acrolein and methacrolein were clarified. The agreement between experiment and calculations validates the use of the chosen DFT methods for kinetic calculations, especially for large systems and cases in which spin contamination is an important issue.

CO Formation in the 254 nm Gas Phase Photolysis of Nitrosobenzene-Oxygen Mixtures at Room Temperature

Large CO yields were detected in the gas phase photolysis of nitrosobenzene at 254 nm in the presence of O2 and N2 by long-path IR absorption in a 200 L photoreactor made of quartz. CO yields were measured at 298 K for partial pressures of O2 between 1 μbar and 1 bar and total pressures from 10 μbar to 1 bar. Depending on reaction conditions, the CO yield varied between ≤ 1 % (at p N2 = 1 bar, p O2 ≤ 120 μbar) and ≈ 100 % (for pO2 = 20 μbar, p N2 = 0). The dependencies of the CO yield (i) from the O2 partial pressure in O2-N2-mixtures at a total pressure of 1 bar, (ii) from the O2 pressure in the absence of N2, and (iii) from the total pressure at constant O2 pressures show very different behaviour. They can be reproduced well by a mechanism involving vibrationally excited phenyl (C6H5*), phenylperoxy (C6H5O2*) and phenoxyl (C6H5O*) radicals where collisional relaxation of the excited radicals competes with O2 addition for C6H5* and with both unimolecular decomposition and reaction with O2 for C6H5O2* and/or C6H5O*. The measured CO yields allow to select reaction conditions where nearly exclusively either highly vibrationally excited C6H5, C6H5O2 and/or C6H5O radicals (for CO yields ≈ 1) or thermally distributed C6H5, C6H5O2 and/or C6H5O radicals (for CO yields ≤ 1 %) are formed.