Reaction Of CH3O2 Research Articles

Coupled evolution of BrOx‐ClOx‐HOx‐NOx chemistry during bromine‐catalyzed ozone depletion events in the arctic boundary layer

Extensive chemical characterization of ozone (O3) depletion events in the Arctic boundary layer during the TOPSE aircraft mission in March–May 2000 enables analysis of the coupled chemical evolution of bromine (BrOx), chlorine (ClOx), hydrogen oxide (HOx) and nitrogen oxide (NOx) radicals during these events. We project the TOPSE observations onto an O3 chemical coordinate to construct a chronology of radical chemistry during O3 depletion events, and we compare this chronology to results from a photochemical model simulation. Comparison of observed trends in ethyne (oxidized by Br) and ethane (oxidized by Cl) indicates that ClOx chemistry is only active during the early stage of O3 depletion (O3 > 10 ppbv). We attribute this result to the suppression of BrCl regeneration as O3 decreases. Formaldehyde and peroxy radical concentrations decline by factors of 4 and 2 respectively during O3 depletion and we explain both trends on the basis of the reaction of CH2O with Br. Observed NOx concentrations decline abruptly in the early stages of O3 depletion and recover as O3 drops below 10 ppbv. We attribute the initial decline to BrNO3 hydrolysis in aerosol, and the subsequent recovery to suppression of BrNO3 formation as O3 drops. Under halogen‐free conditions we find that HNO4 heterogeneous chemistry could provide a major NOx sink not included in standard models. Halogen radical chemistry in the model can produce under realistic conditions an oscillatory system with a period of 3 days, which we believe is the fastest oscillation ever reported for a chemical system in the atmosphere.

Tunable diode laser measurements of formaldehyde during the TOPSE 2000 study: Distributions, trends, and model comparisons

Airborne measurements of formaldehyde (CH2O) were acquired employing tunable diode laser absorption spectroscopy (TDLAS) during the 2000 Tropospheric Ozone Production About the Spring Equinox (TOPSE) study. This study consisted of seven deployments spanning the time period from 4 February to 23 May 2000 and covered a wide latitudinal band from 40°N to 85°N. The median measured CH2O concentrations, with a few exceptions, did not show any clear temporal trends from February to May in each of five altitude and three latitude bins examined. Detailed measurement–model comparisons were carried out using a variety of approaches employing two different steady state models. Because recent emissions of CH2O and/or its precursors often result in model underpredictions, background conditions were identified using a number of chemical tracers. For background conditions at temperatures warmer than −45°C, the measurement–model agreement on average ranged between −13% and +5% (measurement–model/measurement), which corresponded to mean and median (measurement–model) differences of 3 ± 69 and −6 parts per trillion by volume (pptv), respectively. At very low temperatures starting at around −45°C, significant and persistent (measurement–model) differences were observed from February to early April from southern Canada to the Arctic Ocean in the 6–8 km altitude range. In such cases, measured CH2O was as much as 392 pptv higher than modeled, and the median difference was 132 pptv (83%). Low light conditions as well as cold temperatures may be important in this effect. A number of possible mechanisms involving the reaction of CH3O2 with HO2 to produce CH2O directly were investigated, but in each case the discrepancy was only minimally reduced. Other possibilities were also considered but in each case there was no compelling evidence to support any of the hypotheses. Whatever the cause, the elevated CH2O concentrations significantly impact upper tropospheric HOx levels at high latitudes (>57°N) in the February–April time frame.

Quantum mechanical studies of methyl bromoperoxide isomers and the CH3O + BrO reaction

Ab initio quantum mechanical methods are employed to study methyl bromoperoxide and its isomers which are interesting intermediates in the reaction between methoxy radicals and bromine monoxide. Structural parameters, harmonic frequencies and relative energetics are calculated for all isomeric and conformeric forms. The CH3OOBr isomer is found to be the lowest energy structure followed by CH3OBrO while CH3BrO2 lies much higher in energy. The role of these isomers in the mechanism of the reaction CH3O + BrO is examined. Several transition state structures for the most important reaction channels are also investigated.

Analysis of borderline substitution/electron transfer pathways from direct ab initio MD simulations

Ab initio molecular dynamics simulations were carried out for the borderline reaction pathways in the reaction of CH 2O − with CH 3Cl. The simulations reveal distinctive features of three types of mechanisms passing through the S N2-like transition state (TS): (i) a direct formation of S N2 products, (ii) a direct formation of ET products, and (iii) a two-step formation of ET products via the S N2 valley. The direct formation of the ET product through the S N2-like TS appears to be more favorable at higher temperatures. The two-step process depends on the amount of energy that goes into the C–C stretching mode.

Vibrational mode analysis for the multi-channel reaction of CH 3O+CO

All species involved in the multi-channel reaction of CH 3O with CO have been optimized utilizing the G aussian94 procedure package at the B3LYP/6-311++G** level. The vibrational mode analysis is used to elucidate the relationships of the transition states, intermediate and the products. The extensive investigation shows that the reaction mechanism is reliable.

Enhancement of methanol selectivity in the products of direct selective oxidation of methane in CH 4-O 2-NO with Cu-ZnO/SiO 2

Enhancement of methanol selectivity in the products of the direct selective oxidation of methane with CH 4-O 2-NO in a gas-phase reaction was examined using a Cu-ZnO/SiO 2 catalyst. Two types of Cu-ZnO/SiO 2 catalysts were prepared with different compositions: 3:7:90 (denoted as CZS-1) and 5:5:90 (CZS-2) as the weight percent ratios of CuO:ZnO:SiO 2. Both CH 3OH and CH 2O were observed as C 1-oxygenates at 550 °C in the gas-phase selective oxidation of CH 4 in CH 4-O 2-NO without a catalyst, but then only CH 3OH was observed in the presence of the CZS-1, in addition to the direct gas-phase selective oxidation of methane. The catalyst temperature was 250 °C and the reaction pressure was 0.4 MPa. CH 3OH, CO and CO 2 selectivities increased and CH 2O selectivity decreased in the presence of the Cu-ZnO/SiO 2, in comparison to the values in the absence of the catalyst. We, therefore, suggested that three reactions: i.e. the hydrogenation reaction of CH 2O, the dissociation reaction of CH 2O, and the WGSR, progressed simultaneously on the Cu-ZnO/SiO 2 catalyst in CH 4-O 2-NO. Since the selectivity of CH 3OH did not vary during the variation of reaction temperatures from 150 °C up to 350 °C over the Cu-ZnO/SiO 2 catalyst, we assumed that SiO 2 was a favorable support to enhance the selectivity of CH 3OH in a wider catalyst temperature region.

Kinetic study of the reaction of CH3O with Br and Br2

The title reactions have been studied at room temperature by applying the discharge flow method coupled with laser induced fluorescence detection of methoxy radicals and resonance fluorescence detection of bromine atoms. The following rate constants were determined: CH3O + Br O products (1) k1 (298 K) = (3.4 ± 0.4 (1σ)) × 1013 cm3 mol-1 s-1, CH3O + Br2 O products (2) k2 (298 K) £ 5 × 108 cm3 mol-1 s-1.

Discharge-flow studies of the kinetics of the reactions of CH3O with Cl, Br, I, ClO, BrO and IO using laser-induced fluorescence and resonance-fluorescence detection

A low-pressure discharge-flow system employing LIF detection for CH3O, and resonance-fluorescence detection for Cl, Br and I atoms, was used to investigate the kinetics of the reactions of CH3O with X and XO (where X = Cl, Br and I) at T = 298 K. The rate constants obtained are kCl = (7.6 ± 1.4) × 10−11 cm3 molecule−1 s−1, kBr = (5.3 ± 0.9) × 10−11 cm3 molecule−1 s−1, kI = (8.5 ± 2.3) × 10−11 cm3 molecule−1 s−1, kClO = (3.4 ± 0.7) × 10−11 cm3 molecule−1 s−1, kBrO = (5.5 ± 1.0) × 10−11 cm3 molecule−1 s−1 and kIO = (4 ± 2) × 10−11 cm3 molecule−1 s−1. It is likely that the major channel in the reaction of CH3O with X yields HX and CH2O, while, in the reaction with XO, the main products are HOX and CH2O.

Improvement of the Catalytic Performance of Pd/WO3/ZrO2 in the Selective NO–CH4–O2 Reaction by the Addition of Water Vapor

Abstract The Pd/WO3/ZrO2 catalyst exhibited activity for the NO–CH4–O2 reaction in the presence of water vapor when WO3 monolayer covered the ZrO2 surface. The selectivity of methane consumption for NO reduction was considerably improved by the addition of 10% of water vapor.

Effects of aerosols on tropospheric oxidants: A global model study

The global distributions of sulfate and soot particles in the atmosphere are calculated, and the effect of aerosol particles on tropospheric oxidants is studied using a global chemical/transport/aerosol model. The model is developed in the framework of the National Center for Atmospheric Research (NCAR) global three‐dimensional chemical/transport model (Model for Ozone and Related Chemical Tracers (MOZART)). In addition to the gas‐phase photochemistry implemented in the MOZART model, the present study also accounts for the formation of sulfate and black carbon aerosols as well as for heterogeneous reactions on particles. The simulated global sulfate aerosol distributions and seasonal variation are compared with observations. The seasonal variation of sulfate aerosols is in agreement with measurements, except in the Arctic region. The calculated vertical profiles of sulfate aerosol agree well with the observations over North America. In the case of black carbon the calculated surface distribution is in fair agreement with observations. The effects of aerosol formation and heterogeneous reactions on the surface of sulfate aerosols are studied. The model calculations show the following: (1) The concentration of H2O2 is reduced when sulfate aerosols are formed due to the reaction of SO2 + H2O2 in cloud droplets. The gas‐phase reaction SO2 + OH converts OH to HO2, but the reduction of OH and enhancement of HO2 are insignificant (<3%). (2) The heterogeneous reaction of HO2 on the surface of sulfate aerosols produces up to 10% reduction of hydroperoxyl radical (HO2) with an uptake coefficient of 0.2. However, this uptake coefficient could be overestimated, and the results should be regard as an upper limit estimation. (3) The N2O5 reaction on the surface of sulfate aerosols leads to an 80% reduction of NOx at middle to high latitudes during winter. Because ozone production efficiency is low in winter, ozone decreases by only 10% as a result of this reaction. However, during summer the N2O5 reaction reduces NOx by 15% and O3 by 8–10% at middle to high latitudes. (4) The heterogeneous reaction of CH2O on sulfate aerosols with an upper limit uptake coefficient (γ = 0.01) leads to an 80 to 90% decrease in CH2O and 8 to 10% reduction of HO2 at middle to high latitudes during winter. Many uncertainties remain in our understanding of heterogeneous chemical processes and in the estimate of kinetic parameters. This model study should therefore be regarded as exploratory and subject to further improvements before final conclusions can be made.

Mechanistic Study of CO2 Photoreduction in Ti Silicalite Molecular Sieve by FT-IR Spectroscopy

The initial products of the photoreduction of gaseous CO2 in Ti silicalite molecular sieve using methanol as electron donor have been monitored by in-situ FT-IR spectroscopy. Reaction was induced by 266 nm excitation of the Ti+IV−O-II → Ti+III−O-I ligand-to-metal charge-transfer transition of the framework center. HCO2H, CO, and HCO2CH3 were the observed products. CO escaped instantaneously into the gas phase and was recorded at high spectral resolution. The origin of the products was elucidated by infrared analysis of experiments with C18O2, 13CO2, and 13CH3OH. The results show that CO originates from secondary photolysis of HCO2H, while HCO2CH3 emerges mainly from spontaneous Tishchenko reaction of CH2O, the initial oxidation product of methanol. The key finding is that formic acid is the primary 2-electron reduction product of CO2 at the LMCT-excited Ti centers. This implies that C−H bond formation occurs in the initial steps of CO2 activation at the gas−micropore interface.

Kinetics and Mechanism for the Reaction of Phenyl Radical with Formaldehyde

The kinetics and mechanism for the C6H5 + CH2O reaction were investigated by the cavity ringdown spectrometric (CRDS) and pulsed laser photolysis/mass spectrometric (PLP/MS) methods at temperatures between 298 and 1083 K. With the CRDS method, the rate constant was measured by monitoring the decay times of injected probing photons in the absence (tc0) and presence (tc) of the C6H5 radical. In the PLP/MS experiment at higher temperatures, the rate constant was determined by kinetic modeling of the absolute yields of C6H6. The values of the rate constants obtained by the two different methods agree closely, suggesting that the C6H5 + CH2O → C6H6 + CHO reaction 1 is the dominant channel. A weighted least-squares analysis of the two sets of data gave k1 = (8.55 ± 0.25) × 104 T2.19±0.25 exp[−(19 ± 13)/T] cm3 mol-1 s-1 for the temperature range studied. The mechanism for the C6H5 + CH2O reaction was also elucidated with a quantum-chemical calculation employing a hybrid density functional theory (B3LYP) using the aug-cc-PVTZ basis set. The theory predicts the barriers for the abstraction producing C6H6 and the addition giving C6H5CH2O and C6H5OCH2 to be 0.8, 1.4, and 9.1 kcal/mol, respectively. The rate constant calculated for the H-abstraction process using the canonical variational transition-state theory with a 1.1 kcal/mol barrier agrees closely with the experimental result over the entire range of temperatures studied.

Reactions of H3O+(H2O)n with Formaldehyde and Acetaldehyde

Rate constants were measured as a function of temperature for the reactions of H3O+(H2O)0-4 with CH3CHO and H3O+(H2O)0-5 with CH2O. The CH3CHO reactions all occur at the collision rate to within the experimental error. The n = 0 ion reacts via proton transfer, while the clusters react via ligand switching. In the CH2O reactions, n = 0 reacts at 90% of the collision rate and proceeds by proton transfer. The n = 1 reaction is somewhat less efficient and proceeds by ligand exchange. The efficiencies for both reactions are independent of temperature. The n = 2−5 reactions are progressively slower, except for the point at the highest temperature at which a particular cluster has been studied for n = 3−5. Thermal dissociation is postulated to be the reason for the upturn at higher temperatures. Otherwise, the reactions involve mostly ligand exchange with some association occurring at low temperature. The use of these results in model calculations of particle formation in jet engine exhaust is discussed.

A computational study of the mechanism for the C6H5+CH2O reaction

The mechanism for the C6H5 + CH2O reaction has been investigated with hybrid density functional quantum-chemical and statistical theory calculations. The results reveal three possible reaction channels: (1) The abstraction reaction producing C6H6 + HCO; (2) addition to the C atom yielding C6H5CH2O and (3) addition to the O atom giving C6H5OCH2. The barriers for these 3 reactions, calculated at the B3LYP/aug-cc-pvtz level of theory using the geometry optimized with B3LYP/cc-pvdz are 0.8, 1.4 and 9.1 kcal mol−1, respectively. The C6H5CH2O radical can fragment to form C6H5CHO + H with a barrier of 19.4 kcal mol−1. It can also undergo isomerization reactions ia two cyclic epoxy intermediates to give C6H5OCH2 with a maximum barrier of 20.4 kcal mol−1. Transition-state theory calculations using the predicted energy barriers and structures for the rate constants of the abstraction reaction (1) lead to very good agreement with our recently measured values, while the result of RRKM calculations for the isomerization/decomposition of C6H5OCH2 to C6H5CHO + H also agrees quantitatively with available experimental data.

Kinetics and mechanism of the reaction of a quasi-aromatic copper(II) complex, [Cu(PnAO)-6H]0, with formaldehyde

The kinetics of the reaction of CH2O with a quasi-aromatic metal complex (1,1,2,8,9,9-hexamethyl-4,6-dioxa-5-hydro-3,7,10,14-tetraazacyclotetradecane-2,7,10,12-tetraene)copper(II), [Cu(PnAO)-6H]0 (AH), have been studied spectrophotometrically under neutral conditions in 1/3(v/v) MeOH–H2O. The Cu, 2N, 3C quasi-aromatic heterocyclic ring in AH is highly reactive towards CH2O at the central-aromatic-carbon atom, C(12) and the following reaction mechanism is proposed. $${\text{AH + CH}}_{\text{2}} {\text{O}}\xrightarrow{{{\text{k}}_{\text{1}} }}{\text{A}}---{\text{CH}}_{\text{2}} OH\xrightarrow[{ + AH}]{{k_2 }}A---CH_2 ---A$$ $${\text{A}}---{\text{CH}}_{\text{2}} ---{\text{A + CH}}_{\text{2}} O2A---CH_2 OH$$ The compounds AH, A—CH2OH and A—CH2—A were isolated and identified by i.r spectroscopy and by elemental analysis. The kinetic data supported the proposed reaction sequence. All reactions were second order overall. The rate constants and corresponding activation parameters for every step were obtained and are discussed.

Kinetics of the reaction of CH 3O 2 radicals with NO 2

The kinetics of the gas-phase reaction of CH 3O 2 radicals with NO 2 were studied at 295 K in 0.5–14 atm of SF 6 diluent using pulse radiolysis combined with time-resolved UV–VIS spectroscopy. Rate data were obtained by following the loss of CH 3O 2 using a monitoring wavelength of 260 nm. The results were well described using the Troe formula with F c=0.40±0.09, k 0=(1.46±0.50)×10 −30 cm 6 molecule −1 s −1 and k ∞=(1.81±0.25)×10 −11 cm 3 molecule −1 s −1. These results are compared to the previous data and discussed in terms of reactivity trends and the atmospheric chemistry of peroxy radicals.

Microphysics and chemistry of sulphate aerosols at warm stratospheric temperatures

Observations of high NOx/NOy ratios (overall 40% larger than modelled values) during the Polar Ozone Loss in the Arctic Region in Summer campaign have led us to re‐examine the heterogeneous chemistry of stratospheric aerosol particles during the polar summer period, using the Integrated MicroPhysics and Aerosol Chemistry on Trajectories model. The warm summer temperatures (up to 235 K) imply very concentrated sulphuric acid solutions (80 wt %). On the one hand, these solutions are more likely to freeze, into sulphuric acid monohydrate (SAM), reducing the efficiency of the N2O5 hydrolysis reaction. Including this freezing process increases NOx/NOy ratios but does not improve model/measurement agreement: in polar spring, SAM formation causes the NOx/NOy ratio to be overpredicted whereas freezing has a much smaller effect on nitrogen chemistry during the continuous solar exposure of polar summer. On the other hand, if sulphate aerosols remain liquid, the high acidity may promote acid‐catalysed reactions. The most important reaction is CH2O + HNO3, which effectively increases NOx/NOy ratios across a wide range of conditions, improving agreement with measurements. Furthermore, the production of HONO can either enhance gas‐phase OH concentrations or promote secondary liquid reactions, including HONO + HNO3 and HONO + HCl. Primary uncertainties include the uptake coefficient of CH2O relevant to reaction with HNO3, the amount of HONO available for secondary reaction, and the relative rates of HONO reaction with HNO3 and HCl. The fate of the formic acid product, whose presence in the stratosphere may be an indicator for the CH2O reaction, and the impact on the stratospheric hydrogen budget are also discussed.

FT-IR kinetic and product study of the Br-initiated oxidation of dimethyl sulfide

An FT-IR kinetic and product study of the Br-atom-initiated oxidation of dimethyl sulfide (DMS) has been performed in a large-volume reaction chamber at 298 K and 1000-mbar total pressure as a function of the bath gas composition (N2 + O2). In the kinetic investigations using the relative kinetic method, considerable scatter was observed between individual determinations of the rate coefficient, suggesting the possibility of interference from secondary chemistry in the reaction system involving dimethyl sulfoxide (DMSO) formation. Despite the experimental difficulties, an overall bimolecular rate coefficient for the reaction of Br atoms with DMS under atmospheric conditions at 298 K of ≤1 × 10−13 cm3 molecule−1 s−1 can be deduced. The major sulfur products observed included SO2, CH3SBr, and DMSO. The kinetic observations in combination with the product studies under the conditions employed are consistent with rapid addition of Br atoms to DMS forming an adduct that mainly re-forms reactants but can also decompose unimolecularly to form CH3SBr and CH3 radicals. The observed formation of DMSO is attributed to reactions of BrO radicals with DMS rather than reaction of the Br–DMS adduct with O2 as has been previously speculated and is thought to be responsible for the variability of the measured rate coefficient. The reaction CH3O2 + Br → BrO + CH3O is postulated as the source of BrO radicals. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 883–893, 1999

Deuterium isotope effects on gas phase ion-molecule hydrogen-bonding interactions: Alcohol-alkoxide and alcohol-chloride adduct ions

Fourier Transform Ion Cyclotron Resonance (FT-ICR) and High Pressure Mass Spectrometric (HPMS) measurements of the deuterium isotope effect and kinetics of adduct ion formation have been used to probe the nature of the potential describing the motion of the hydrogen in gas phase ion-molecule hydrogen-bonding interactions. Hydrogen-bonding systems reported in this paper are alkoxide ion and chloride ion solvated by one molecule of alcohol, ROH●OR - and ROH●Cl -. Significant differences in the isotope effects were observed for the two systems. These differences are explained on the basis of the differing hydrogen bond strengths of the adduct ions, and the ability of the chloride ion to partake in multiple site, or chelate, interactions. In addition, HPMS studies of the kinetics of the reaction of CH 3O − with CH 3OH reveal that a double minimum potential energy surface may be appropriate for describing the adduct ion formation. These experimental studies have been supplemented by ab initio calculations to determine adduct ion structures as well as to permit statistical thermodynamic calculations of the isotope effect.

Combustion Rate of Burning Graphite in a Stagnation Flow of Water Vapor

The combustion rate of a burning graphite rod in a stagnation flow of water vapor is measured experimentally as a function of the surface temperature, with the stagnation velocity gradient taken as a parameter. The combustion rate, which is obtained from the surface regression rate in the forward stagnation region and the density change in the test specimen, is then used to determine surface kinetic parameters for the global C-H2O reaction. Relative combustion rates for oxidizers (O2, H2O, and CO2) are also examined, finding that H2O is a medium oxidizer even at high surface temperatures. It is also pointed out that there is a possibility that the CO2 produced by the water gas shift reaction in the gas phase participates in the surface reaction as an additional oxidizer and enhances the combustion rate in H2O flow.