The reaction of CH3O2 radicals with NO2

The rate constant for the reaction CH₃O₂ + NO₂ → (products) has been measured directly by flash photolysis and kinetic spectroscopy. At room temperature and at total pressures between 53 and 580 Torr, k₃ = (9.2 ± 0.4) × 10⁸ liter/mole sec so that the rate of formation of the probable primary product peroxymethyl nitrate (CH₃O₂NO₂) may be significant in urban atmospheres.     

UV absorption spectrum of CF3CFClO2 and kinetics of the self reaction of CF3CFCl and CF3CFClO2 and the reactions of CF3CFClO2 with NO and NO2

The ultraviolet absorption spectrum of CF₃CFClO₂ and the kinetics of the self reactions of CF₃CFCl and CF₃CFClO₂ radicals and the reactions of CF₃CFClO₂ with NO and NO₂ have been studied in the gas phase at 295 K by pulse radiolysis/transient UV absorption spectroscopy. The UV absorption cross section of CF₃CFCl radicals was measured to be (1.78 ± 0.22) × 10^?18 cm² molecule^?1 at 220 nm. The UV spectrum of CF₃CFClO₂ radicals was quantified from 220 nm to 290 nm. The absorption cross section at 250 nm was determined to be (1.67 ± 0.21) × 10^?18 cm² molecule^?1. The rate constants for the self reactions of CF₃CFCl and CF₃CFClO₂ radicals were (2.6 ± 0.4) × 10^?12 cm³ molecule^?1 s^?1 and (2.6 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1, respectively. The reactivity of CF₃CFClO₂ radicals towards NO and NO₂ was determined to (1.5 ± 0.6) × 10^?11 cm³ molecule^?1 s^?1 and (5.9 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1, respectively. Finally, the rate constant for the reaction of F atoms with CF₃CFClH was determined to (8 ± 2) × 10^?13 cm³ molecule^?1 s^?1. Results are discussed in the context of the atmospheric chemistry of HCFC-124, CF₃CFClH. © 1994 John Wiley & Sons, Inc.     

Kinetics of the reaction of CH3O with NO2

Rate coefficients for the reaction of CH₃O with NO₂ were measured over the temperature range 220–473 K and over the pressure range 0.6–4.0 Torr using a flow reactor apparatus with laser-induced fluorescence (LIF) detection of CH₃O. The results were fitted to extract recombination and disproportionation rate constants. Combined with previous indirect studies at higher pressure, they suggest that the reaction proceeds not through a single complex but by separate paths, with disproportionation occurring by direct H-atom abstraction.     

Pressure dependence of the rate coefficients and product yields for the reaction of CH3CO radicals with O2

Relative rate coefficients for the reaction of acetyl (CH₃CO) radicals with O₂ (k₄) and Cl₂ (k₇) have been obtained at 298 K and 228 K as a function of total pressure, using FTIR/environmental chamber techniques. Measured values of k₄/k₇ were placed on an absolute basis using k₇=2.8×10⁻¹¹ exp(−47/T) cm³ molec⁻¹ s⁻¹. At 298 K, the value of k₄ is constant ((7±2)×10⁻¹³ cm³ molec⁻¹ s⁻¹) at pressures from 0.1 to 2 torr, then increases to a high pressure limiting value of (3.2±0.6)×10⁻¹² cm³ molec⁻¹ s⁻¹, which is approached at pressures above 300 torr. At 228 K, the low-pressure value of k₄ increases by about 20–30%, while the high pressure value remains unchanged. Experiments designed to elucidate the products of reaction (4) as a function of pressure at 298 K indicate that the reaction occurs via a concerted mechanism in which CH₃CO radicals combine with O₂ to give an excited acetylperoxy radical (CH₃COO₂*) which is increasingly stabilized at high pressure at the expense of a low pressure decomposition channel. The yield of acetylperoxy radicals from reaction (4) decreases from >95% at pressures above 100 torr, to about 90% at 60 torr, and 50% at 6 torr. Indirect evidence for formation of OH radicals from the low pressure decomposition is presented, although the carbon-containing coproduct(s) of this channel could not be identified. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 655–663, 1997.     

Pressure and temperature dependence of methyl nitrate formation in the CH3O2 + NO reaction

The branching ratio β = k(1b)/k(1a) for the formation of methyl nitrate, CH(3)ONO(2), in the gas-phase CH(3)O(2) + NO reaction, CH(3)O(2) + NO → CH(3)O + NO(2) (1a), CH(3)O(2) + NO → CH(3)ONO(2) (1b), has been determined over the pressure and temperature ranges 50-500 Torr and 223-300 K, respectively, using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. At 298 K, the CH(3)ONO(2) yield has been found to increase linearly with pressure from 0.33 ± 0.16% at 50 Torr to 0.80 ± 0.54% at 500 Torr (errors are 2σ). Decrease of temperature from 300 to 220 K leads to an increase of β by a factor of about 3 in the 100-200 Torr range. These data correspond to a value of β ≈ 1.0 ± 0.7% over the pressure and temperature ranges of the whole troposphere. Atmospheric concentrations of CH(3)ONO(2) roughly estimated using results of this work are in reasonable agreement with those observed in polluted environments and significantly higher compared with measurements in upper troposphere and lower stratosphere.     

Temperature and pressure dependence of the rate constants of the reaction of NO3 radical with CH3SCH3

The reaction of nitrate radical with dimethyl sulfide was studied with cavity ring-down spectroscopy in 20-200 Torr of N2 diluent in the temperature range of 283-318 K. The rate constant for this reaction, k(1), is found to be temperature dependent and pressure independent: k1 = 4.5(-2.8)(+4.0) x 10(-13) exp[(310 +/- 220)/T] cm3 molecule(-1) s(-1). The uncertainties are two standard deviations from regression analyses. The present rate constants are in good agreement with those reported by Daykin and Wine (Int. J. Chem. Kinet. 1990, 22, 1083) and may be used in the atmospheric model calculation. Theoretical calculations were carried out to verify the existence of an intermediate complex.     

Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals,the reaction of CH3OCH2 radicals with O2, and the thermal decomposition of CH3OCH2 radicals

The rate constant for the reaction of CH₃OCH₂ radicals with O₂ (reaction (1)) and the self reaction of CH₃OCH₂ radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorption spectroscopy. k₁ was studied at 296K over the pressure range 0.025–1 bar and in the temperature range 296–473K at 18 bar total pressure. Reaction (1) is known to proceed through the following mechanism: CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# → CH₂OCH₂O₂H^# → 2HCHO + OH (k_prod) CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# + M → CH₃OCH₂O₂ + M (k_RO2) k = k_RO2 + k_prod, where k_RO2 is the rate constant for peroxy radical production and k_prod is the rate constant for formaldehyde production. The k₁ values obtained at 296K together with the available literature values for k₁ determined at low pressures were fitted using a modified Lindemann mechanism and the following parameters were obtained: k_RO2,0 = (9.4 ± 4.2) × 10⁻³⁰ cm⁶ molecule⁻² s⁻¹, k_RO2,∞ = (1.14 ± 0.04) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and k_prod,0 = (6.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, where k_RO2,0 and k_RO2,∞ are the overall termolecular and bimolecular rate constants for formation of CH₃OCH₂O₂ radicals and k_prod,0 represents the bimolecular rate constant for the reaction of CH₃OCH₂ radicals with O₂ to yield formaldehyde in the limit of low pressure. k_RO2,∞ = (1.07 ± 0.08) × 10⁻¹¹ exp(−(46 ± 27)/T) cm³ molecule⁻¹ s⁻¹ was determined at 18 bar total pressure over the temperature range 296–473K. At 1 bar total pressure and 296K, k₅ = (4.1 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and at 18 bar total pressure over the temperature range 296–523K, k₅ = (4.7 ± 0.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. As a part of this study the decay rate of CH₃OCH₂ radicals was used to study the thermal decomposition of CH₃OCH₂ radicals in the temperature range 573–666K at 18 bar total pressure. The observed decay rates of CH₃OCH₂ radicals were consistent with the literature value of k₂ = 1.6 × 10¹³exp(−12800/T)s⁻¹. The results are discussed in the context of dimethyl ether as an alternative diesel fuel. © 1997 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reaction of CH3O with NO

The kinetics of the reaction of CH₃O with NO and the branching ratio for HCHO product formation, obtained as Γ_HCHO = (Rate of HCHO formation) / (Rate of CH₃O decay), have been studied using a discharge flow reactor. Laser induced fluorescence has been used to monitor the decay of the CH₃O radical and the build-up of the HCHO product. Overall rate constants and product branching ratios were measured at room temperature over the pressure range of 0.72–8.5 torr He. Three reaction mechanisms were considered which differed in the routes of HCHO formation: (i) direct disproportionation; (ii) via an energized collision complex; or (iii) both reaction routes. It has been shown that data on the pressure dependence of the overall rate constant are not sufficient to distinguish between these mechanisms. In addition, an accurate value of Γ is required. Analysis of the available experimental data provided 0.0 and about 0.1 as the lower and upper limit for Γ, respectively. Since the rate constants derived for CH₃ONO formation were not sensitive to the value assumed for Γ, k = (1.69 ± 0.69) × 10^?29 cm⁶ molecule^?2 s^?1 and k = (2.45 ± 0.31) × 10^?11 cm³ molecule^?1 s^?1 could be derived. The rate constant obtained for formaldehyde formation when extrapolated to zero pressure is k = (3.15 ± 0.92) × 10^?12 cm³ molecule^?1 s^?1. © 1994 John Wiley & Sons, Inc.     

Non-RRKM dynamics in the CH3O2 + NO reaction system

Quasi-classical trajectory (QCT) calculations on a model potential energy surface (PES) show strong deviations from statistical Rice-Ramsperger-Kassel-Marcus (RRKM) rate theory for the decomposition reaction (1) CH3OONO* --> CH3O + NO2, where the highly excited CH3OONO* was formed by (2) CH3O2 + NO --> CH3OONO*. The model PES accurately describes the vibrational frequencies, structures, and thermochemistry of the cis- and trans-CH3OONO isomers; it includes cis-trans isomerization in addition to reactions 1 and 2 but does not include nitrate formation, which is too slow to affect the decay rate of CH3OONO*. The QCT results give a strongly time-dependent rate constant for decomposition and damped oscillations in the decomposition rate, not predicted by statistical rate theory. Anharmonicity is shown to play an important role in reducing the rate constant by a factor of 10 smaller than predicted using classical harmonic RRKM theory (microcanonical variational transition state theory). Master equation simulations of organic nitrate yields published previously by two groups assumed that RRKM theory is accurate for reactions 1 and 2 but required surprising parametrizations to fit experimental nitrate yield data. In the present work, it is hypothesized that the non-RRKM rate of reaction (1) and vibrational anharmonicity are at least partly responsible for the surprising parameters.     

UV absorption spectra and kinetics of the self reaction of CFCl2CH2O2 and CF2ClCH2O2 radicals in the gas phase at 298 K

The ultraviolet absorption spectra of the peroxy radicals derived from hydrochlorofluorocarbons 141b and 142b, (CFCl₂CH₂O₂ and CF₂ClCH₂O₂, respectively), and the kinetics of their self reactions have been studied in the gas phase at 298 K using a pulse radiolysis technique. Absorption cross sections were quantified over the wavelength range 220–300 nm. Measured absorption cross sections at 250 nm were indistinguishable within the experimental uncertainties (≈10%) and yield; Errors represent the sum of statistical uncertainty and our estimate of potential systematic errors. Our absorption cross section data were then used to derive the observed self reaction rate constants for reactions (1) and (2), defined as ?d[RO₂]/dt = 2k[RO₂]² (R = CFCl₂CH₂ or CF₂ClCH₂), of k_1obs = (4.36 ± 0.64) × 10^?12 and k_2obs = (4.13 ± 0.58) × 10^?12 cm³ molecule^?1 s^?1, quoted errors represent 2σ. These results are discussed with respect to previous studies of the absorption spectra and kinetics of peroxy radicals.     

Temperature dependence of pentyl nitrate formation from the reaction of pentyl peroxy radicals with NO

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary congruent with tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past.     

Quantum mechanical investigation of the atmospheric reaction CH3O2 + NO

The important stationary points on the potential energy surface of the reaction CH(3)O(2) + NO have been investigated using ab initio and density functional theory techniques. The optimizations were carried out at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory while the energetics have been refined using the G2MP2, G3//B3LYP, and CCSD(T) methodologies. The calculations allow the proper characterization of the transition state barriers that determine the fate of the nascent association conformeric minima of methyl peroxynitrite. The main products, CH(3)O + NO(2), are formed through either rearrangement of the trans-conformer to methyl nitrate and its subsequent dissociation or via the breaking of the peroxy bond of the cis-conformer to CH(3)O + NO(2) radical pair. The important consequences of the proposed mechanism are (a) the allowance on energetic grounds for nitrate formation parallel to radical propagation under favorable external conditions and (b) the confirmation of the conformational preference of the homolytic cleavage of the peroxy bond, discussed in previous literature.     

Room temperature rate coefficient for the reaction between CH3O2 and NO3

The molecular modulation spectroscopic technique was employed to study the kinetics of NO₃ radicals produced in the 253.7 nm photolysis of flowing gas mixtures of HNO₃/CH₄/O₂ at room temperature. By computer fitting of the NO₃ temporal behavior, a rate coefficient of (2.3 ± 0.7) × 10^?12 cm³ molecule^?1 s^?1 was obtained for the reaction between NO₃ and CH₃O₂ at 298 K.     

Rate and equilibrium constant of the reaction of 1-methylvinoxy radicals with O2: CH3COCH2 + O2<--> CH3COCH2O2

The reaction of 1-methylvinoxy radicals, CH3COCH2, with molecular oxygen has been investigated by experimental and theoretical methods as a function of temperature (291-520 K) and pressure (0.042-10 bar He). Experiments have been performed by laser photolysis coupled to a detection of 1-methylvinoxy radicals by laser-induced fluorescence LIF. The potential energy surface calculations were performed using ab inito molecular orbital theory at the G3MP2B3 and CBSQB3 level of theory based on the density function theory optimized geometries. Derived molecular properties of the characteristic points of the potential energy surface were used to describe the mechanism and kinetics of the reaction under investigation. At 295 K, no pressure dependence of the rate constant for the association reaction has been observed: k(1,298K) = (1.18 +/- 0.04) x 10(-12) cm3 s(-1). Biexponential decays have been observed in the temperature range 459-520 K and have been interpreted as an equilibrium reaction. The temperature-dependent equilibrium constants have been extracted from these decays and a standard reaction enthalpy of deltaH(r,298K) = -105.0 +/- 2.0 kJ mol(-1) and entropy of deltaS(r,298K) = -143.0 +/- 4.0 J mol(-1) K(-1) were derived, in excellent agreement with the theoretical results. Consistent heats of formation for the vinoxy and the 1-methylvinoxy radical as well as their O2 adducts are recommended based on our complementary experimental and theoretical study deltaH(f,298K) = 13.0 +/- 2.0, -32. 9+/- 2.0, -85.9 +/- 4.0, and -142.1 +/- 4.0 kJ mol(-1) for CH2CHO, CH3COCH2 radicals, and their adducts, respectively.     

Atmospheric chemistry of acetone: Kinetic study of the CH3C(O)CH2O2+NO/NO2 reactions and decomposition of CH3C(O)CH2O2NO2

Pulse radiolysis was used to study the kinetics of the reactions of CH₃C(O)CH₂O₂ radicals with NO and NO₂ at 295 K. By monitoring the rate of formation and decay of NO₂ using its absorption at 400 and 450 nm the rate constants k(CH₃C(O)CH₂O₂+NO)=(8±2)×10⁻¹² and k(CH₃C(O)CH₂O₂+NO₂)=(6.4±0.6)×10⁻¹² cm³ molecule⁻¹ s⁻¹ were determined. Long path length Fourier transform infrared spectrometers were used to investigate the IR spectrum and thermal stability of the peroxynitrate, CH₃C(O)CH₂O₂NO₂. A value of k₋₆≈3 s⁻¹ was determined for the rate of thermal decomposition of CH₃C(O)CH₂O₂NO₂ in 700 torr total pressure of O₂ diluent at 295 K. When combined with lower temperature studies (250–275 K) a decomposition rate of k₋₆=1.9×10¹⁶ exp (−10830/T) s⁻¹ is determined. Density functional theory was used to calculate the IR spectrum of CH₃C(O)CH₂O₂NO₂. Finally, the rate constants for reactions of the CH₃C(O)CH₂ radical with NO and NO₂ were determined to be k(CH₃C(O)CH₂+NO)=(2.6±0.3)×10⁻¹¹ and k(CH₃C(O)CH₂+NO₂)=(1.6±0.4)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹. The results are discussed in the context of the atmospheric chemistry of acetone and the long range atmospheric transport of NO_x. © John Wiley & Sons, Inc. Int J Chem Kinet: 30: 475–489, 1998     

Temperature and pressure dependence of the rate coefficient for the reaction between ClO and CH3O2 in the gas-phase

A temperature and pressure kinetic study for the CH(3)O(2) + ClO reaction has been performed using the turbulent flow technique with a chemical ionisation mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of CH(3)O(2) + ClO reaction: k(10)(T) = (1.96(?0.24)(+0.28)) × 10(-11) exp[(-626 ± 35)/T] cm(3) molecule(-1) s(-1) where the uncertainty associated with the rate coefficient is given at the one standard deviation level. Over a range of pressure (100-200 Torr) and temperature (298-223 K) no pressure dependence is observed. The smaller rate coefficients measured at lower temperatures compared with both previous low temperature studies are believed to arise through the reduction of secondary chemistry and greater sensitivity in terms of reactant detection (hence much lower initial concentrations were employed). These new data reduce the effectiveness of ozone loss cycles involving reaction of CH(3)O(2) + ClO in the polar stratosphere by around a factor of 1.5 and restrict the importance of the reaction to the tropical and extra-tropical clean marine environments in the troposphere.     

Direct kinetics study of the temperature dependence of the CH2O branching channel for the CH3O2 + HO2 reaction

A direct kinetics study of the temperature dependence of the CH₂O branching channel for the CH₃O₂ + HO₂ reaction has been performed using the turbulent flow technique with high‐pressure chemical ionization mass spectrometry for the detection of reactants and products. The temperature dependence of the CH₂O‐producing channel rate constant was investigated between 298 and 218 K at a pressure of 100 Torr, and the data were fitted to the following Arrhenius expression: 1.6 × 10^?15 × exp[(1730 ± 130)/T] cm³ molecule^?1 s^?1. Using the Arrhenius expression for the overall rate of the CH₃O₂ + HO₂ reaction and this result, the 298 K branching ratio for the CH₂O producing channel is measured to be 0.11, and the branching ratio is calculated to increase to a value of 0.31 at 218 K, the lowest temperature accessed in this study. The results are compared to the analogous CH₃O₂ + CH₃O₂ reaction and the potential atmospheric ramifications of significant CH₂O production from the CH₃O₂ + HO₂ reaction are discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 363–376, 2001     

The reaction of CH3O2 with SO2

Mixtures of Cl₂, CH₄, and O₂ were flash photolyzed at room temperature and pressures of ∽60–760 Torr to produce CH₃O₂. The CH₃O₂ radicals decay by the second-order process with k₆ = (3.7 ± 0.3) × 10^?13 cm³/sec in good agreement with other studies. This value ignores any removal by secondary radicals produced as a result of reaction (6), and therefore the true value might be as much as 30% lower. The value is independent of total pressure or the presence of H₂O vapor. With SO₂ also present, the CH₃O₂ decay becomes pseudo first order at sufficiently high SO₂ pressure which indicates the reaction The value of (8.2 ± 0.5) × 10^?15 cm³/sec at about 1 atm total pressure (mostly CH₄) was found for CH₃O₂ removal by SO₂, in good agreement with another recent measurement. This value can be equated with k₁, unless the products rapidly remove another CH₃O₂ radical, in which case k₁ would be a factor of 2 smaller.     

Kinetics and mechanism of the reaction of CF3 radicals with NO2

The reaction of CF₃ with NO₂ was studied at 296 ± 2K using two different absolute techniques. Absolute rate constants of (1.6 ± 0.3) × 10⁻¹¹ and (2.1 _−0.3⁺⁰⁷) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were derived by IR fluorescence and UV absorption spectroscopy, respectively. The reaction proceeds via two reaction channels: CF₃ + NO₂ → CF₂O + FNO, (70 ± 12)% and CF₃ + NO₂ → CF₃O + NO, (30 ± 12)%. An upper limit of 11% for formation of other reaction products was determined. The overall rate constant was within the uncertainty independent of total pressure between 0.4 to 760 torr. © 1996 John Wiley & Sons, Inc.     

CH3O·2+NO气相反应的密度泛函理论研究

用密度泛函方法分别研究了单态和三态 CH3 O·2 NO CH3 O· NO2 气相反应 .结果表明 ,反应中 NO进攻 CH3 O·2 经过了一个顺反异构化的过程 ,摘取 CH3 O·2 的端基氧 .整个反应是吸热反应 ,理论计算吸热值为 5 0 .93k J/ mol,单态为多通道多步骤反应 ,决定速度步骤的能垒为 1 90 .6 1 k J/ mol.而三态为单通道反应 ,其决定速度步骤的能垒为 1 6 3.31 k J/ mol.三态反应为最佳反应通道 .该反应的研究将为保护臭氧层及大气环境提供重要的理论依据 .