- Competition between hydrogen and deuterium abstraction by methyl radicals in isotopomerically mixed methanol glasses
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Rate parameters are reported for hydrogen and deuterium abstraction of methyl radicals embedded in glassy mixtures of CH3OH and CD3OD.The mole fraction of CH3OH in these isotopomeric mixture is 0, 0.05, 0.075, 0.10, 0.15, or 1.The nonexponential time dependence of the radical concentration is analyzed in terms of distributions of first-order rate constants.For the isotopomerically pure matrices, lognormal distributions describe the decay satisfactorily.The large difference between characteristic H and D transfer rate constants indicates tunneling.In the mixtures, there is competition between H and D abstraction processes which depends on the local structure about a radical, so that the corresponding rate parameters contain information about this structure.On the basis of earlier work , the analysis begins with the assumption that the structure about a radical resembles one of the crystalline phases of methanol.The entire set of decay curves is described by a (disordered) β-phase structure in which the radical replaces a methanol molecule and is located near the position associated with a methyl group.However, this static picture is inadequate because the radical can diffuse through the glass on the time scale of the kinetic measurements.Diffusion allows the radical to encounter more CH3OH molecules than would be expected for the static structure on a statistical basis - the effective mole fraction of CH3OH in the mixtures is higher than the analytical concentration.For the xH = 0.05 mixture, we estimate that on the average the radical encounters approximately 26 methanol molecules before abstraction occurs.This corresponds to diffusion over roughly 1100pm through the lattice.
- Doba, Takahisa,Ingold, K. U.,Lusztyk, Janusz,Siebrand, Willem,Wildman, Timothy A.
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- ESR Spectroscopic Detection of Methoxyl Radicals Formed in the Photochemical Gas-Phase Reaction of Methane and Water
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During the photolysis of methane-water gas mixtures, a radical has been trapped by both α-phenyl-N-tert-butylnitrone and 5,5-dimethyl-1-pyrroline 1-oxide.The analysis of the ESR parameters along with the control experiments and the determination of reaction products strongly suggests the detection of methoxyl radical.It is revealed that the yield of methanol and methoxyl radical depends on the initial concentration of methane and the reaction temperature, and the observed difference between their yields is mainly ascribed to the reaction CH3O. + CH4 --> CH3OH + .CH3, which is favorable at higher concentration of initial methane and at higher reaction temperature.Furthermore, the preferential trapping of the methoxyl radical by the spin traps is discussed.
- Migita, Catharina T.,Chaki,. Satoshi,Ogura, Kotaro
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- Kinetics and Mechanism of the Reaction of CH3 and CH3O with ClO and OClO at 298 K
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A discharge-flow system equipped with a laser-induced fluorescence cell to detect the methoxyl radical and a quadrupole mass spectrometer to detect both the chlorine monoxide and chlorine dioxide radicals has been used to measure the rate constants for the reactions CH3 + ClO -> products (1) CH3O + ClO -> products (2) CH3 + OClO -> products (3) CH3O + OClO -> products (4) at T 298 K and P = 1-3 Torr.The observed products of these reactions are CH3O for reaction (1), HOCl for reaction (2), and CH3O and ClO for reaction (3).For reaction (4), CH3OCl is a possible product.The rate constants derived for reactions (1)-(4) are: k1 = (1.3 +/- 0.4)E-10 cm3 molecule -1 s-1; k2 = (1.3 +/- 0.3)E-11 cm3 molecule-1 s-1; k3 = (1.6 +/- 0.3)E-11 cm3 molecule-1 s-1; k4 = (1.5 +/- 0.5)E-12 cm3 molecule-1 s-1.The likely mechanism for (1)-(4) are briefly discussed.
- Biggs, Peter,Canosa-Mas, Carlos E.,Fracheboud, Jean-Marc,Marston, George,Shallcross, Dudley E.,Wayne, Richard P.
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- Kinetics of the Methoxy Radical Decomposition Reaction: Theory and Experiment
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The rate constant for the unimolecular decomposition of the methoxy radical, CH3O + M --> CH2O + H + M, is determined both theoretically and experimentally.In the theoretical calculations, potential energy surface information is obtained from ab initio multiconfiguration SCF and multireference configuration interaction calculations using basis sets of up to triple-ζ plus polarization quality.The zero point corrected forward and reverse barriers are calculated to be 25.6 and 8.0 kcal/mol, respectively.RRKM rate calculations are performed incorporating a quantum correction due to tunneling through an Eckart barrier fit to represent the MRCI/TZP energetics and the shape of the MC-SCF/DZP vibrationally adiabatic potential energy curve in the saddle pont region.The calculated values compare closely with experimental data derived from kinetic modeling of CO formation rates measured in the thermal decomposition of methyl nitrate at 550-700 K in a static cell and at 1060-1620 K in shock waves.
- Page, M.,Lin, M. C.,He, Yisheng,Choudhury, T. K.
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- Kinetic studies of the reactions CH3+NO2→products, CH3O+NO2→products, and OH+CH3C(O)CH3→CH3C(O)OH+CH3, over a range of temperature and pressure
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The title reactions were investigated using pulsed laser photolysis combined with pulsed laser induced fluorescence detection of CH3O to determine the rate coefficients for CH3+NO2→products (3) and CH3O+NO2→products (5) as a function of temperature and pressure, and to estimate the yield of CH3 (and thus the yield of CH3C(O)OH) from the reaction of OH with CH3C(O)CH3 (2) at two different temperatures. Reaction 3 has both bimolecular and termolecular components: a simplified falloff parametrization with Fcent = 0.6 gives k3b0 = (3.2±1.3)×10-28(T/297)-0.3 cm6 s-1 and k3b∞ = (4.3±0.4)×10-11(T/297)-1.2 cm3 s-1 with CH3NO2 the likely product. The rate constant for the bimolecular reaction pathway to form CH3O+NO (3a) was found to be 1.9×10-11 cm-3 s-1. The low- and high-pressure limiting rate coefficients for reaction between CH3O and NO2 to form CH3ONO2 (5b) were derived as k5b0 = (5.3±0.3)×10-29(T/297)-4.4 cm6 s-1 and k5b∞ = (1.9±0.05)×10-11(T/297)-1.9 cm3 s-1, respectively. Although the final result is associated with some experimental uncertainty, we find that CH3 is formed in the reaction between OH and CH3C(O)CH3 at ≈50% yield at room temperature and 30% at 233 K.
- Wollenhaupt, Matthias,Crowley, John N.
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- Chemiluminescence Studies of the Reaction between O(3P) Atoms and Methyl Ioide
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The reaction between O(3P) atoms and CH3I was studied in a fast discharge-flow system.A threshold behavior with respect to the ratio of initial reactant concentrations in the chemiluminescence due to I2 (B2Π0+u -> X1Σ+g) was observed.The reactions in Chart 1 were considered significant in our interpretation.Fitting of temporal profiles produced an order of magnitude for k1 of 1010 cm3 mol-1 s-1 at room temperature.Chemiluminescence from OH(A2Σ -> X2Π)(0,0), OH(X)(2->1,1->0), CO(X)(1->0), and CO2(X)(v3=1->0) was also observed, in common with observations of O(3P) reactions with hydrocarbons.
- Kwong, Bess C. L.,Tse, R. S.
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- Competing pathways for methoxy decomposition on oxygen-covered Mo(110)
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The reactions of methanol (CH3OH) are investigated on a range of oxygen overlayers on Mo(110), with θO from ~0.5 to >1 ML, using a combination of vibrational spectroscopies and temperature-programmed reaction. Infrared spectroscopy identifies a common, tilted methoxy intermediate at high temperature on all overlayers studied; electron energy loss spectroscopy shows that this intermediate decomposes to deposit oxygen exclusively in high-coordination sites. While C-O bond scission to evolve gas-phase methyl radicals is the only reaction observed for methoxy on highly oxidized Mo(110), on the surface oxygen overlayers competition between dehydrogenation and methyl evolution is highly sensitive to oxygen coverage. The enhanced selectivity for hydrocarbon formation from methanol reaction on oxygen-modified Mo(110) relative to the clean surface is attributed to inhibition of dehydrogenation pathways rather than to marked changes in the C-O bond potential of methoxy.
- Queeney,Friend
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- Kinetics of the Reactions of CH3O and CD3O with NO
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The kinetics of the reactions of CH3O and CD3O with NO have been studiedusing a discharge flow reactor.CH3O and CD3O were detected using laser-excited fluorescence (LEF) near 300 nm.Total rate coefficients for the reaction of CH3O with NO were measured as a function of temperature (220-473 K) and presure (0.75-5.0 Torr) of He or Ar.Total rate coefficients for the CD3O + NO reaction were measured at ca. 294 K over the pressure range 0.75-5.0 Torr He.Using molecular-beam mass spectrometry, the CH3ONO zield of the CH3O + NO reaction was measured at 297 K (0.5 and 1.0 percentTorr) and 223 K (1.0 Torr), showing that disproportionation to H2CO + HNO is the major channel at low pressures.These results were combined to obtain the following expressions for the disproportionation and low-pressure recombination rate coefficients.For the CH3O + NO reaction, kDISP=(1.3 +/- 0,4) x 10-12 exp cm3s-1, kolll=(1.8 +/- 1.3) x 10-29 (Τ/300)-(3.2+/-0.5) cm6 s-1.For the CD3O + NO reaction at 294 +/- 2 K, kDISP =(3.0 +/- 0.4) x 10-12 cm3 s-1, and kolll = (2.5 +/- 0.4) x 10-29 cm6 s-1.While the uncertainty associated with the measured rate coefficients is ca. +/- l5percent, the product channel-specific rate coefficients obtained from these expressions are less certain, ca. +/- 50percent.Using MBMS, the CH3NO2 yield of the CH3 + NO2 reaction was measured at 297 K )0.5 and 1.0 Torr) and 223 K (1.0 Torr).Combined with the previously reported rate coefficient, these results indicate a low-pressure, third-order recombination rate coefficient of (6 +/- 2) x 10 -29cm 6 s-1 in He at 297 K.
- McCaulley, James A.,Moyle, Alfred M.,Golde, Michael F.,Anderson, Stuart M.,Kaufman, Frederick
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- High-temperature shock tube study of the reactions CH3 + OH → products and CH3OH + Ar → products
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The reaction between methyl and hydroxyl radicals has been studied in reflected shock wave experiments using narrow-linewidth OH laser absorption. OH radicals were generated by the rapid thermal decomposition of tert-butyl hydroperoxide. Two different species were used as CH3 radical precursors, azomethane and methyl iodide. The overall rate coefficient of the CH3 + OH reaction was determined in the temperature range 1081-1426 K under conditions of chemical isolation. The experimental data are in good agreement with a recent theoretical study of the reaction. The decomposition of methanol to methyl and OH radicals was also investigated behind reflected shock waves. The current measurements are in good agreement with a recent experimental study and a master equation simulation.
- Vasudevan, Venkatesh,Cook, Robert D.,Hanson, Ronald K.,Bowman, Craig T.,Golden, David M.
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- Near-infrared cavity ring-down spectroscopic study of the reaction of methylperoxy radical with nitrogen monoxide
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Time-resolved near-infrared cavity ring-down spectroscopy was applied to the kinetics of the gas-phase reaction of CH3O2 with NO at 100 Torr total pressure and 298 K. After flash photolysis of the CH 4/Cl2/Osub
- Enami, Shinichi,Yamanaka, Takashi,Kawasaki, Masahiro
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body text
p. 80 - 81
(2009/11/30)
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- Barrierless electron transfer bond fragmentation reactions
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The ultrafast N-O bond fragmentation in a series of N-methoxypyridyl radicals, formed by one-electron reduction of the corresponding N-methoxypyridiniums, has been investigated as potentially barrierless electron-transfer-initiated chemical reactions. A model for the reaction involving the electronic and geometric factors that control the shape of the potential energy surface for the reaction is described. On the basis of this model, molecular structural features appropriate for ultrafast reactivity are proposed. Femtosecond kinetic measurements on these reactions are consistent with a kinetic definition of an essentially barrierless reaction, i.e., that the lifetime of the radical is a few vibrational periods of the fragmenting bond, for the p-methoxy-N-methoxypyridyl radical.
- Lorance, Edward D.,Kramer, Wolfgang H.,Gould, Ian R.
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p. 14071 - 14078
(2007/10/03)
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- Oxidation of alkyl ions, CnH2n+1+ (n = 1-5), in reactions with O2 and O3 in the gas phase
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Rate constants and product ion branching fractions are reported for the reactions of CH3+, C2H5+, s-C3H7+, s-C4H9+, t-C4H9+, and t-C5H11+ with O2 and O3 at 300 K in a variable-temperature selected-ion flow tube (VT-SIFT). The reaction rate constant for CH3+ with O3 is large and approximately equal to the thermal energy capture rate constant given by the Su-Chesnavich equation. The C2H5+, s-C3H7+, and s-C4H9+ ions are somewhat less reactive, reacting at approximately 7-46% of the thermal capture rate. The HCO+ and C2H3O+ ions are the major products in these reactions. The t-C4H9+ and t-C5H11+ ions are found to be unreactive, with rate constants -12 cm3 s-1, which is the present detection limit of our apparatus using this ozone source. Ozone is a singlet in its ground state, and ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that reactant complexes can be formed, decreasing in stability with the size of alkyl chains attached to the cationic carbon atom. The decreasing reactivity of the alkyl ions with increasing order of the carbocation is attributed to a greatly reduced O3 binding energy. The ions listed above do not undergo two-body reactions with O2, k -13 cm3 s-1, despite the availability of reaction channels with exothermicities of several hudnred kilojoules per mole. Ab initio calculations at the B3LYP/6-31G(d) level of theory indicate that the O2 reaction systems form weak complexes with large C-O bond distances (repulsive at smaller distances) on the lowest energy triplet potential energy surface. Access to the singlet surface is required for bond formation; however, this surface is not accessible at thermal energies.
- Williams, Skip,Knighton,Midey, Anthony J.,Viggiano,Irle, Stephan,Wang, Qingfang,Morokuma, Keiji
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p. 1980 - 1989
(2007/10/03)
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- Kinetics of reductive N-O bond fragmentation: The role of a conical intersection
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N-alkoxyheterocycles can act as powerful one-electron acceptors in photochemical electrontransfer reactions. One-electron reduction of these species results in formation of a radical that undergoes N-O bond fragmentation to form an alkoxy radical and a neutral heterocycle. The kinetics of this N-O bond fragmentation reaction have been determined for a series of radicals with varying substituents and extents of delocalization. Rate constants varying over 7 orders of magnitude are obtained. A reaction potential energy surface is described that involves avoidance of a conical intersection. A molecular basis for the variation of the reaction rate constant with radical structure is given in terms of the relationship between the energies of the important molecular orbitals and the reaction potential energy surface. Ab initio and density functional electronic structure calculations provide support for the proposed reaction energy surface.
- Lorance, Edward D.,Kramer, Wolfgang H.,Gould, Ian R.
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p. 15225 - 15238
(2007/10/03)
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- Flow reactor studies of methyl radical oxidation reactions in methane-perturbed moist carbon monoxide oxidation at high pressure with model sensitivity analysis
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New rate constant determinations for the reactions (1), (2), (3) were made at 1000 K by fitting species profiles from high-pressure flow reactor on moist CO oxidation perturbed with methane. The experiments were selected to minimize the uncertainty in the
- Scire Jr., James J.,Yetter, Richard A.,Dryer, Frederick L.
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- Kinetics of the cross reactions of CH3O2 and C2H5O2 radicals with selected peroxy radicals
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The kinetics of the reactions of selected peroxy radicals (RO2) with CH3O2 and with C2H5O2 have been investigated using two techniques: excimer-laser photolysis and conventional flash photolysis, both coupled with UV absorption spectrometry. Radicals were generated either by photolysis of molecular chlorine in the presence of suitable hydrocarbons or by photolysis of the appropriate alkyl chloride. All such cross-reaction kinetics were investigated at 760 Torr total pressure and room temperature except for the reaction of the allylperoxy radical with CH3O2, for which the rate constant was determined between 291 and 423 K, resulting in the following rate expression: k15 = (2.8 ± 0.7) × 10-13 exp[(515 ± 75)/T] cm3 molecule-1 s-1. Values of (2.0 ± 0.5) × 10-13, (1.5 ± 0.5) × 10-12, (9.0 ± 0.15) × 10-14, -12, (2.5 ± 0.5) × 10-12, and (8.2 ± 0.6) × 10-12 (units of cm3 molecule-1 s-1) have been obtained for the reactions of CH3O2 radicals with C2H5O2, neo-C5H11O2, c-C6H11O2, C6H5CH2O2, CH2ClO2, and CH3C(O)O2, respectively, and (1.0 ± 0.3) × 10-12, (5.6 ± 0.8) × 10-13, (4.0 ± 0.2) × 10-14, and (1.0 ± 0.3) × 10-11 (units of cm3 molecule-1 s-1) for the reactions of C2H5O2 with CH2=CHCH2O2, neo-C5H11O2, c-C6H11O2, and CH3C(O)O2 radicals, respectively. These rate constants were obtained by numerical simulations of the complete reaction mechanisms, which were deduced from the known mechanisms of the corresponding peroxy radical self-reactions. A systematic analysis of propagation of errors was carried out for each reaction to quantify the sensitivity of the cross-reaction rate constant to the parameters used in kinetic simulations. The rate constant for a given cross reaction is generally found to be between the rate constants for the self-reactions of RO2 and CH3O2 (or C2H5O2). However, when the RO2 self-reaction is fast, the cross reaction with CH3O2 (or C2H5O2) is also fast, with similar rate constants for both reactions, suggesting that these particular peroxy radical cross reactions can play a significant role in the chemistry of hydrocarbon oxidation processes in the troposphere and in low-temperature combustion. Relationships between cross-reaction and self-reaction rate constants are suggested.
- Villenave, Eric,Lesclaux, Robert
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p. 14372 - 14382
(2007/10/03)
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- Kinetics and products of the reactions of CH3O2 with Cl and ClO
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The mechanisms of the reactions CH3O2 + Cl → products (1) and CH3O2 + ClO → products (2) have been studied at 298 K using a discharge flow reactor coupled to laser induced fluorescence for the detection of CH3O, observed as a product of both reactions, and mass spectrometry for the detection of other species. In the kinetic study of reaction (1), the channel producing the Criegee radical, CH2O2, has been found to be preponderant compared to the CH3O forming channel : CH3O2 + Cl → CH3O + ClO (1a), k1a = (2.0 ± 0.4) × 10-11 cm3 molecule-1 s-1 and CH3O2 + Cl → CH2O2 + HCl (1b), k1b = (2.0 ± 0.4) × 1010 cm3 molecule-1 s-1. The study of reaction (2) aimed at identifying the reaction pathways : CH3O2 + ClO → CH3O + ClOO (2a) and CH3O2 + ClO → CH3OCl + O2 (2b). The branching ratio for the first channel has been determined : α2a = 0.3 ± 0.1. These results are compared with other recent literature data and their atmospheric implication is briefly discussed.
- Daele,Poulet
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p. 1081 - 1099
(2007/10/03)
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- Determination of Rate Constants and Product Branching Ratios for the Reactions of CH302 and CH30 with Cl Atoms at Room Temperature
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Reactions of CH302 and of CH30 radicals with Cl atoms have been investigated at 300 K using a discharge flow system with LIF and mass-spectrometric detection.CH302 + Cl -> 0Cl + CH30 (1a), -> HCl + CH202 (1b), CH30 + Cl -> HCl + HCHO (2).Rate constants k1 = (1.15 +/- 0.3)*10-10 and k2 = (1.00 +/- 0.2)*10-10, in units cm3 molecule-1 s-1, have been obtained observing pseudo-first-order decays of CH302 and CH30 radicals, respectively.The branching ratio k1a/k1 = 0.51 +/- 0.09 has been found from the determination of 0Cl yields.Formation of the Criegee intermediate CH202 in reaction (1b) was inferred from C02 generation.A lower limit of 0.8 +/- 0.5 was derived for the C02 yield from unimolecular CH202 decomposition.Possible reaction pathways for steps (1a) and (1b) are suggested on the basis of ab initio model calculations.The calculations include the estimation of structures and energies of intermediate CH302Cl collision complexes and the decomposition of CH300Cl to yield products. - Keywords: Ab Initio Calculations / Chemical Kinetics / Elementary Reactions / Mass Spectrometry / Radicals
- Jungkamp, T. P. W.,Kukui, A.,Schindler, R. N.
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p. 1057 - 1066
(2007/10/02)
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- Temperature-Dependent Rate Constants and Product Branching Ratios for the Gas-Phase Reaction Between CH3O2 and ClO
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Kinetic and product branching data have been measured for the reaction of CH3O2 radicals with ClO in the temperature range 225-355 K using the discharge-flow/mass spectrometry technique.The pressure-independent overall reaction rate constant is described by k(12)(225-355 K) = (3.25 +/- 0.50) * 10-12 exp((-114 +/- 38)/T) cm3 molecule-1 s-1.Two reaction channels were identified, leading to CH3O + ClOO (12a) and CH3OCl + O2 (12b), respectively.The branching ratios, α12a = k12a/k12 and α12b = k12b/k12, are also independent of pressure and are described by α12a = (1.51 +/- 0.56) exp((-218 +/- 93)/T) and α12b = (0.080 +/- 0.059) exp((-377 +/- 178)/T).These expressions yield roughly equal rate constants of k12a and k12b of ca. 1 * 10-12 cm3 molecule-1 s-1 at the low temperatures prevalent in the polar winter and early springtime stratosphere.We thus identify CH3OCl as a potentially important species in ozone hole chemistry.
- Helleis, Frank,Crowley, John N.,Moortgat, Geert K.
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p. 11464 - 11473
(2007/10/02)
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- Time-resolved Spectroscopy and Charge-transfer Photochemistry of Aromatic EDA Complexes with X-Pyridinium Cations
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Direct photoexcitation of 1: 1 aromatic EDA complexes with various N-substituted X-pyridinium cations (X = nitro, fluoro, methoxy and acetoxy) is achieved by the specific irradiation of their charge-transfer (CT) absorption bands.Time-resolved picosecond spectroscopy refers to charge-transfer activation by the identification of the aromatic cation radical as the initial transient (T1) formed in a photoinduced electron-transfer together with the X-pyridinyl radical.The homolytic fragmentation of the latter varies with the X-substituent in the order X = NO2 > F > AcO >CH3O, and the addition of X. to the aromatic donors leads to a series of cyclohexadienyl adducts that are identified as longer-lived transients (T2) by time-resolved (nanosecond/microsecond) spectroscopy.The phototransients T1 and T2 together account for the different types of aromatic product (resulting from ring substitution, side-chain substitution and dimerization) that are generated by steady-state CT photochemistry of the aromatic EDA complexes with X-pyridinium cations.
- Bockman, T. M.,Lee, K. Y.,Kochi, J. K.
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p. 1581 - 1594
(2007/10/02)
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- Kinetics of Reactions of CH3O2 and HOCH2CH2O2 Radicals Produced by the Photolysis of Iodomethane and 2-Iodoethanol
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The molecular modulation technique coupled with UV absorption spectroscopy has been used to investigate the UV spectra and kinetics of reactions of the methylperoxy radical (CH3O2) and the 2-hydroxyethylperoxy radical (HOCH2CH2O2), generated by the 254-nm photolysis of the organic iodides CH3I and HOCH2CH2I: RI + hν (λ = 254 nm) -> R + I (7) and R + O2 + M -> RO2 + M (8).Measurements of the UV spectra of both RO2 radicals were complicated by the production of additional transient species absorbing strongly at wavelengths above 240 nm.These are believed to be CH3OOIand HOCH2CH2OOI formed as intermediates in the RO2-chaperoned recombination of iodine atoms.Both CH3O2 and HOCH2CH2O2 were found to obey second-order kinetic behaviour owing to removal by a series of reactions initiated by the self-reactions: CH3O2 + CH3O2 -> products (9) and HOCH2CH2O2 + HOCH2CH2O2 -> products (10).The parameter k9obs/? (where k9obs is the observed second-order rate coefficient) had a value of (1.01 +/- 0.09)E5 cm s-1 at 230 nm, independent of pressure in the range 10.8-760 Torr, at 298 K.Additional measurements made over the temperature range 268-350 K indicated that this parameter displays a weak negative temperature dependence.E/R was found to have a value of -220 +/- 72 K at 760 Torr, and a value of -92 +/- 53 K at 10.8 Torr.The parameter k10obs/? had a value of (6.8 +/- 0.4)E5 cm s-1 at 230 nm (p = 760 Torr, T = 298 K).Assuming the photolysis of HOCH2CH2I leads exclusively to the production of HOCH2CH2O2, the following values of ?(230 nm) and k10obs were concluded: ?(230 nm) = (2.35 +/- 0.25)E-18 cm2 molecule-1 and k10obs = (1.60 +/- 0.17)E-12 cm3 molecule-1 s-1.At lower pressures, HO2 was also generated in the HOCH2CH2I system in significant quantities, enabling investigation of reaction 11 at 10 Torr and 298 K: HOCH2CH2O2 + HO2 -> products (11).A value of k11 = (4.8 +/- 1.5)E-12 cm3 molecule-1 s-1 was concluded from these measurements, based on the above value of ?(230 nm).
- Jenkin, Michael E.,Cox, Richard A.
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p. 3229 - 3237
(2007/10/02)
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- Combustion Reactions in Methane-Air Premixed Flames
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A reaction scheme reproducing methane combustion by computer simulation under a wide range of stoichiometric ratio has been proposed and applied to flat methane-air premixed flames to elucidate factors determining their propagation mechanisms and the role of C2 reaction pathways.Main methane combustion reactions, which progress sequentially from the fuel to carbon dioxide, are inhibited at low temperatures dur to retarded production of oxygen atoms which are necessary for the oxidation of methyl radicals.The role of C2 routes have been investigated by comparison of the results obtained by use of the C2 scheme and a simplified C1 scheme.Some of the methyl radicals are once recombined to ethane but most of the produced ethane is dissociated again to methyl via ethyl radicals.Then the C2 routes work as a temporary storage of species and hinder the main combustion reactions which go through C1 species.
- Fukutani, Seishiro,Sakaguchi, Keizo,Kunioshi, Nilson,Jinno, Hiroshi
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p. 1623 - 1631
(2007/10/02)
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- Photodissociation of Alkyl Nitrites in a Molecular Beam. Primary and Secondary Reactions
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The translational energy distributions P(ET) for the 248-nm photodissociation products (NO + RO) of isopropyl nitrite and tert-butyl nitrite have been measured with a molecular beam time-of-flight (TOF) apparatus.Previous experiments with methyl nitrite and ethyl nitrite have been repeated with higher resolution.The average photofragment translation energies of these four alkyl nitrites are in good agreement with those predicted by an impulsive model that treats the NO as a rigid fragment and the alkoxy radical as a soft fragment.Hence, and in contrast to the vibrational predissociation on the S1 potential energy surface, S2 dissociation is direct and involves no significant "vibrational-translational" coupling between the reaction coordinate rO-N and the rN=O coordinate.The width of the experimental P(ET) distributions decreases with increasing size of the alkoxy substituent.This result is discussed in terms of an anticorrelation between the internal energies of a fragment pair.Furthermore, the spontaneous secondary dissociation of isopropoxy and tert-butoxy photofragments was observed which yields CH3 radicals and acetaldehyde or acetone, respectively.The unimolecular decay of these alkoxy radicals confirms their relatively high internal energy as deduced from the primary P(ET) and it is shown that this decay occurs on a submicrosecond time scale.
- Effenhauser, C. S.,Felder, P.,Huber, J. Robert
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p. 296 - 302
(2007/10/02)
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- Generation, Thermodynamics, and Chemistry of the Diphenylcarbene Anion Radical (Ph2C.-)
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Dissociative electron attachment with Ph2C=N produced Ph2C.- (m/z 166).The reactions of Ph2C.- with potential proton donors of known gas-phase acidity were used to bracket PA(Ph2C.-) = 380 +/- 2 kcal mol-1 from which ΔHf0(Ph2C.-) = 81.8 +/- 2 kcal mol-1 was calculated.The reactions of Ph2C.- with CH3OH and C2H5OH proceeded with major and minor amounts, respectively, of a H2.+-transfer channel, forming Ph2CH2, RCHO, and an electron.The kinetic nucleophilicity of Ph2C.- in SN2 displacement reactions with CH3X and C2H5X molecules was shown to be medium, which requires a significant intrinsic barrier in these reaction.The reactions of Ph2C.- with various aldehydes, ketones, and esters were fast and established two principal product-forming channels: (1) H+ transfer if the neutral reactant contains activated C-H bonds and (2) carbonyl addition followed by radical β-fragmentation of one of the groups originally attached to the carbonyl carbon.The order for the ease of radical β-fragmentation in the tetrahedral intermediates was RO > alkyl >> H, and CO2CH3 > CH3.Since the reactions of Ph2C.- with the simple esters HCO2CH3 and CH3CO2CH3 were fast, it should now be possible to examine the reactions of carbonyl-containing organic molecules, which are expected to react slower than these esters and obtain their relative reactivities.
- McDonald, Richard N.,Gung, Wei Yi
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p. 7328 - 7334
(2007/10/02)
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- Infrared Laser Techniques to Study the Structure and Dynamics of Transient Species
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Infrared multiple photon laser induced chemistry has moved over the past ten years between the extremes of heralding truly intramolecular selective chemistry and being merely a laboratory curiosity.The maturing of infrared multiple photon excitation (IRMPE) now finds IRMPE used routinely in a variety of experiments including spectroscopy, chemical kinetics, thermodynamics and laser driven synthesis.Continuously tunable infrared laser sources now allow a much wider range of molecules to be excited, and the photophysics of IRMPE to be probed in greater detail, than was possible with only limited line tunability.Examples of both the pfotophysics of IRMPE and applications to kinetics and spectroscopy are discussed, including vibrational photodissociation spectra of isolated metal clusters.
- Woodin, R. L.,Kaldor, A.
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p. 269 - 276
(2007/10/02)
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- Reaction of F Atoms with Methyl Nitrite. Infrared Spectroscopic Evidence for the Stabilization of FON in an Argon Matrix
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When the products of the reaction between F atoms formed in a microwave discharge and methyl or methyl-d3 nitrite are frozen in a large excess of argon at 14 K, prominent absorptions of FNO appear in the infrared spectrum of the solid deposit, demonstrating that NO abstraction occurs.Two absorptions previously attributed to FON are also very prominent, supporting the identification of that isomer, but the assignment of the NO-stretching fundamental of FON was not confirmed.The appearance of infrared absorptions of isolated and hydrogen-bonded HF indicates that the more exothermic reaction channel involving H-atom abstraction also occurs.The absorptions of H2CO and NO, products of the decomposition of CH2ONO, are prominent, with indirect evidence that a small concentration of CH2ONO may have been stabilized.Factors influencing the relative contributions of the two observed F-atom reaction channels are considered.
- Jacox, Marilyn E.
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p. 4940 - 4945
(2007/10/02)
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- Gas-Phase Nucleophilic Reactivities of Phenylnitrene (PhN-*) and Sulfur Anion Radicals (S-/.) at sp3 and Carbonyl Carbon
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The reactions of PhN-/. with a series of carbonyl-containing molecules (aldehydes, ketones, and esters) were shown to proceed via an addition/fragmentation mechanism, PhN-* + R2C=O -> -)R2> -> PhN=C(O-)R + *R, producing various acyl anilide anion products.In several cases, the tetrahedral intermediate anion radicals were observed as minor ions.The intrinsic reactivity of the carbonyl-containing molecules was aldehydes > ketones > esters, where similar R groups were involved.The overall exothermicities of these reactions did not appear to play the major role in determining the relative rates (krelC=O) for these reactions.From the reaction of PhN-* with cyclobutanone, a new type of anion radical, PhN=C(O-)CH2* (m/z 133) (+ C2H4) was produced; the loss of C2H4 was considered due to the ring strain in the ketone.With cyclopentanone, cyclohexanone, and cycloheptanone, the anion radicals PhN=C(O-)(CH2)n* (n = 4-6) were the exclusive product ions.PhN-* was shown to be a poor nucleophile in SN2 displacement reactions with CH3X molecules (X = Cl, Br, O2CCF3).S-* was shown to exhibit modest SN2 nucleophilicity with CH3Cl and CH3Br.The reactions of S-* with CF3CO2R proceed via both SN2 displacement and carbonyl addition/fragmentation mechanisms: with R = CH3, the anion products were 65percent CF3CO2- and 35percent CF3COS-; from R = C2H5, the product ions were 4percent CF3CO2- and 96percent CF3COS-.These data yield the ratio kCH3/kC2H5 = 16 for SN2 displacement by S-* at these alkyl groups.The reactions of PhN-* with CO2, COS, CS2, and O2 are also reported.The reaction of PhN-* with CS2 to produce S-* as a major channel was used as the source of this atomic anion radical.In several reactions occuring at nearly the collison limit, selectivity was observed for (a) which of two reaction centers were attacked to give products and (b) which of two mechanisms would be dominant in the overall reaction.
- McDonald, Richard N.,Chowdhury, A. Kesem
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p. 198 - 207
(2007/10/02)
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- Oxidation of Sulfur Dioxide by Methylperoxy Radicals
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This study was made to resolve the apparent discrepancy between the finite rate constants observed for the CH3O2 reaction with SO2 in high-intensity flash photolysis and near zero values observed recently by us and others for this reaction in NO-free, CH3O2-SO2 experiments at low intensity.
- Kan, Charles S.,Calvert, Jack G.,Shaw, John H.
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p. 1126 - 1132
(2007/10/02)
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