- Photolysis of Acetyl Benzoyl Peroxide Isolated in an Argon Matrix: The Stability of the Benzoyloxy and Acetoxy Radicals toward Decarboxylation
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Exposure of matrix-isolated acetyl benzoyl peroxide to UV light (λ > 2800 angstroem) produces methyl benzoate, the methyl-phenyl radical pair, and carbon dioxide.The results of the low-temperature irradiation reveal that methyl benzoate is formed by recombination of the benzoyloxy-methyl radical pair.Since phenyl acetate was not detected during the low-temperature irradiation, it was concluded that the lifetime of the acetoxy-phenyl radical pair is too short to allow concentrations that could be detected by infrared spectroscopy.Ab initio calculations are used to provide an explanation for the dramatic lifetime differences between the benzoyloxy and acetoxy radicals.In addition, it was shown that electronic excitation of the phenyl radical does not induce a chemical reaction with carbon dioxide.Arguments based on experiment and theory are presented to explain this unexpected result.
- Pacansky, J.,Brown, D. W.
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- 2′-deoxyguanosine (DG) oxidation and strand-break formation in DNA by the radicals released in the photolysis of N-tert-butoxy-2-pyridone. Are tert-butoxyl or methyl radicals responsible for the photooxidative damage in aqueous media?
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Chemical equation presented The photolysis of pyridone 3b (photo-Fenton reagent) in benzene releases tert-butoxyl radicals, which have been trapped by DMPO and EPR-spectrally identified. In aqueous solution, however, the fragmentation of the tert-butoxyl into methyl radicals prevails and the former radicals are of no direct consequence in the photooxidation of 2′-deoxyguanosine (dG) and pBR 322 DNA. The photooxidative damage of nucleic acids is caused by the oxyl radical species generated from the methyl radicals with oxygen.
- Adam, Waldemar,Marquardt, Stefan,Kemmer, Diana,Saha-Moller, Chantu R.,Schreier, Peter
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- Photoinduced reactions of methyl radical in solid parahydrogen
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Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H2 molecules appreciably but that the radical in the electronic excited state of B(2A1′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 a(1A1) and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give methane.
- Fushitani, Mizuho,Sogoshi, Norihito,Wakabayashi, Tomonari,Momose, Takamasa,Shida, Tadamasa
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- Microwave kinetic spectroscopy of reaction intermediates: O + ethylene reaction at low pressure
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A microwave spectroscopic method has been developed to study elementary reactions in real time through in situ observation of rotational spectra of reaction intermediates such as free radicals with lifetime as short as 1 ms.This method was applied to the O(3P) + ethylene reaction in order to assess the roles of (a) vinoxy + H and (b) CH3 + CHO channels in the initial process.The reaction was initiated by irradiating an N2O/C2H4 mixture containing a trace amount of mercury with the 253.7 nm mercury resonance line, and the time evolution of vinoxy, HCO, and H2CO was followed by measuring their microwave absorption intensities as functions of time.The branching ratio was thus determined to be 0.4 +/- 0.1 and 0.5 +/- 0.1 for (a) and (b), respectively, at the sample pressure of 30 mTorr.The present result agrees with those obtained by Hunziker et al. using much higher pressures of samples, but is not compatible with the observation of Buss et al. that (a) is dominant in collision-free conditions.
- Endo, Yasuki,Tsuchiya, Soji,Yamada, Chikashi,Hirota, Eizi,Koda, Seiichiro
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- The gaseous reaction of vinyl radical with oxygen
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Time-resolved Fourier transform infrared (TR-FTIR) emission spectroscopy was used to explore the gaseous reaction of C2H3 vinyl radical. As a result, several new primary and secondary products of the C2H3+O
- Wang,Wang,He,Kong
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- Rate parameters for the reaction of atomic hydrogen with dimethyl ether and dimethyl sulfide
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Absolute rate constants for the reaction of atomic hydrogen with dimethyl ether (DME) and dimethyl sulfide (DMS) were obtained using the flash photolysis-resonance fluorescence technique.Under conditions where secondary reactions are avoided, rate constants for the H + DME reaction over the temperature range 273-426 K are well represented by the Arrhenius expression k1 = (4.38 +/- 0.59) * 10-12 exp( - 1956 +/- 43/T) cm3 molecule-1 s-1.The corresponding Arrhenius expression for the H + DMS reaction over the temperature range 212-500 K is k2 = (1.30 +/- 0.43) * 10-11 exp( -1118 +/- 81/T) cm3 molecule-1 s-1.The Arrhenius plot for k2 shows signs of curvature, however, and separate Arrhenius expressions are derived for the data above and below room temperature.These results are discussed and comparisons are made with previous determinations which employed flow discharge and product analysis techniques.
- Lee, J. H.,Machen, R. C.,Nava, D. F.,Stief, L. J.
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- Photosensitized Dissociation of Di-tert-butyl Peroxide. Energy Transfer to a Repulsive Excited State
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Energy transfer from a variety of aromatic hydrocarbons and ketones to di-tert-butyl peroxide has been examined by using nanosecond laser flash photolysis techniques.Triplet energy transfer to the peroxide leads to its efficient cleavage into two tert-butoxy radicals.Representative rate constants for triplet quenching in benzene at 25 deg C are 7.9*1E6, 3.4*1E6, and 7.0*1E4 M-1 s-1 for p-methoxypropiophenone, benzophenone, and benzanthracene, respectively.The rate of transfer for p-methoxypropiophenone (ET ca. 72.5 kcal/mol) is approximately temperature independent; for lower energy sensitizers ca. 0.17 kcal/mol activation energy is required for each kilocalorie per mole decrease in triplet energy.No evidence indicating exciplex intermediacy was found.A model for energy transfer to a repulsive state of peroxide is proposed in which no activation energy is required if the sensitiser meets the energy requirements at the O-O equilibrium distance.For sensitizers of lower triplet energy, energy transfer to a repulsive state is proposed to occur from a thermally activated ground state having a greater than equilibrium oxygen-oxygen bond lenght.The same mechanism may apply in other systems where the acceptor lacks low-lying excited states.A few rate constants for the quenching of singlet sensitizers have also been determined by using fluorescence techniques.
- Scaiano, J. C.,Wubbels, Gene G.
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- Time-Resolved Observation of Sequential Bond Cleavage in a Gas-Phase Azoalkane
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The gas-phase photodissociation of an unsymmetrical azoalkane, 3-(methylazo)-3-methyl-1-butene, was studied with time-resolved coherent anti-Stokes Raman spectroscopy (CARS) to probe for product formation.Appearance kinetics were measured for all three primary photoproducts.The results indicate a two-step dissociation mechanism in which the 1,1-dimethylallyl fragment is formed within 2 ns of excitation, whereas the methyl radical and N2 are formed through decay of reaction intermediate having a lifetime of 12 +/- 2 ns.These findings mark the first direct demonstration of a stepwise homolysis mechanism for an azoalkane.
- Adams, J. Stephen,Burton, Katherine A.,Andrews, B. Kim,Weisman, R. Bruce,Engel, Paul S.
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- Experimental and theoretical studies of gas phase NO3 and OH radical reactions with formaldehyde, acetaldehyde and their isotopomers
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Formaldehyde and acetaldehyde are among the most abundant carbonyls in the atmosphere. The vapor phase reactions of formaldehyde, formaldehyde-d2, 13C-formaldehyde, acetaldehyde, acetaldehyde-1-d1, acetaldehyde-2,2,2-d3, and acetaldehyde-d4 with NO3 and OH radicals were studied at 298 K and 1013 mbar using long-path FTIR detection. The OH and NO3 radicals reacted with formaldehyde and acetaldehyde entirely through Hald-abstraction. The acetaldehyde with OH proceeded via two pathways - CH3CHO + OH → CH3CO + H2O and CH3CHO + OH → CH3 + CO + H2O - with a branching ratio of ≈ 9:1 at 298 K. In the acetaldehyde reaction with NO3, the latter reaction path was not important in atmospheric conditions. Theoretical calculations results apparently reproduced all essential features of the experimental kinetic data for the parent compounds. Theoretical analysis of the kinetics of CH3CHO + OH indicated a complex and a temperature dependent reaction mechanism with significant dominance of the formation of the adduct at low temperatures. However, the observed kinetic isotope effects for reactions of NO3 were much smaller than those calculated by conventional transition state theory. This was also attributed to the formation of pre-reactions adducts.
- D'Anna, Barbara,Bakken, Vebjorn,Beukes, Jon Are,Nielsen, Claus J.,Brudnik, Katarzyna,Jodkowski, Jerzy T.
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- Absolute rate constants and Arrhenius parameters for the addition of the methyl radical to unsaturated compounds: The methyl affinities revisited
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Absolute rate constants and their Arrhenius parameters are reported for the addition of the methyl radical to 21 monosubstituted and 1,1- disubstituted alkenes and to 6 benzenes in 1,1,2-trifluoro-1,2,2- trichloroethane solution. They are used to convert relative reaction rates known as methyl affinities from the work of M. Szwarc and others for about 250 additional unsaturated compounds to the absolute scale. An analysis shows that the addition rates depend on the reaction enthalpy but also indicates a moderate nucleophilic polar effect for the liquid-phase reactions. It is pointed out that this polar effect may be smaller in the gas phase.
- Zytowski, Torsten,Fischer, Hanns
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- Infrared spectroscopic studies on photolysis of methyl iodide and its clusters in solid parahydrogen
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Methyl iodide is trapped as the monomer and as clusters in the parahydrogen, known as a quantum crystal, at temperatures below about 8 K.UV illumination of the deposited sample at about 5 K causes the dispersal of clusters and the production of the methyl radical, methane, and ethane as evidenced by their infrared absorption spectra.Thermal annealing of the photolyzed sample at temperatures up to 11 K results in the disappearance of the methyl radical, the enhancement of ethane, and the regeneration of methyl iodide.When the initial concentration of the iodide is small, the clusters in the deposited sample are suppressed.For such a sample the UV excitation produces the methyl radical and methane but the formation of ethane is negligibly small.Relevance of the present work to studies of photolysis in gaseous clusters of methyl iodide is discussed.
- Momose, Takamasa,Miki, Masaaki,Uchida, Mikio,Shimizu, Takayuki,Yoshizawa, Isamu,Shida, Tadamasa
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- Thermal and Low-Energy Electron-Driven Chemistry of Biacetyl on Ag(111)
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The thermal and electron-induced chemistry of biacetyl (CH3COCOCH3) on Ag(111) has been studied using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy.No thermal decomposition of biacetyl occurs, confirming that Ag(111) is inert with respect to the breaking of C-C, C=O, and C-H bonds.There are five molecular biacetyl desorption peaks in TPD-180, 178, 174, 188, and 215 K.The peak at 180 K is attributed to monolayer adsorption, and its saturation peak area is used to scale other TPD biacetyl peaks.The peak at 188 K is assigned to multilayers and that at 215 K to desorption from defect sites.The peaks at 174 and 188 K are discussed in terms of coverage dependent reorientation and bilayers.Nonthermal excitation pathways by which the surface chemistry of biacetyl may be directed were explored by irradiating 1 ML of biacetyl with 50 eV electrons.During irradiation, CO, CH3, ketene (H2C=C=O), and C2H6 desorb.After irradiation, five new post-irradiation TPD peaks appear.These are identified as H2 at 210 K, CH4 at 235 and 315 K, H2C=C=O at 240 K, and reaction-limited CH3COCOCH3 at 440 K.XPS shows C(a) and O(a) remain on the surface after heating to 700 K.
- Pylant, E. D.,Hubbard, M. J.,White, J. M.
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- Studies on the Reaction of Acetaldehyde and Acetyl Radicals with Atomic Hydrogen
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The reaction of acetaldehyde and acetyl radical with atomic hydrogen was studied with a discharge flow tube equipped with an electron-impact mass spectrometer.Experiments were performed under the condition of excess atomic hydrogen at 299 +/- 5 K.A rate constant for the reaction CH3CHO + H -> CH3CO + H2 (1) of (11 +/- 2) * 10-14 cm3 molecule-1 s-1 was obtained.The branching fractions for the subsequent reactions (2a) CH3CO + H -> CH3 + HCO and (2b) CH3CO + H -> CH2CO + H2 are found to be 65 +/- 4percent and 35 +/- 2percent, respectively.
- Ohmori, Kenji,Miyoshi, Akira,Matsui, Hiroyuki,Washida, Nobuaki
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- The β C-C bond scission in alkoxy radicals: Thermal unimolecular decomposition of t-butoxy radicals
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The temperature and pressure dependence of the unimolecular decomposition of t-butoxy radicals was studied by the laser photolysis/laser induced fluorescence technique. Experiments have been performed at total pressures between 0.04 and 60 bar of helium and in the temperature range 323- 383 K. The low and the high pressure limiting rate constants as well as the broadening factor F(c) have been extracted from a complete falloff analysis of the experimental results: k0 = [He] x 1.5 x 10-8 exp(- 38.5 kJ mol- 1/RT) cm3 s-1, k∞ = 1.0 x 1014 exp(-60.5 kJ mol-1/RT) s-1, and F(c) = 0.87 - T/870 K. We anticipate an uncertainty for these rate constants of ± 30%. Important features of the potential energy surface have been computed by ab initio methods. The Arrhenius parameters for the high pressure limiting rate constant for the β C-C bond scission of t-butoxy radicals have been computed from the properties of a transition state based on the results of G2(MP2) ab initio calculation. The results from density functional theory (DFT) with a small basis set (B3LYP/SVP) are very similar. Excellent agreement between the calculated and the experimental rate constants has been found. We suggest a common pre-exponential factor for β C-C bond scission rate constants of all alkoxy radicals of A = 10(14±0.3) s-1. Thus we express the high pressure limiting rate constant for ethoxy and i-propoxy radicals by k∞ = 1.0 x 1014 exp(-78.2 kJ mol-1/RT) and 1.0 x 1014 exp(-63.1 kJ mol-1/RT) s-1, respectively. For the reverse reactions, the addition of CH3 radicals to CH2O, CH3CHO, and (CH3)2CO, we obtained activation enthalpies of 32, 42, and 52 kJ mol-1, respectively.
- Fittschen, Christa,Hippler, Horst,Viskolcz, Bela
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- Kinetic Studies of the Catalytic Oxidation of Methane. 2. Methyl Radical Recombination and Ethane Formation over 1percent Sr/La2O3
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The early stages of the catalytic oxidative conversion of methane to ethane over 1percent Sr/La2O3 have been studied at 1086 K, using a heatable tubular flow reactor (containing the catalyst) coupled to a photoionization mass spectrometer.Temporally resolved concentrations of CH3, C2H6, and C2H4 were recorded along the catalyst bed.Conditions used resulted in negligible loss of the reactants (CH4 and O2, C2H6.The small amount of C2H4 formed (5 +/- 3percent of the C2H6) can largely be accounted for by known gas-phase processes.Conversion of CH3 to C2H6 was stoichiometric, > 75percent.No indication was found of any heterogeneous "deep oxidation" of methyl radicals or of any heterogeneous conversion of methane to C2 compounds (C2H6 or C2H4).Rate constants for the CH3 + CH3 reaction under the conditions of these experiments (which were used in the kinetic modeling of the experimental results) were obtained inseparate experiments using the same reactor but with the catalyst removed, experiments in which the CH3 radicals were produced by laser photolysis of acetone and the decay of CH3 was monitored in time-resolved experiments.
- Feng, Y.,Niiranen, J.,Gutman, D.
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- Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra
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Methyl and methylene fragments are achieved using a complete set of quantum yields for different photodissociation channels. The results depicted that the hydrogen (H) atoms are more abundant in photofragments than deuterium (D) atoms. The ultraviolet absorption spectrum of methane is identical at different temperatures. The quantum yields of H atoms are determined by the measurement of the ratio of areas under the laser-induced fluorescence excitation curves.
- Wang, Jen-Han,Liu, Kopin,Min, Zhiyuan,Su, Hongmei,Bersohn, Richard,Preses, Jack,Larese, John Z.
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- Application of multi-photon ionization mass spectrometry to the study of the reactions O + C2H4, F + C3H6, F + c-C3H6, F + CH3OH, H + CH2OH and O + CH3O
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The experimental arrangement of a time-of-flight mass spectrometer with multi-photon ionization and electron impact ionization is described. This set up was combined with a discharge flow reactor and a molecular beam sampling device. The detection of the radicals CH3, C3H5, CH2OH and their deuterated analogs by the wave-length selective multi-photon ionization allowed the identification of primary products of elementary reactions in the gas-phase (around 1 mbar).
- Heinemann-Fiedler,Hoyermann
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- Modeling the decomposition of nitromethane, induced by shock heating
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The decomposition of nitromethane was studied over the temperature range 1000-1100 K in reflected shock waves. CH3NO2 and the reaction products were analyzed by gas chromatography. The derived gross rate constant and activation energy for the disappearance of CH3NO2 is consistent with that of Gl?nzer and Troe. A reaction mechanism consisting of 99 chemical reactions was developed to simulate the experimental data of the present study and that of Hsu and Lin. Good agreement between experiments and simulations was achieved. It appears that significant amounts of CH3NO2 are destroyed through secondary reactions that involved highly reactive free radicals (H, OH, and CH3), suggesting the need for redeterming the true unimolecular decay rate constant for CH3NO2. For improvement of the performance of the model, several other rate constants also need to be determined. The final section is a preliminary report on a spectrophotometric technique for measuring the loss of nitromethane due to pyrolysis by recording its absorption of UV radiation, directed axially along a small diameter shock tube. Although only semiquantitative data were obtained, this novel procedure merits discussion.
- Zhang, Yi-Xue,Bauer
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- Ultraviolet photodissociation study of CH3SCH3 and CH3SSCH3
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The photodissociation processes of CH3SSCH3 at 248 and 193 nm and CH3SCH3 at 193 nm have been studied by translational spectroscopy.When excited at 248 nm, CH3SSCH3 undergoes a simple S-S bond scission to produce two CH3S fragments with an average translational energy of 33 kcal/mol.The angular distribution of the product with respect to the polarized laser is measured and fitted with an anisotropy parameter β=1.2.It indicates that the dissociation is a fast, direct process.At 193 nm, there is only a simple dissociation channel for CH3SCH3, while CH3SSCH3 undergoes a predominant C-S bond scission with the S-S bond scission as a minor channel.No angular dependence for the primary products from both CH3SCH3 and CH3SSCH3 has been observed.The observation of S+2 time-of-flight spectra shows that a major fraction of CH3S2, which is internally excited when produced, undergoes spontaneous dissociation to form slow S2 and CH3 radicals.Results obtained for the dissociation processes of both molecules at 193 nm are rationalized and comparisons with previous investigations are also included.
- Lee, Y. R.,Chiu, C. L.,Lin, S. M.
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- Diffuse Reflectance Infrared Fourier-transform Study of the Plasma Hydrogenation of Diamond Surfaces
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Plasma hydrogenation of diamond surfaces was investigated using a diffuse reflectance infrared Fourier-transform (DRIFT) technique.Hydrogenation was carried out under microwave plasma conditions similar to those used for the chemical vapour deposition of diamond.Surface species chemisorbed on the diamond surface were characterized by DRIFT spectroscopy.The number of hydrogen-carbon bonds increased and the structure of the chemisorbed species on the diamond surfaces changed on increasing the temperature of the plasma hydrogenation.
- Ando, Toshihiro,Ishii, Motohiko,Kamo, Mutsukazu,Sato, Yoichiro
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- First experimental observation on different ionic states of the CH3SS radical: A HeI photoelectron spectrum
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The photoelectron (PE) spectrum of methyl disulfide radical beam produced by pyrolysis was developed. Different ionic states of methyl disulfide radical were calculated to study the photoelectron spectroscopy (PES) bands and density functional theory (DFT). The analysis of the five sharp peaks of dimethyl disulfide PE spectrum showed that the G2 calculations correctly predicted the ionization energies of different ionic states and also improved the DFT calculations.
- Maofa, Ge,Jing, Wang,Zheng, Sun,Xinjiang, Zhu,Dianxun, Wang
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- Photofragment imaging of methane
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The channels leading to the formation of atomic and/or molecular hydrogen in the photolysis of methane are studied. The H-atom elimination is investigated following photolysis at Lyman-α. The H2 elimination is investigated following two-photon
- Heck,Zare,Chandler
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- UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy
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The photodissociation of the allyl radical was studied by photofragment translational spectroscopy following excitation to the C(pver-tilde) (22B1) and A(over-tilde)(12B1) states. At 248 nm excitation, two different primary channels were detected: H-atom loss and CH3 elimination. The overall shape of the P(ET) for the H-atom loss channel suggests a statistical dissociation from the ground potential energy surface of the C3H5 system.
- Stranges,Stemmler,Yang,Chesko,Suits,Lee
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- Solvent effects on the competitive β-scission and hydrogen atom abstraction reactions of the cumyloxyl radical. resolution of a long-standing problem
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Laser flash photolysis (LPF) techniques with detection in the infrared and in UV-visible regions of the spectrum have been used in combination with detailed product studies to assess solvent effects on the hydrogen abstraction and β-scission reactions of cumyloxyl radicals. The variation in the ratio of the products of these two competing processes in solvents with different polarities is due to the solvent's influence on β-scission. The rate constants for β-scission at 30°C, kβCumO, were (2.63 ± 0.24) × 105, and (19.6 ± 3.4 × 105s-1 in CCl4, C6H6, C6H5Cl, (CH3)3COH, CH3CN, and CH3COOH, respectively. The rate constant for hydrogen abstraction from cyclohexane were essentially identical in these six solvents, viz., kaCumO = (1.24 ± 0.12) × 106 M-1 s-1. There is a reasonably good linear correlation between log (kβCumO/s-1) and certain cybotactic solvent parameters indicating that solvent effect on kβCumO are due to a localized interaction between the transition state for β-scission and adjacent solvent.
- Avila, David V.,Brown,Ingold,Lusztyk
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- Reaction of Acetaldehyde and Acetyl Radical with Atomic and Molecular Oxygen
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The reaction of acetaldehyde with atomic oxygen, O(3P), was studied by the discharge-flow-photoinization mass spectrometry method at room temperature (295 +/- 4 K) for the conditions of excess atomic oxygen.The rate constant for reaction 1, CH3CHO + O -> CH3CO + OH, was (3.9 +/- 0.3) x 10-13 cm3 molecule-1 s-1.Branching fractions for the subsequent reactions 2a, CH3CO + O -> CH2CO + OH, and 2b, CH3CO + O -> CH3CO + O -> CH3 + CO2, were determined to be 22 +/- 5 percent and 76 +/- 24 percent, respectively.The experiments in the presence of molecular oxygen show that the ratio of rate constants for reaction 3, CH3CO + O2 + M -> CH3CO3 + M, to reaction 2 is (6.3 +/- 0.5) x 10-3 .From the rate constant for reaction 3 obtained by McDade et al., the rate constant for reaction 2 was calculated: k2 = (3.2 +/- 0.7) x 10-10 cm3 molecule-1 s-1.When the concentration of molecular oxygen increased, the yields of both CH3 and CH2CO decreased.From this decrease, the branching fractions for reactions 4a, CH3CO3 + O -> CH2CO + OH + 02, and 4b, CH3CO3 + O -> CH3 + CO2 + O2, were determined: A discussion of the other products from CH3CO3 + O is presented.
- Miyoshi, Akira,Matsui, Hiroyuki,Washida, Nobuaki
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- Direct studies on the decomposition of the tert-butoxy radical and its reaction with NO
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The first laser induced fluorescence (LIF) spectrum for the tert-butoxy radical is reported following radical generation by excimer laser photolysis of tert-butyl nitrite. The laser flash photolysis-LIF technique is used to measure the temperature dependence of the rate coefficient for the reaction with NO (T = 200-390 K): (CH3)3CO + NO → (CH3)3CONO (3) which can be represented in either Arrhenius or AT(-n) format: k3 = (7.8 ± 1.8) x 10- 12 exp(2850 ± 290 J mol-1/RT) cm3 molecule-1 s-1 k3 = (4.17 ± 0.12) x 10-11(T/200)(-1.27±0.07) cm3 molecule-1 s-1 and the tert- butoxy decomposition. (CH3)3CO→CH3COCH3 + CH3 (2). Whilst the former reaction shows no pressure dependence (p = 70-500 Torr of helium), the latter reaction is in the fall-off regime over the entire range of experimental conditions (T = 303-393 K, p = 13-600 Tort helium). The fall-off data were fitted using an inverse Laplace transformation/master equation model to give the following Arrhenius parameters: k2/(∞)(T) = (1.4±0.6) x 1013 exp[- (57±2) kJ mol-1/RT]. These Arrhenius parameters are significantly lower than previous indirect measurements or calculations and the atmospheric and mechanistic implications are discussed. Finally, reaction (3) was also studied by monitoring the temporal dependence of the NO LIF signal following the photolysis of tert-butylnitrite with no additional NO. The results are in good agreement with the tert-butoxy monitoring and allow for an estimation of the rate parameters for the tert-butoxy self reaction.
- Blitz,Pilling,Robertson,Seakins
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- Cathodic cleavage of heteroarylalkylsulfones: A facile route to long chain aliphatic sulfinates and relevant sulfones
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Heteroarylalkylsulfones - mainly pyridylalkylsulfones - exhibit a cathodic cleavage reaction producing alkanesulfinate anion in high yield. This reaction is tested with long chain alkyl groups and allows an easy synthesis of aliphatic sulfinic acids.
- Delaunay,Mabon,Chaquiq El Badre,Orliac,Simonet
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- Kinetics of the Capture of Methyl Radicals by Carbon Monoxide in Aqueous Solution
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The title reaction has a rate constant of (2.0 +/- 0.3)E6 dm3 mol-1 s-1 at 25 deg C, which is fast enough to make it a useful route for C-C bond formation even at ambient temperatures and pressures.
- Bakac, Andreja,Espenson, James H.
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- Adsorption and Decomposition of Methanol on NiAl(110)
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Methanol adsorption and reaction on NiAl(110) have been investigated with temperature-programmed desorption, X-ray and ultraviolet photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy.Methanol chemisorbs associatively (at least relatively high coverage) on NiAl(110) at 120 K and transforms into surface methoxy by 200 K.Decomposition of the methoxy overlayer leads to the evolution of gaseous H2, CO, CH4, and CH3 radicals, in addition to the deposition of surface oxygen and carbonaceous species.The formation of C2H4 is also observed after a high coverage of methoxy decomposes on NiAl(119).Methyl radical desorption occurs near 570 K and CH4 desorbs near 350 K from NiAl(110).The low-temperature CH4 desorption peak indicates that the C-O bond of a fraction of surface methoxy is cleaved below 350 K on NiAl(110), in contrast to monometallic Al and Ni, emphasizing the unique reactivity of this alloy surface.
- Sheu, Bor-Ru,Chaturvedi, S.,Strongin, D. R.
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- CH3(.) vs. CF3(.): Relative Rates of Formation from β-Scission
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The relative rates of formation of CH3(.) and CF3(.) from β-scission reactions of trifluoro-t-butoxy radical (1) have been measured to be about 10:1 by thermolysing both trifluoro-t-butyl hypochlorite (4) and bis(trifluoro-t-butyl) peroxydicarbonate (14) in CCl4 at 160 and 140 deg C.
- Jiang, Xi-Kui,Li, Xing-Ya,Wang, Ke-Yang
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- Kinetics of the Reaction of CH3S with O3 at 298 K
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Laser-induced fluorescence was used to detect CH3S radicals in the laser flash photolysis of CH3SH-O3-H2O-He(SF6) mixtures.Evidence was found for a reaction between CH3S and O3, with a rate coefficient (4.1+/-2.0)x10E-12 cm3 molecule-1s-1.The yield of CH3S from the reaction OH + CH3SH was determined to be 1.1+/-0.2.Our results suggest that the CH3S + O3 reaction could be the major sink for CH3S in the atmosphere.
- Tyndall, G. S.,Ravishankara, A. R.
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- Thermal decomposition of tetramethylsilane and tetramethylgermane by flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry
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Thermal decomposition of tetramethylsilane (TMS) and tetramethylgermane (TMG) was studied on a short time scale of 20100 ms using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Primary decomposition of TMS and TMG occurred via loss of a methyl radical to form ?Si(CH3)3 and ?Ge(CH3)3, respectively. Both the ?Si(CH3)3 and ?Ge(CH3)3 radicals underwent secondary loss of a second methyl radical to form :Si(CH3)2 and :Ge(CH3)2, respectively. A previously unobserved secondary decomposition process in TMS involving loss of H atom from ?Si(CH3)3 followed by elimination of H2 to form SiC3H8, SiC3H6, and SiC3H4 was also identified. Sequential loss of the third and fourth methyl radical with significant formation of Ge and Ge2 was observed in the TMG pyrolysis. Loss of a third methyl radical in the TMS pyrolysis was not significant, while Si and SiC products were possibly produced. Secondary reactions of methyl to form unsaturated CxHy species, particularly in the TMG decomposition, were also observed.
- Lemieux, Jessy M.,Zhang, Jingsong
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- Photo-tautomerization of acetaldehyde to vinyl alcohol: A potential route to tropospheric acids
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Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers). The model quantitatively reproduces the experiments and shows unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the crucial first step. When collisions at atmospheric pressure are included, the model quantitatively reproduces previously reported quantum yields for photodissociation at all pressures and wavelengths. The model also predicts that 21 ± 4% of the initially excited acetaldehyde forms stable vinyl alcohol, a known precursor to organic acid formation, which may help to account for the production of organic acids in the troposphere.
- Andrews, Duncan U.,Heazlewood, Brianna R.,Maccarone, Alan T.,Conroy, Trent,Payne, Richard J.,Jordan, Meredith J. T.,Kable, Scott H.
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p. 1203 - 1206
(2012/10/29)
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- Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2Π) radical with C2H2, C2H4, C3H4 (methylacetylene and allene), C3H6 (
-
The reactions of the CH radical with several unsaturated hydrocarbons C2H2 (acetylene), C2H4 (ethylene), C3H4 (methyl-acetylene and allene), C3H 6 (propene) and C
- Loison, Jean-Christophe,Bergeat, Astrid
-
supporting information; body text
p. 655 - 664
(2009/05/07)
-
- Phenol-based lipophilic fluorescent antioxidant indicators: A rational approach
-
(Chemical Equation Presented) The reactivity, electrochemistry, and photophysics of the novel antioxidant indicator B-TOH, a BODIPY-α- tocopherol adduct, were investigated. We also studied a newly prepared BODIPY-3,5-di-tert-butyl-4-hydroxybenzoic acid ad
- Krumova, Katerina,Oleynik, Paul,Karam, Pierre,Cosa, Gonzalo
-
scheme or table
p. 3641 - 3651
(2009/10/02)
-
- Kinetics of reactions of CN with chlorinated methanes
-
The kinetics of reactions of CN with the chlorinated methanes CH3Cl, CH2Cl2, CHCl3 and CCl4 were investigated over the temperature range 298-573 K, using laser induced fluorescence (LIF) spectroscopy. At 298 K, rate constants of 9.0 ± 0.3 × 10-13, 8.8 ± 0.4 × 10-13, 9.0 ± 0.5 × 10-13 and 4.3 ± 0.6 × 10-13 cm3 molecule-1 s-1 were measured, respectively. A small positive temperature dependence was observed, as well as kinetic isotope effects of kH/kD ~ 2.14-2.25. These data along with product detection experiments strongly suggest that hydrogen abstraction dominates these reactions.
- Samant, Vaishali,Hershberger, John F.
-
experimental part
p. 64 - 67
(2009/02/04)
-
- Dynamics of the gas-phase reactions of chloride ion with fluoromethane: High excess translational activation energy for an endothermic SN2 reaction
-
Guided ion beam tandem mass spectrometry techniques are used to examine the competing product channels in the reaction of Cl- with CH3F in the center-of-mass collision energy range 0.05-27 eV. Four anionic reaction products are detected: F-, CH2Cl-, FCl- and CHCl-. The endothermic SN2 reaction Cl- + CH3F → CH3Cl + F- has an energy threshold of E0 = 181 ± 14 kJ/mol, exhibiting a 52 ± 16 kJ/mol effective barrier in excess of the reaction endothermicity. The potential energy of the SN2 transition state is well below the energy of the products. Dynamical impedances to the activation of the SN2 reaction are discussed, including angular momentum constraints, orientational effects, and the inefficiency of translational energy in promoting the reaction. The fluorine abstraction reaction to form CH3 + FCl- exhibits a 146 ± 33 kJ/mol effective barrier above the reaction endothermicity. Direct proton transfer to form HCl is highly inefficient, but HF elimination is observed above 268 ± 95 kJ/mol. Potential energy surfaces for the reactions are calculated using the CCSD(T)/aug-cc-pVDZ and HF/6-31+G(d) methods and used to interpret the dynamics.
- Angel, Laurence A.,Garcia, Sandra P.,Ervin, Kent M.
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p. 336 - 345
(2007/10/03)
-
- Spectral properties and absolute rate constants for β-scission of ring-substituted cumyloxyl radicals. A laser flash photolysis study
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A laser flash photolysis study of the spectral properties and β-scission reactions of a series of ring-substituted cumyloxyl radicals has been carried out. All cumyloxyl radicals display a broad absorption band in the visible region of the spectrum, which decays on the microsecond time scale, leading to a strong increase in absorption in the UV region of the spectrum, which is attributed to the corresponding acetophenone formed after β-scission of the cumyloxyl radicals. The position of the visible absorption band is red-shifted by the presence of electron-donating ring substituents, while a blue-shift is observed in the presence of electron-withdrawing ring substituents, suggesting that + R ring substituents promote charge separation in the excited cumyloxyl radical through stabilization of the partial positive charge on the aromatic ring of an incipient radical zwitterion. Along this line, an excellent Hammett-type correlation between the experimentally measured energies at the visible absorption maxima of the cumyloxyl radicals and σ+ substituent constants is obtained. A red-shift is also observed on going from MeCN to MeCN/H2O for all cumyloxyl radicals, pointing toward a specific effect of water. The ring substitution does not influence to a significant extent the rate constants for β-scission of the cumyloxyl radicals, which varies between 7.1 × 105 and 1.1 × 106 s-1, a result that suggests that cumyloxyl radical β-scission is not governed by the stability of the resulting acetophenone. Finally, κβ increases on going from MeCN to the more polar MeCN/H2O 1:1 for all cumyloxyl radicals, an observation that reflects the increased stabilization of the transition state for β-scission through increased solvation of the incipient acetophenone product.
- Baciocchi, Enrico,Bietti, Massimo,Salamone, Michela,Steenken, Steen
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p. 2266 - 2270
(2007/10/03)
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- Absolute rate constants for some reactions of the triethylamineboryl radical and the borane radical anion
-
Laser flash photolysis (LFP) of dj-tert-butyl peroxide or dicumyl peroxide at ambient temperatures in the presence of Et3N→BH3 or BH4- generated the title radicals which were found to have broad, featureless absorptions in the visible region. Rate constants for H-atom abstraction from Et3N→BH3 by cumyloxyl radicals show a small solvent dependence, e.g. 12 × 107 and 2.2 × 107 dm3 mol-1 s-1 in isooctane and acetonitrile, respectively. Rate constants for halogen atom abstraction by Et3N→BH2. and BH3.- from a number of chlorides and bromides were determined by LFP and by competitive kinetics, e.g., for Et3N→BH2. + CCl4/PhCH2Cl/CH3(CH2)2Cl, k = 4.4 × 109/1.1 × 107/5.1 × 105 dm3 mol-1 s-1 and for BH3.- + CCl4/PhCH2Cl, k = 2.0 × 109/3.0 × 107 dm3 mol-1 s-1. Rates of addition of Et3N→BH2. to 1-and 1,1-substituted olefins increase dramatically as the electron affinity of the olefin increases, confirming the nucleophilic character of amine-boryl radicals. A comparison of the present results with literature data for the addition of olefins of four nucleophilic carbon-centered radicals proves that Et3N→BH2. is by far the most nucleophilic radical for which kinetic data are available. A few rate constants for abstraction of hydrogen from electron-deficient carbon by Et3N→BH2. are also reported.
- Sheeller, Brad,Ingold, Keith U.
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p. 480 - 486
(2007/10/03)
-
- Determination of Rh-C bond dissociation energy in methyl(porphyrinato)rhodium(III) complexes: A new application of photoacoustic calorimetry
-
The photolysis of RhCH3(ttp) and RhCH3(tmp) (ttp = 5,10,15,20-tetratolylporphyrin, tmp = 5,10,15,20-tetramesitylporphyrin) was studied in methanol at ambient temperature: the quantum yields for photolysis were determined to be 0.51 and 0.54, respectively, and the Rh-C bond dissociation energies (227 and 219 kJ mol-1, respectively) were measured by photoacoustic calorimetry, which were larger than those of Co-C bond.
- Li, Gang,Zhang, Fei Fei,Pi, Na,Chen, Hui Lan,Zhang, Shu Yi,Chan, Kin Shing
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p. 284 - 285
(2007/10/03)
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- Photofragment translational spectroscopy of 1,2-butadiene at 193 nm
-
The dissociation dynamics of 1,2-butadiene at 193 nm were analyzed by photofragment translational spectroscopy. Tunable vacuum ultraviolet (VUV) synchrotron radiation at the Advanced Light Source (ALS) was used for ionization of scattered photoproducts. I
- Robinson, Jason C.,Sun, Weizhong,Harris, Sean A.,Qi, Fei,Neumark, Daniel M.
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p. 8359 - 8365
(2007/10/03)
-
- Detection of1CH2 radicals in hydrocarbon pyrolysis behind shock waves using FM spectroscopy
-
Singlet methylene radical (1CH2) concentrations were measured for the first time in the pyrolysis of methane (CH4) and ethane (C2H6) behind shock waves. The very sensitive frequency modulation (FM) spectroscopy, already established for sensitve detection of amino radicals (NH2)[1,2], was used for that purpose. Applying computer simulations using a complex reaction mechanism the experimental 1CH2 signals were fitted and rate coefficients of different reaction channels were obtained. For the reaction channel (1a) CH4 + M → CH3 + H + M an extented Arrhenius expression of k1a = 6.5·1018-(T/298)-1.70·exp[-366kJ mol-1/RT] cm3mol-1s-1(±50%), and futhermore rate coefficients for (7) 3CH2+CH3 → H+C2H4 with k7 = 3.2 · 1013cm3mol-1 s-1 (±40%), (11) 3CH2 + H → CH + H2 with ku = 7.9 · 1013 cm3 mol-1 s-1 (±40%), and for the intersystem crossing via (6)1CH2+M → 3CH2+M with k6 = 1.40 · 1010 · (T/K)0.9cm3mol-1s-1 (±40%) were determined. The experimental conditions ranged from 1900 to 4000 K with corresponding pressures between 0.23 to 0.54 bar. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Deppe, Joachim,Wagner, Heinz Gg.
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p. 1501 - 1525
(2007/10/03)
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- Investigation of the thermal decomposition of ketene and of the reaction CH2 + H2 ? CH3 + H
-
Using frequency modulation (FM) spectroscopy singlet methylene radicals have been detected for the first time behind shock waves. The thermal decomposition of ketene served as source for metylene radicals at temperatures from 1905 to 2780 K and pressures around 450 mbar. For the unimolecular decomposition reaction, (1) CH2CO+M → CH2 +CO+M, the rate constants obtained are: k1 = (9.5±5.7) · 1015 · exp[(-244±25) kJ mol-1/RT] cm3mol-1 s-1. As a first study of a methylene reaction at high temperatures by diretly tracing methylene the reaction of methylene with hydrogen, (8+9) 1.3CH2 + H2 → CH3 + H, was investigated at temperatures from 1930 to 2455 K and pressures around 500 mbar. For the total rate constant of the singlet and triplet methylene reaction a temperature independent value was obtained: log(kg+9/(cm3mol-1s-1)) = 13.89±0.26. A comparison with low temperature literature data and the systematics of activation energies of triplet methylene reactions allowed a consistent description of singlet and triplet contributions and of the forward and reverse reaction. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Friedrichs, Gernot,Wagner, Heinz Gg.
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p. 1601 - 1623
(2007/10/03)
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- The direct production of CO(v=1-9) in the reaction of O(3P) with the ethyl radical
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A new product channel that yields vibrationally excited CO(υ=1-9) in the reaction of the ethyl radical with O(3P) is experimentally observed by time-resolved Fourier transform infrared emission spectroscopy. The branching ratios for the different vibrational states are estimated to be 0.21+/-0.06, 0.27+/-0.03, 0.14+/-0.02, 0.08+/-0.02, 0.07+/-0.02, 0.07+/-0.02, 0.06+/-0.02, 0.05+/-0.02, and 0.05+/-0.02 for υ=1-9, respectively. Previously, only the CH3+H2CO, CH3CHO+ H, and C2H4+OH channels were known. Kinetics tests are provided to verify that the CO is produced directly in the reaction and not from secondary chemistry. The two possible new product channels are CO+CH4+H and CO+CH3+H2. The implications of this previously unexplored reaction channel for combustion chemistry and the possible mechanisms for this reaction are discussed.
- Reid, Jonathan P.,Marcy, Timothy P.,Kuehn, Seppe,Leone, Stephen R.
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p. 4572 - 4580
(2007/10/03)
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- Kinetics of the Cl(2PJ) + CH4 Reaction: Effects of Secondary Chemistry below 300 K
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Absolute rate data for the Cl(2PJ) + CH4 → HCl + CH3 reaction have been obtained from 218 to 298 K by using the discharge flow resonance fluorescence technique in helium at 1 Torr total pressure. The result at 298 K is (10.1 ± 0.6) × 10-14 cm3 molecule-1 s-1. The temperature dependence in Arrhenius form is (6.5 ± 0.9) × 10-12 exp[(-1235 ± 34)/T]. The errors given are one standard deviation; overall experimental error is estimated at ± 15%. Because of the relatively large disagreement among earlier measurements at low temperatures, the results were examined for possible effects of non-Boltzmann spin distribution and vibrational excitation of CH4, secondary chemistry of CH3 radicals, and impurities in the Cl atom and CH4 sources. There was no significant change in the observed rate constant when an efficient spin quencher, CF4, was added, and estimates indicate that vibrational partitioning in CH4 should be at the ambient reactor temperature before the start of the reaction. The results were also independent of the source of Cl atoms (microwave discharge or thermal decomposition of Cl2) and whether CH4 was purified in situ. However, the observed rate constant did depend on initial Cl atom concentrations and to a lesser extent on CH4 concentrations. Numerical simulations were used to assess the importance of secondary chemistry over a range of reactant concentrations.
- Wang,Keyser, Leon F.
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p. 7460 - 7469
(2007/10/03)
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- Experimental and theoretical studies of the reaction of the phenyl radical with methane
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The kinetics of the metathetical reaction of phenyl radical with methane has been studied theoretically and experimentally. The rate constants determined by two complementary methods, pyrolysis/Fourier transform infrared spectrometry and pulsed laser photolysis/mass spectrometry in the temperature range 600-980 K, give the Arrhenius equation: K1 = 1012.78 ± 0.13 exp[(-6201 ± 225)/T] cm3/(mol s). At the best theoretical level employed (G2M(CC,MP2)), the barrier for the reaction at 0 K is E10 = 9.3 kcal/mol. The rate constant k1 calculated from theoretical molecular parameters fits experimental data if the barrier height is increased to 10.5 kcal/mol. The fitted barrier is well within the 2-3 kcal/mol accuracy of the G2M method for the present open-shell, seven-heavy-atom system. Because of the relatively high reaction barrier and the predicted high imaginary frequency (1551 cm-1), tunneling corrections resulted in a significant enhancement in the calculated rate constant, 150% at 500 K and 7% at 2000 K. The theoretical result also correlates well with recently reported shock-tube data measured in the temperature range 1050-1450 K by UV absorption spectrometry. Kinetic analysis of the toluene formation data obtained from the photolysis of acetophenone without and with added H2 and CR4 gave the rate constant for the recombination of CH3 and C6H5, k2 = (1.38 ± 0.08) x 1013 exp[-(23 ± 36)/T] cm3/(mol s) for the temperature range 300-980 K.
- Tokmakov,Park,Gheyas,Lin
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p. 3636 - 3645
(2007/10/03)
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- Thermal stability or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and related N-alkoxyamines
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The carbon-oxygen bond dissociation enthalpies, BDE(C-O), in several N- alkoxyamine derivatives based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) have been determined in the liquid phase by photoacoustic calorimetry. The BDE(C-O) and the BDE(C-H) in the corresponding hydrocarbons follow a linear correlation: BDE(C-O) = 1.04BDE(C-H) - 62.1 kcal mol-1. When an electronegative element adjacent to the C-O bond is present, as in tetrahydrofuryl, a substantial deviation from the correlation is noticed. Due to the anomeric interaction the (THF) N-alkoxyamine is stabilized by an additional 14 kcal mol-1. The Arrhenius expression for the homolytic decomposition of methyl-TEMPO in the gas phase obeys k/s-1 = 1015.3 exp(-45.3/RT) to yield a BDE(C-O) of 47 ± 1 kcal mol-1 at 298 K. Furthermore, a high reactivity of TEMPO toward hydrogen donors, 1,4- cyclohexadiene or 9,10-dihydroanthracene, has been observed. Above 380 K, TEMPO is converted into the hydroxyamine 2,2,6,6-tetramethyl-1-piperidinol (TEMPOH) and the amine 2,2,6,6-tetramethylpiperidine (TEMPH). An acid- catalyzed mechanism for TEMPO deoxygenation is proposed.
- Ciriano, Maria Victoria,Korth, Hans-Gert,Van Scheppingen, Wibo B.,Mulder, Peter
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p. 6375 - 6381
(2007/10/03)
-
- Chemical structure of methane and ethane flames
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Premixed laminar CH4/O2/Ar (Φ) = 0.69; 1; 1.18) and C2H6/O2/Ar (Φ = 1) flat flames have been studied at low-pressure. The experimental results were compared with predictions from a detailed kinetic mechanism including 65 species involved in 454 reactions. Particular attention was given for the key intermediate species CH3, CH2O, C2H5, C2H4, C2H2, C3H8 and C3H6. In methane flames, the fraction of methyl radicals leading to the C2 oxidation route is found to be very low. The C1/C2 coupling is mainly via CH3 + CH3 → C2H5 + H reaction and less significantly via CH3 + CH3 → C2H4 + H2 and CH3 + CH3 → C2H6. In an ethane flame, the linkage between the C2 and C1 oxidation routes occurs muchmore easily, mainly via C2H5 + H → 2 CH3. However, the reaction of vinyl radicals with molecular oxygen in C2H3 + O2 → CH2O + HCO contributes significantly to the formation of CH2O and HCO. Finally, the C3 chemistry begins with the formation of both C3H6 and C3H8 via C2H3 + CH3 → C3H6 and C2H5 + CH3 +M → C3H8 +M.
- Crunelle,Turbiez,Pauwels
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p. 1146 - 1171
(2007/10/03)
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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
-
p. 6067 - 6074
(2007/10/03)
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- Solvent effect on the rate of β-scission of the tert-butoxyl radical
-
The transient absorption spectrum of the tert-butoxyl radical in the UV region was obtained by the laser flash photolysis technique. The rate constants for β-scission and self-termination reactions of tert-butoxyl radicals were measured in five solvents; the Arrhenius parameters of the rate constant for β-scission kβ were determined. It was shown that both the solvent polarity and ability for hydrogen bonding accelerate the reaction of β-scission. The solvent effect on the rate constant of the β-scission reaction is discussed in terms of a simple Onzager-Betcher model, a point dipole model, and a model of the H-bonded complex of the radical with the solvent molecule.
- Tsentalovich, Yuri P.,Kulik, Leonid V.,Gritsan, Nina P.,Yurkovskaya, Alexandra V.
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p. 7975 - 7980
(2007/10/03)
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- p-benzyne
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A noble gas matrix at low temperature was used to investigate the photochemical behavior of diacetyl terephthaloyl diperoxide and dipropionyl terephthaloyl diperoxide as well as 1,4-diiodobenzene. All three photoreactions formed small quantities of a compound with IR absorption bands at 725 and 980 cm-1, which disappeared when the matrix was annealed. These bands correspond to the most intense of the calculated bands (B3LYP) for 1,4-didehydrobenzene (p-benzyne) (1). That decomposition of the peroxides in fact leads to 1 is confirmed by vapor-phase pyrolysis experiments in which (Z)-2 was obtained in high yield.
- Marquardt, Ralph,Balster, Andreas,Sander, Wolfram,Kraka, Elfi,Cremer, Dieter,Radziszewski, J. George
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p. 955 - 958
(2007/10/03)
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- Collisional deactivation of vibrationally highly excited benzyl radicals
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The collisional deactivalion of vibrationally highly excited benzyl radicals in the ground electronic state has been investigated. Vibrationally excited benzyl radicals have been generated using the fast dissociation of vibrationally excited ethylbenzene prepared from UV absorption followed by fast internal conversion. Subsequent to complete collisional deactivation the benzyl radicals have been reexcited by the absorption of a further UV-photon also followed by fast internal conversion. Once again the collisional deactivation of the benzyl radicals has been monitored by time-resolved UV absorption spectroscopy. Average energies transferred per collision have been determined by comparison of absorption-time profiles with different initial vibrational energies. It was found that the rate of collisional deactivation of the open shell benzyl radical is almost identical to the rate of collisional deactivation of the closed shell hydrocarbons of similar size. WILEY-VCH Verlag GmbH, 1997.
- Damm,Deckert,Hippler
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p. 1901 - 1908
(2007/10/03)
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- Scavenging of hydrocarbon radicals from flames with dimethyl-bisulfide II. Hydrocarbon radicals in fuel-rich low-pressure flames of acetylene, ethylene, 1,3-butadiene and methane with oxygen
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The new technique of free-jet condensation/scavenging with dimethyl-disulfide has been applied for the quantitative analysis of hydrocarbon radicals and carbenes in flat premixed hydrocarbon/oxygen flames burning with fuel-rich mixtures at 27 mbar. The results are reported as profiles of mole fractions of the radicals. The limit of detectability was about l-10~7mole fraction. Qualitatively the radicals are very similar with the four fuels, but there are large differences in their quantities. While in the methane flame only C1 and C2 radicals were present in detectable concentration, higher radicals up to naphthyl could be detected with the unsaturated fuels. Although methyl could not be determined quantitatively, it was found to be the major hydrocarbon radical in all flames. Methoxy radical was only found in the methane flame. Whereas, for example, C2, C3H2 and C3H are typical high-temperature species, C2H is surprisingly formed at the beginning of the oxidation zone at relatively low temperature. Vinyl and the vinyl-type radicals C2nH3 (n = I to 4) are in equilibrium with acetylene and the polyynes (C2nH2) at maximum flame temperature and in the burned gas. Since phenyl peaks always after benzene, it is concluded that it is mainly a degradation product from benzene and other lower aromatics. The mechanism of formation of C2H and C2, the equilibria of the C2nH3 and the relation between aliphatic and aromatic radicals are discussed. VCH Verlagsgesellschaft mbH, 1997.
- Hausmann,Homann
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p. 651 - 667
(2007/10/03)
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- Absolute reaction rate of chlorine atoms with iodomethane
-
The reaction of atomic chlorine with iodomethane was studied in the gas phase with the very low pressure reactor technique over the temperature range 273-363 K. The absolute rate constant was given by the expression k = (1.33 ± 0.49) × 10-11 exp[-(5.73 ± 1.00 kJ mol-1)/RT] cm3 molecule-1 s-1. The reaction proceeds through an intermediate weakly bound adduct CH3I-Cl and results in the formation of HCl and iodomethyl radical CH2I. The kinetic isotope effect kH/kD of the reaction was independent of temperature and found to be 1.09 ± 0.4.
- Kambanis, Kyriakos G.,Lazarou, Yannis G.,Papagiannakopoulos, Panos
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p. 498 - 504
(2007/10/03)
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- Thermal dissociation of acetophenone molecular ions activated by infrared radiation
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The thermal dissociation of the molecular ions of acetophenone (C6H5COCH3?+ → C6H5CO+ + ?CH3) and acetophenone-d3 (C6H5COCD3?+ → C6H5CO+ + ?CD3) induced by broad band infrared radiation has been studied in the cell of an FT-ICR spectrometer. Rate constants in the range of 0.5-10 s-1 have been obtained for the system of ions exposed to a radiation source equivalent to blackbody temperatures between 1100 and 1600 K. The unimolecular dissociation is almost pressure independent in the 4 × 10-8 to 5 × 10-7 Torr range indicating that the most important mechanism is of a noncollisional nature. Activation energies obtained from Arrhenius-type plots yield 46.6 ± 2.0 kJ mol-1 for acetophenone and 44.9 ± 2.2 kJ mol-1 for acetophenone-d3. The dissociation process has been modeled by a Monte Carlo simulation and by numerical solution of the master equation of a process which takes into account interaction with the background radiation field through absorption and emission. These calculations reveal that meaningful activation energies can be obtained from these experiments even though the exact radiance viewed by the ions is not known. Solution of the master equation reveals that the experimental activation energies are consistent with a dissociation energy of 80.5 kJ mol-1 for the acetophenone molecular ion. This result is used to derive a heat of formation of 745 kJ mor-1 for the C6H5CO+ ion.
- Sena, Marcelo,Riveros, Jose M.
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p. 4384 - 4391
(2007/10/03)
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- Laser photofragmentation time-of-flight mass spectrometric study of acetophenone at 193 and 248 nm
-
The photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm is studied using the time-of-flight mass spectrometric technique. It is found that the dissociation is dominated by processes (1) and (2): C6H5COCH3+hν→C6H5CO+CH3 (1), C6H5+CH3CO (2) and, C6H5CH3+CO. At 193 nm, processes (1) and (2) occur with comparable cross sections. The cross section for process (3) at 193 nm is estimated to be 3CO and C6H5CO radicals initially formed at 193 nm by processes (1) and (2) are found to undergo further dissociation according to processes (6) and (8). At 248 nm, process (1) is overwhelmingly the dominant channel. The branching ratios for process (1) : process (2) : process (3) are estimated as 1.0:0.01:0.0008. The energy releases for these dissociation processes are also determined.
- Zhao,Cheung,Liao,Liao,Ng
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p. 7230 - 7241
(2007/10/03)
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- Electron transfer as a possible initial step in nucleophilic addition elimination reactions between (radical) anions and carbonyl compounds in the gas phase
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The reactions of the HO-, CH3S-, CH2S- and CH2=C(CH3)-CH2- ions with three ketones (CF3COR; R=CH3, CF3, C6H5) and three esters of trifluoroacetic acid (CF3CO2R; R=CH3, C2H5 and C6H5) have been studied with use of Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. All four negative ions react exclusively by proton transfer with CF3COCH3. With the other substrates, the HO- ion reacts by various pathways, such as proton transfer, SN2 substitution, E2 elimination and attack on the carbonyl group. The CH3S- ion is unreactive towards CF3COC6H5 but is able to react by hydride transfer, SN2, E2 and/or carbonyl attack with the remaining neutral species. The CH2S- radical anion reacts by electron transfer to afford stable molecular radical anions of CF3COCF3 and CF3COC6H5, whereas the main reaction with the two esters, CF3CO2CH3 and CF3CO2C2H5, is dissociative electron transfer leading to CF3CO2- and CF3- ions. The CH2=C(CH3)-CH2- anion displays a more complex reactivity pattern involving electron transfer, SN2, E2 as well as attack on the carbonyl group. Direct evidence for the occurrence of electron transfer as the initial step in an overall BAC2 type process has not been obtained for the systems studied. The reaction of the CH2S- ion with CF3CO2C6H5 was observed, however, to yield exclusively a CF3COCHS-. radical anion. Based upon the absence of a BAC2 process in the reaction of CH2S- with the methyl and ethyl esters of trifluoroacetic acid in combination with the facile occurrence of electron transfer from this radical anion, it is suggested that the CF3COCHS-. ion is formed by an initial electron transfer followed by coupling between the CH2S molecule and the CF3CO2C6H5- radical anion and subsequent loss of C6H5OH from the collision complex.
- Staneke, Paul O.,Ingemann, Steen,Nibbering, Nico M. M.
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p. 179 - 184
(2007/10/03)
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- 12. Gas-phase reactions of aliphatic alcohols with 'bare' FeO+
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Ion/molecule reactions of 'bare' FeO- with linear and branched aliphatic alcohols have been examined by Fourier -transform ion-cyclotron resonance mass spectrometry. Depending on the chain length of the alcohol, three different types of reactions can be distinguished: i) Oxidation of the alcohols in the α-positions, to yield the corresponding carbonl-Fe+ complexes, involves an initial O-H bond activation of the alcohol resulting in the formation of RO-Fe+-OH as the central intermediate. ii) The formation of Fe(OH)+2, concomitant by loss of the corresponding neutral alkenes, competes with the generation of neutral OFeOH and a carbocation R+ These couples point to the existence of an intracomplex acid-base equilibrium and are connected with each other by a proton transfer from either acid to the other, e.g. i-C3H+7 + OFeOH?C3H6 + Fe(OH)+2. The process is driven by the Lewis acidity of FeO+ and starts with the abstraction of a hydroxide anion from the alcohol. iii) For longer alcohols, e.g. pentanol, functionalization of non-activated C-H bonds which are remote from the O functionality is observed. Here, the OH group of the alcohol serves as an anchor, which directs the reactive metal-oxide cation toward a particular site of the hydrocarbon chain.
- Schroeder, Detlef,Wesendrup, Ralf,Schalley, Christoph A.,Zummack, Waltraud,Schwarz, Helmut
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p. 123 - 132
(2007/10/03)
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- Temperature Dependence and Product Distribution for the Reaction of CF3O Radicals with Methane
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The rate coefficient for the reaction (1) CF3O + CH4 --> CF3OH + CH3 has been determined using a combined laser photolysis/LIF technique.From measurements in the temperature range 235-401 K the Arrhenius expression k1 = (3.3 +/- 0.8)E-12 exp. cm3/s, corresponding to k1 = 2.5E-14 cm3/s at 298 K is obtained.The products of reaction (1) have been identified by independent experiments using a steady-state/FTIR technique.It is concluded that the main primary product of reaction (1) is CF3OH which decomposes under our experimental conditions to yield CF2O and HF.An upper limit of the decomposition rate coefficient for (7) CF3OH --> CF2O + HF at 298 K of k7 -1 is determined. - Keywords: CF3O radicals / Laser photolysis / LIF technique / H abstraction from CH4 / Decomposition of CF3OH
- Bednarek, G.,Kohlmann, J. P.,Saathoff, H.,Zellner, R.
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- Time-Resolved REMPI Detection of Methyl Radicals Generated in Laser-Induced Unimolecular Reactions
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The specific rate constants for the photofragmentation of four substituted benzene molecules (ethylbenzene, tert-butylbenzene, toluene, benotrifluoride) following pulsed laser excitation at λ=193 nm were measured in a molecular beam experiment by monitoring the change of the intensity of the nascent products as a function of time delay between excitation and detection laser pulses.In the case of toluene (C6H5CH3), ethylbenzene (C6H5CH2CH3), and tert-butylbenzene (C6H5C(CH3)3) a (3+1)REMPI process a Λ=450.8 nm was used to ionize the CH3 products, which were detected in a time-of-flight mass spectrometer.Following dissociation of benzotrifluoride (C6H5CF3) the trifluormethyl radical CF3 was detected at λ=455 nm.The measured specific rate constants can be modeled satisfactorily by using the SACM (statistic adiabatic channel model) statistical theory.
- Froechtenicht, Ralf,Hartmann, Matthias
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p. 16290 - 16295
(2007/10/02)
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- Electron-Induced Reactions in Methanol Ultrathin Films Studied by Temperature-Programmed Desorption: A Useful Method To Study Radiation Chemistry
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The exposure of multilayers of an adsorbate to low energy (a previously unknown methanol radiolysis product: methoxymethanol (CH3OCH2OH).Moreover, this technique allows study of the dependence on initial electron energy, providing additional insight into the physical processes underlying radiation chemistry.
- Harris, Tracy D.,Lee, Do H.,Blumberg, Meaghan Q.,Arumainayagam, Christopher R.
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p. 9530 - 9535
(2007/10/02)
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- Competing Bond Fission and Molecular Elimination Channels in the Photodissociation of CH3NH2 at 222 nm
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This paper presents the first experimental investigation under collisionless conditions of the competing photodisssociation channels of methylamine excited in the first ultraviolet absorption band.Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H2 elimination.The data, taken on photofragments from both methylamine and methylamine-d2, elucidate the mechanism for each competing reaction.Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl(umbrella) deformation or with two quanta in the amino torsion vibration.The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry.The photofragment kinetic energy distributions measured for CH3ND2 show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation.The reaction coordinates for N-H and C-N fission are analyzed in comparison to that for ammonia dissociation from the state and with reference to ab iniitio calculations of cuts along the excited state potential energy surface of methylamine which show these reactions traverse a small barrier in the excited state from a Rydberg/valence avoided crossing and then encounter a conical intersection in the exit channel.The measured kinetic energy distribution of the C-N bond fission photofragments indicates that the NH2 (ND2) product is formed in the 2A1 state; the C-N fission reactive trajectories thus remain on the upper adiabat as they traverse the conical intersection.The mechanism for C-H bond fission is less clear; most of the kinetic energy distribution indicates the reaction evolves on a potential energy surface with no barrier to the reverse reaction, consistent with dissociation along the excited state surface or upon internal conversion to the ground state, but some of the distribution reflects more substantial partitioning to relative translation, indicating that some molecules may dissociate via a repulsive triplet surface.In general, the photofragment angular distributions were anisotropic, but the measured β ca. -0.4 +/- 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation.We end with analyzing why in methylamine three other primary dissociation channels effectively compete with N-H fission while in CH3OH and CH3SH primarily O-H and S-H fission, respectively, dominate.
- Waschewsky, G. C. G.,Kitchen, D. C.,Browning, P. W.,Butler, L. J.
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p. 2635 - 2645
(2007/10/02)
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- Oxidative Coupling of Methane over Lanthana Catalysts II. A Mechanistic Study Using Isotope Transient Kinetics
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The elementary steps dealing with the methane and oxygen activation in the reaction of oxidative coupling of methane (OCM) over lanthana catalysts have been investigated by means of state-of-the-art transient kinetic techniques.Methane and oxygen activation over lanthana catalysts are shown to combine reversible and irreversible steps, parallel for methane and consecutive for oxygen.In both processes, the surface residence time of activated species per active site is found to be below the time resolution of 1 ms, characteristic of the temporal analysis of products reactor.Evidence of a strong interaction between gaseous and lattice oxygen and of an inhibiting effect of carbon dioxide on both methane and oxygen activation is also provided.Specific sites pertaining to each of these routes are proposed, accounting for the various kinetic effects which are observed under the OCM conditions.
- Lacombe, S.,Zanthoff, H.,Mirodatos, C.
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p. 106 - 116
(2007/10/03)
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- The photodissociation of toluene studied by forward photofragment translational spectroscopy
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The translational energies of different fragments produced in laser induced unimolecular reactions are investigated in a molecular beam experiment.The time-of-flight of the photoproducts is measured in the forward direction with a mass specrometer.By operating the ion source at low electron impact energies it is possible to nearly eliminate fragmentation and observe the nascent products at their parent mass.With this technique the primary and secondary reaction channels can be identified and branching ratios and product translational energy distribution down to small kinetic energies can be measured.The method is illustrated in a study of the photodissociation of toluene (methylbenzene) at three excitation energies (Eex=51 590, 52 240, and 63 240 cm-1).From the measured product time-of-flight spectra the branching ratio as well as the translational energy distributions of the product molecules benzyl+H and phenyl+CH3 in the two reaction channels were determined.The measured product energy distributions are compared with RRK (Rice-Ramsperger-Kassel) and SACM (statistic adiabatic channel model) statistical theories.The experimental results agree very well with the SACM theory for the phenyl, but not for the benzyl products.This is attributed to a barrier in the exit channel for the latter product.
- Froechtenicht, Ralf
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p. 4850 - 4859
(2007/10/02)
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- The Formation of Gas Phase Benzyl Radicals during the Reaction of Toluene and Nitrous Oxide over Li-MgO and Sr-La2O3 Coupling Catalysts
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Surface-generated gas-phase benzyl radicals have been detected during the reaction of toluene and nitrous oxide over Li-MgO and Sr-La2O3 catalysts.
- Xu, Mingting,Lunsford, Jack H.
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p. 1203 - 1204
(2007/10/02)
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- Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical
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The absolute rate constant and primary reaction products have been determined at T=298 K for the atom-radical reaction N(4S)+C2H5 in a discharge flow system with collision-free sampling to a mass spectrometer.The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source.The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C2H5 or C2D5 as a function of time in the presence of excess N atoms.The result isk=(1.1+/-0.3)*10-10 cm3 molecule-1 s-1.For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD3, D2CN and C2D4 for the N+C2D5 reaction.The product identification is based on the unambiguous combination of product m/z values, the shift of the m/z peaks observed for the N+C2D5 reaction products with respect to the N+C2H5 reaction products an the photoionization threshold measured for the major products.The observed products are consistent with the occurrence of the reaction channels D2CN+CD3(2a) and C2D4+ND(2c).Formation of C2D4 product via channel (2c) accounts for approximately 65percent of the C2D5 reacted.Most, if not all, of the remaining 35percent is probably accounted for by channel (2a).These rate constant and product results are compared with those for the N+CH3 reaction as well as other atom+C2H5 reactions.The role of the N+C2H5 reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered.In addition, the appearance energy for the formation of C2D3+ from C2D5 was determined from photoionization threshold measurements,AE0(C2D3+,C2D5)=239.5 kcal mol-1.From this, values are derived for the zero Kelvin heats of formation of C2D3+ (266 kcal mol-1) and C2D3 (71.6 kcal mol-1).
- Stief, L. J.,Nesbitt, F. L.,Payne, W. A.,Kuo, S. C.,Tao, W.,Klemm, R. B.
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p. 5309 - 5316
(2007/10/02)
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