- Cooperative Effect of Surface Sites on the Laser Flash Photolysis of 1,1-Diphenylacetone and 1,1,3,3-Tetraphenylacetone Adsorbed on Layered Clays. Generation of Radicals and Carbocations
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Laser flash photolysis of 1,1-diphenyl- and 1,1,3,3-tetraphenylacetone adsorbed onto a series of clays has been carried out under dry conditions.The photochemical behavior of the guests was investigated by using time-resolved diffuse reflectance techniques.Deposition of the organic compounds onto the solids was performed by slow removal of the solvent from dilute solutions at low temperature.Distinct transient spectra in the microsecond time domain were obtained for the guest-host composites.The formation of the diphenylmethyl radical (DPM.) was observed with montmorillonites SWy-1 (λmax = 310 nm) and STx-1 (λmax = 340 nm).The decay of DPM. on the bidimensional geometry of the layered clays was found to be much slower than within a tridimensional large-pore zeolite.By contrast, in the case of pillared Al13SWy-1 and Al13STx-1 clays, additional reflectance at 450 nm characteristic of the diphenylmethyl cation (DPM+) was present.This intermediate presumably arises from the cooperative interaction with the acid sites of the clay.These results show that these ketones are convenient probes to report on the chemical and topological properties of solid surfaces.
- Cozens, Frances L.,Garcia, Hermengildo,Gessner, F.,Scaiano, J. C.
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- Laser photofragmentation time-of-flight mass spectrometric study of acetophenone at 193 and 248 nm
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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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- Kinetics of Intramolecular Alkyl Radical Attack on Sulfur in Disulfides and Thioesters
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The 4-(alkyldithio)butyl radicals 8a and c, and the 4-(phenyldithio)butyl radical 8b, generated from the corresponding esters of N-hydroxypyridine-2(1H)-thione, undergo fast exo-cyclisation by SHi attack at sulfur.Similarly, the 5-(alkyldithio)pentyl radical 8d undergoes 1,6-ring formation.The rate constants for cyclisation were determined by photolysis of the radical precursors in the presence of appropriate thiols.Butyl and pentyl radicals bearing ω-acetylthio or ω-benzoylthio substituents also undergo ring closure but much more slowly.The kinetics of these intramolecular SH2 reactions are discussed and compared with those for the intermolecular attack of hexyl radicals on diphenyl disulfide and on dibutyl disulfide.
- Beckwith, Athelstan L. J.,Duggan, Sandhya A. M.
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p. 1509 - 1518
(2007/10/02)
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- Studies of NO3 Radical Reactions with Some Atmospheric Organic Compounds at Low Pressures
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Rate constantsfor the reactions of NO3 with trans-2-butene (1), isoprene (2), α-pinene (3), and acetaldehyde (4) have been measured as a function of temperature at low preessures in a fast flow system with LIF detection of the NO3 reractant and NO2 product.Rate costants for reaction 1 were found to be independent of pressure from 0.44 to 4.5 Torr, and the Arrhenius plot was curved.The data were fit by the four-parameter equation, k1(T = 204-378 K) = (12.78 +/-0.36) x 1E-12 exp (where all the error limits are the 95 percent confidence levels including a factor systematic error, and the units are cm3 molecule -1 s-1).The data for isoprene were fit by a normal Arrhenius equation, k2(T = 251-381 K) = (3.03 +/- 0.45) x 1E-12 exp.The Arrhenius plots for α-pinene and acetaldehyde were linear, and the fits gave k3(T = 261-384 K) = (1.19 +/- 0.31) x 1E-12 exp and k4(T = 264-374 K) = (1.44 +/- 0.18) x 1E-12 exp.The efficiencies for the conversion of NO3 to NO2 were determined for reactions of trans-2-butene and isoprene at low pressures and room temperature in He, N2, and O2 carrier gases, and at 360 K in 1 Torr of He.The measured yields of NO2 decreased with increasing size of the organic reactant, with increasing pressure, with decreasing temperature, and with increasing deactivation efficiency of the carrier gas.These observations and the temperature dependencies of the rate constants indicate that reactions 1-3 proceed by way of an addition mechanism.From the analysis used to determine the NO2 yields, NO2 fluorescence quenching rate constants were determined for isoprene, k = (3.5 +/- 1.1) x 1E-10, and trans-2-butene, k = (3.0 +/- 0.9) x 1E-10.
- Dlugokencky, Edward J.,Howard, Carleton J.
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p. 1091 - 1096
(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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- Kinetics of the Reactions between CH2(3B1)-Radicals and Saturated Hydrocarbons in the Temperature Range 296 K = T = 707 K
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The reaction between CH2-radicals in their ground electronic state (3B1) and n-hexane CH2() + n-C6H14 --> CH3 + C6H13 was studied in a discharge flow system with LMR detection of CH2.In the temperature regime 413 K 4 = 1E(13.22 +/- 0.20)*exp(-3380 +/- 240/T) cm3/mol s.The reaction proceeds both via direct H-atom abstraction by CH2() and via thermal excitation of CH2() to the low-lying singlet state (1A1) followed by fast consecutive reactions of CH2().The contributions due to thermal excitation and singlet reaction were evaluated for the present work as well as for a recent study of the reactions of CH2() with a series of other hydrocarbons.Corrected rate constants kT for the direct reactions of CH2() with the reactants HR = CH4 (1), C2H6 (2), C3H8 (3), n-C6H14 (4), i-C4H10 (5), and CH3CHO (6) in the temperature range 296 K 1T = 4.3E12*exp(-42 kJ mol-1/RT) cm3/mol s, k2T = 6.5E12*exp(-33.1 kJ mol-1/RT) cm3/mol s, k3T = 4.9E12*exp(-27.7 kJ mol-1/RT) cm3/mol s, k4T = 7.8E12*exp(-25.6kJ mol-1/RT) cm3/mol s, k5T = 2.5E12*exp(-22.5 kJ mol-1/RT) cm3/mol s, k6T = 1.7E12*exp(-14.7 kJ mol-1/RT) cm3/mol s.The activation energies for the reactions studied are described by an Evans Polanyi type relation.Arrhenius expressions are proposed for the rate constants of H-atom abstraction by CH2(3B1) from primary, secondary, tertiary, and aldehydic C-H bonds.The results are compared to the isoelectronic reactions of O(3P). - Keywords: Chemical Kinetics / Elementary Reactions / Radicals / Spectroscopy, Laser Magnetic Resonance
- Boehland, T.,Dobe, S.,Temps, F.,Wagner, H. Gg.
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p. 1110 - 1116
(2007/10/02)
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- A Direct Study of the Reactions of CH2 (3B1)-Radicals with Selected Hydrocarbons in the Temperature Range 296 K = T = 705 K
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The kinetics of the reactions of CH2 (3B1)-radicals with five selected organic compounds has been studied in an isothermal discharge flow system in the temperature range 296 .Two basic reaction mechanisms, either direct H-atom abstarction by 3CH2 or thermal excitation of 3CH2 to the low lying 1A1 state followed by consecutive reactions of 1CH2, are of importance.For acetaldehyde, isobutane, and propane direct H-atom abstraction by 3CH2 predominates.After separation of the small contribution attributed to the singlet reaction the following rate constants for the reactions of CH2 (3B1) with acetaldehyde, isobutane, and propane are obtained: .Presuming the reactions of 1CH2 with hydrocarbons are fast the thermal excitation mechanism dominates the reaction system in the cases of methane and ethane.The activation energy of EA(CH4) = 40 +/- 8 kJ/mol measured for methane is concluded to be determined by the singlet-triplet energy splitting in CH2. - Keywords: Chemical Kinetics / Elementary Reactions / Laser Magnetic Resonance / Methylene / Radicals
- Dobe, S.,Boehland, T.,Temps, F.,Wagner, H. Gg.
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p. 432 - 441
(2007/10/02)
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- Study of the Oxidation of Organic Compounds in an Ultrasonic Field. III. Mechanism of Chain Initiation and Breaking in the Oxidation of Acetaldehyde under the Influence of Ultrasound
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We have shown by the chemiluminescence method that during the oxidation of acetaldehyde under the influence of ultrasound radicals are formed additionally within the cavtation bubbles followed by recombination in the gas phase and partial transfer to the liquid phase.Additions of water lower the rate of formation of radicals, and α-naphtol, because of its low partial pressure, reacts with the free radicals only in the liquid phase.The chemiluminescence (ultrasonic after-effect) method was used to determine the recombination rate constant of the peracyl radicals under cavitation conditions and the recombination coefficient (an important characteristic of the effect of ultrasound on matter).
- Starchevskii, V. L.,Vasilina, T. V.,Grundel', L. M.,Margulis, M. A.,Mokryi, E. N.
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p. 1181 - 1184
(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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- Abschaetzung kinetischer und thermodynamischer Daten fuer Wasserstoffabspaltungsreaktionen von niederen Olefinen
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Mit Hilfe der BEBO-Methode sind fuer 500 ... 1200 K die Aktivierungsparameter der H-Uebertragungsreaktionen .R1 + HR2 --> R1H + R2.(R1: H, CH3, C2H5, n-C3H7, i-C3H7, CH3CO, CH2COCH3; R2H: Ethen, Propen, But-1-en) berchnet worden.Unter Verwendung entsprechender Daten der Rueckreaktion wurden daraus ΔRGθ, ΔRHθ und ΔRSθ bestimmt und mit den aus thermochemischen Inkrementen zugaenglichen Werten verglichen.Die Beweiskraft der guten Uebereinstimmung beider Datensaetze fuer die Anwendbarkeit des BEBO-Verfahrens auf H-Abstraktionen von Olefinen wird diskutiert.Ausserdem wird gezeigt, dass bei der Pyrolyse von Olefinen die H-Abstraktion aus der Vinylgruppe bis ueber 1500 K gegenueber der aus den Alkylgruppen zu vernachlaessigen ist.
- Scherzer, K.,Lohse, K.,Loeser, U.
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p. 196 - 204
(2007/10/02)
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