- Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
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Hydrogen abstraction from 1-substituted cyclohexa-2,5-diene-1-carboxylic acids containing linear, branched and cyclic alkyl substituents, as well as allyl, propargyl (prop-2-ynyl), cyanomethyl and benzyl substituents, has been studied by EPR spectroscopy. For each carboxylic acid, EPR spectra of the corresponding cyclohexadienyl radicals were observed at lower temperatures, followed by spectra due to ejected carbon-centred radicals at higher temperatures. Rate constants, for release of the carbon-centred radicals from the cyclohexadienyl radicals, were determined from radical concentration measurements for the above range of substituents. The rate of cyclohexadienyl radical dissociation increased with branching in the 1-alkyl substituent and with electron delocalisation in the ejected carbon-centred radical; 3,5-and 2,6-dimethyl-substitution of the cyclohexadienyl ring led to reductions in the dissociation rate constants. Rate data for abstraction of bisallylic hydrogens from the cyclohexadienyl acids were also obtained for ethyl, n-propyl and isopropyl radicals. These results indicated a sharp drop in the rate of hydrogen abstraction as the degree of branching in the attacking radical increased. Small decreases in the hydrogen abstraction rate constants were observed for cyclohexadienes containing CO2R substituents.
- Jackson, Leon V.,Walton, John C.
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p. 1758 - 1764
(2007/10/03)
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- The efficiency of alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids
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A novel EPR spectroscopic technique has been used to determine kinetic data for alkyl radical generation and hydrogen transfer from 1-alkylcyclohexa-2,5-diene-1-carboxylic acids; the implications of these data for preparative chain reactions of these reagents are inferred.
- Jackson, Leon,Walton, John C.
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p. 7019 - 7021
(2007/10/03)
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- Kinetics and mechanisms of the reactions of chlorine atoms with ethane, propane, and n-butane
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Absolute (flash photolysis) and relative (FTIR-smog and GC) rate techniques were used to study the gas-phase reactions of Cl atoms with ethane (k1), propane (k3), and n-butane (k2). Experiments performed at 298-540 K give k2÷k1=(2.0±0.1)exp((183±20)÷T). At 296 K the reaction of Cl atoms with propane yields of 43 % 1-propyl and 57 % 2-propyl radicals, while the reaction of Cl atoms with n-butane produces 29% 1-butyl and 71% 2-butyl radicals. Butyl radicals were found to react with Cl2 with rates which are 3 times greater than the corresponding reactions with O2.
- Tyndall,Orlando,Wallington,Dill,Kaiser
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- Photodissociation dynamics of n-butylbenzene molecular ion
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Photodissociation dynamics of n-butylbenzene molecular ion has been investigated on a nanosecond time scale. The rate constants for production of C7H8?+ and C7H7+, their branching ratios, and the kinetic energy release distributions have been determined by the photodissociation method using mass-analyzed ion kinetic energy spectrometry. The branching ratios have been found to be in excellent agreement with the previously established results. All the experimental data could be explained with statistical theories such as Rice-Ramsperger-Kassel-Marcus (RRKM) and phase space theories. RRKM fittings for these reactions have been improved. The present result supports the previous suggestion that the dissociation to C7H8?+ occurs via a stepwise McLafferty rearrangement.
- Oh, Seong Tae,Choe, Joong Chul,Kim, Myung Soo
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p. 13367 - 13374
(2007/10/03)
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- Kinetics of the Reactions of the CF3O Radical with Alkanes
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Utilizing the technique of pulsed laser photolysis/pulsed laser induced fluorescence, we have investigated the atmospherically important reactions of the (trifluoromethyl)peroxy radical, CF3O, with several alkanes.The reaction rate coefficients for CF3O + CH4 (k3), C2H6 (k4), C3H8 (k5), (CH3)3CH (k6), and CD4 (k7) were measured, as functions of temperature, to be k3 = (1.92 +/- 0.33) x E-12 exp cm3 molecule-1 s-1, k4 = (4.84 +/- 1.11) x E-12 exp cm3 molecule-1 s-1, k5 = (5.12 +/- 1.12) x E-12 exp cm3 molecule-1 s-1, k6 = (4.32 +/- 0.42) x E-12 exp cm3 molecule-1 s-1, and k7 = (0.91 +/- 0.31) E-12 exp cm3 molecule-1 s-1, respectively.These kinetic data are compared with results from previous studies.The atmospheric implications of these findings are discussed.
- Barone, Stephen B.,Turnipseed, Andrew A.,Ravishankara, A. R.
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p. 4602 - 4608
(2007/10/02)
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- Ion-Molecule Reactions and Thermal Decomposition of Ions in N2-O2-Alkane (C2-C8) Mixtures Studied by Time-Resolved Atmospheric Pressure Ionization Mass
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The experiments were carried out at temperatures ranging from 236 to 569 K.The O2+ ion reacted with n-alkanes (CnH2n+2) via fast nondissociative and dissociative charge-transfer channels, its proportion depending on temperature.The nondissociative product CnH(2n+2)+ subsequently reacted with O2 via a slow H atom transfer path, producing alkyl ions.With increasing temperature the alkane ions CnH(2n+2)+ (n >/= 4) began to decompose thermally, producing olefinic ions and alkanes.The product olefinic ions CmH2m+ (m=4,5,6) reacted with O2 via a slow H atom transfer path, producing alkenyl ions CmH(2m-1)+.The C8H17+ decomposed thermally forming fragment alkyl ions and olefins.The fragment alkyl ions reacted with n-C8H18 reproducing C8H17+, thus leading to a chain mechanism in n-C8H18 decomposition.The equilibrium reaction, C2H5+ + C2H6 ->/+, and the subsequent dissociative rearrangement reaction, C4H11+ -> C4H9+ + H2, were studied.The reactions of NO+ and NOO+, both minor products of the irradiation of N2-O2 mixtures, with alkanes were also studied.The rate constants of the ion-molecule reactions and the unimolecular thermal decomposition reactions and the equilibrium constant were measured.
- Matsuoka, Shingo,Ikezoe, Yasumasa
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p. 1126 - 1133
(2007/10/02)
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- Absolute Rate Expressions for Intramolecular Displacement Reactions of Primary Alkyl Radicals at Sulfur
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Arrhenius parameters are presented for the intramolecular displacement reactions of 4-(alkylthio)butyl radicals, where alkyl = benzyl, propyl, and tert-butyl, giving tetrahydrothiophene and the corresponding alkyl radical (R.).Arrhenius expressions were determined by competition of the rearrangement (displacement) with abstraction of hydrogen from tributylstannane.The Arrhenius parameters for the intermolecular displacement reactions were as follows ., log (A/s-1), Ea (kcal/mol), at the 95percent confidence interval>: benzyl, 9.92 +/- 0.22, 8.63 +/- 0.40; tert-butyl, 10.77 +/- 0.24, 11.36 +/- 0.40; n-propyl, 10.17 +/- 0.56, 12.15 +/- 1.16.The 4-(benzylthio)butyl and 4-(propylthio)butyl radicals undergo a competing intramolecular 1,6-hydrogen migration to form the corresponding α-(butylthio)benzyl and (butylthio)propyl radicals.The Arrhenius parameters -1), Ea (kcal/mol), 95percent confidence interval>, per hydrogen atom abstracted, for these latter two reactions were as follows: 4-(benzylthio)butyl,9.51 +/- 0.28, 8.87 +/- 0.44; 4-(propylthio)butyl, 9.10 +/- 1.9, 9.98 +/- 3.9.The Arrhenius parameters of the intramolecular displacement reaction are consistent with a product-like rate-determining step, as opposed to the formation of an intermediate sulfuranyl radical followed by nonselective ligands loss.INDO calculations predict the SOMO of the trialkylsulfuranyl radicals to be a ?* orbital with electron density distributed about sulfur and two pseupoapical carbons.The pseudoapical C-S-C angle defined by this SOMO suggests a predominance of exocyclic (axial-axial) displacement over endocyclic (equatorial-axial) displacement for the intramolecular displacement reactions forming a tetrahydrothiophene ring.MM2 calculations of internal rotational barriers of tert-butyl, hex-5-en-1-yl, and the 4-(alkylthio)butyl radicals and of the exocyclic ligands of models of the displacement transition structures are presented to aid in the interpretation of displacement A factors.
- Franz, James A.,Roberts, David H.,Ferris, Kim F.
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p. 2256 - 2262
(2007/10/02)
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- ON THE REMOVAL OF METALLIC MIRRORS BY FREE RADICALS.
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Large radicals can be formed by passing chlorinated organic compounds at pressures of a few mm. , through a furnace containing a pellet of sodium and heated to 350-400 degree C. It is found that the only radicals that will remove metallic mirrors (of tellurium or antimony, etc. , previously deposited beyond the furnace) are those that can decompose into methyl or ethyl radicals plus an unsaturated molecule, without undergoing any transmigration of atoms. The authors also found, especially in the case of larger monochlorinated molecules, that there was some decomposition, approximately half, even in the absence of metallic sodium.
- Rice,Tweedell
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p. 995 - 997
(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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- 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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- 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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- Absolute Rate Constants for the Reaction of Triethylsilyl Radicals with Organic Halides
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The absolute rate constants for the reactions of Et3Si. radicals with a number of organic halides have been measured in solution by using laser flash photolysis techniques.The reactivities cover a wide range; e.g., the rate constants at ca. 300 K are 4.3 * 109, 1.1 * 108, 2.5 * 106, and 5 M-1 s-1 for CH3CH2I, C6H5Br, (CH3)3CCl, and C6H5CH2F, respectively.Arrhenius parameters were determined for a few representative substrates.Thus, for CCl4, Ea = (0.78 +/- 0.24) kcal/mol and log (A/M-1 s-1) = 10.2 +/- 0.2, data which can be combined with relative rates from the literature to yield absolute rate constants for some additional substrates.Comparison of the Arrhenius parameters for halogen abstraction from several substrates suggests the involvment of charge-transfer interactions in some of these reactions.Such rate-enhancing interactions can have a greater influence on the preexponential factor than on the activation energy.An explanation for this phenomenon is advanced.Combination of our rate constants with data from literature also allows evaluation of the rates of inversion at the silicon center in triorganosilyl radicals as being in the range (3-12) * 109 s-1 at temperatures from 0 to 80 deg C which implies that a considerable activation barrier is involved in such process.
- Chatgilialoglu, C.,Ingold, K. U.,Scaiano, J. C.
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p. 5123 - 5127
(2007/10/02)
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