- Energetic differences between the five- and six-membered ring hydrocarbons: Strain energies in the parent and radical molecules
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(Chemical Equation Presented) The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for
- Agapito, Filipe,Nunes, Paulo M.,Costa Cabral, Benedito J.,Borges Dos Santos, Rui M.,Martinho Simoes, Jose A.
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- The Kinetics of Radiation-Induced Hydrogen Abstraction by CCl3 and CCl3O2 Radicals from Cyclohexane in the Liquid Phase
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The kinetics of the photolysis-induced free-radical reaction in carbon tetrachloride solution of cyclohexane in the absence and in the presence of oxygen were studied in the temperature range of 100-200 deg C.It was found that the influence of the oxygen on the chloroform formation is significant, and a new mechanism which explains the results was suggested.The rate constants for the reactions .CCl3 + O2 -> .CCl3O2 (k6), .CCl3O2 -> COCl2 + ClO (k7), and .CCl3O2 + C6H12 -> CHCl3 + .C6H11 + O2 (k8) were found as log (k6/(M-1s-1)) = 5.05 +/- 0.07 and log 8/k7)/(M-1s-1)> = (3.0808 +/- 0.0973) - (3.7080 +/- 0.1857)/θ, where θ = 2.303RT kcal/mol.
- Alfassi, Zeev B.,Mosseri, Shlomo
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- The reaction of O(3P) with cyclohexane clusters
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The effect of cluster formation on the reactivity of cyclohexane was investigated.When single molecules react with O(3P) the products are the OH radical and cyclohexyl radical.In contrast, we found that when small clusters react with O(3/
- Rudich, Y.,Hurwitz, Y.,Lifson, S.,Naaman, R.
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- HYDROGEN-ATOM ABSTRACTION FROM ALKANES BY OH. 6. CYCLOPENTANE CYCLOHEXANE
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Absolute rate coefficients for the reactions of the hydroxyl radical with cyclopentane-h10 (k1),cyclopentane-d10 (k2),cyclohexane-h12 (k3), and cyclohexane-d12 (k4) we
- Droege, August T.,Tully, Frank P.
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- HYDROGEN ABSTRACTION REACTIONS BY NH2(X2B1)-RADICALS FROM HYDROCARBONS IN THE GAS PHASE.
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Rate constants for the reactions of the NH//2(X**2B//1) radical with ethane, propane and cyclohexane were measured in an isothermal flow system. Kinetic data were obtained at 4 mbar helium pressure in the high temperature range given below. left bracket NH//2 right bracket (t)-profiles were mapped at pseudo-first order conditions left bracket NH//2 right bracket //0 VM LS TH left bracket RH right bracket //0 with the laser induced fluorescence technique. The temperature dependence of the rate constants is represented by certain Arrhenius-expressions. A correlation of the activation energy with the energy of the broken C-H bond of various hydrocarbons is discussed.
- Ehbrecht,Hack,Rouveirolles,Wagner
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- The dynamics of O(3P) + deuterated hydrocarbons: Influences on product rotation and fine-structure state partitioning
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The dynamics of the reactions O(3P) + CH4/CD4 and O(3P) + cyclo-C6H12/C6D12 were experimentally studied. The novel dynamical experimental results confirmed the main fe
- McKendrick,Ausfelder,Kelso
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- Kinetic and Mechanistic Studies of the Reactions of Cyclopentylperoxy and Cyclohexylperoxy Radicals with HO2
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The kinetics and mechanism of the reactions c-C5H9O2 + HO2 -> c-C5H9OOH + O2 (1) and c-C6H11O2 + HO2 -> c-C6H11OOH + O2 (2) have been studied using both the flash photolysis/UV absorption and continuous photolysis/FTIR product analysis techniques.End prod
- Rowley, David M.,Lesclaux, Robert,Lightfoot, Phillip, D.,Noziere, Barbara,Wallington, Timothy J.,Hurley, Michael D.
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- Using toluates as simple and versatile radical precursors
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(Chemical Equation Presented) The viability of the toluate moiety as a radical precursor has been examined by studying deoxygenation and cyclization reactions.
- Lam, Kevin,Marko, Istvan E.
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scheme or table
p. 2773 - 2776
(2009/05/30)
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- Chemistry of the t-butoxyl radical: Evidence that most hydrogen abstractions from carbon are entropy-controlled
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Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical (tBuO·) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an an
- Finn, Meghan,Friedline, Robert,Suleman, N. Kamrudin,Wohl, Christopher J.,Tanko, James M.
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p. 7578 - 7584
(2007/10/03)
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- Absolute rate constants for some reactions of the triethylamineboryl radical and the borane radical anion
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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)
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- Entropy control of the cross-reaction between carbon-centered and nitroxide radicals
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Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from 5 M-1 s-1 to 2.3 × 109 M-1 s-1 and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.
- Sobek,Martschke,Fischer
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p. 2849 - 2857
(2007/10/03)
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- Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes
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The reactions of the photoexcited aromatic ketones, xanthone and 1-azaxanthone, with polyalkylbenzene donors yields the corresponding ketyl radicals as detected by nanosecond laser flash photolysis. On the basis of formation of these photoreduced products, the quenching of the photoexcited species is expected to occur either by a one-step hydrogen abstraction from the donor, electron transfer followed by proton transfer from the donor, or by formation of a charge-transfer type encounter complex prior to hydrogen atom transfer. The reactions of triplet xanthone and triplet 1-azaxanthone with polyalkylbenzene donors in acetonitrile were investigated to probe the effect of the nature of the triplet state and the redox properties on the relative importance of each quenching pathway. Determination of bimolecular rate constants, as well as analysis of kinetic isotope effects and ketyl radical yields, suggests that for both xanthone and 1-azaxanthone the quenching process is dominated by formation of charge-transfer encounter complexes between excited-state aromatic ketone acceptor and ground-state polyalkylbenzene donor. The reactivites of the xanthone π,π* triplet and 1-azaxanthone n,π* triplet toward these donors is shown to be governed by their reduction potentials, with their electronic configuration being unimportant to the kinetics of encounter complex formation. The only exception to this is found when sterically encumbered polyalkylbenzene donors are employed. Results with these compounds suggest that π,π* and n,π* states form encounter complexes of different structure which affects their ability to react with hindered donors. Additionally, product yields with all of the donors are controlled by both the extent of charge transfer within encounter complexes and the encounter complex structure.
- Coenjarts,Scaiano
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p. 3635 - 3641
(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)
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- Absolute rate constants for the β-scission and hydrogen abstraction reactions of the tert-butoxyl radical and for several radical rearrangements: Evaluating delayed radical formations by time-resolved electron spin resonance
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An analysis is presented for the determination of absolute rate constants for radical transformation reactions in complex reaction systems by time-resolved ESR during intermittent photochemical radical production. The technique is applied to obtain absolu
- Weber, Matthias,Fischer, Hanns
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p. 7381 - 7388
(2007/10/03)
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- Structure and reactivity of perfluorinated branched α-ketoradicals
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Fluoroaliphatic hydroxyketoradicals prepared by photochemical reduction of the corresponding α-diketones and (i-C3F7)2CC(O)CF3 react with hydrogen abstraction (according to ESR data). The hydroxyketor
- Tumanskii,Shaposhnikova,Avetisyan,Sterlin
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- Reactions of Atomic Oxygen (3P) with Selected Alkanes
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Rate constants for the reactions of atomic oxygen (3P) with selected alkanes (C2-C6 straight chain alkanes, c-C6H12, neo-C5H12, and i-C4H10) have been determined directly by a laser photolysis-shock tube-atomic resonance absorption method at hi
- Miyoshi, Akira,Tsuchiya, Kentaro,Yamauchi, Noboru,Matsui, Hiroyuki
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p. 11452 - 11458
(2007/10/02)
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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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p. 466 - 470
(2007/10/02)
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- COMPETITION RATE CONSTANTS FOR ADDITION OF CYCLOHEXYL RADICAL TO AMIDE SUBSTITUTED ALKENES: ORIGINS OF ACYCLIC STEREOSELECTION IN RADICAL ADDITIONS
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Competition rate constants were determined for the addition of cyclohexyl radical to α,β unsaturated amides derived from pyrrolidine and trans-2,5-dimethylpyrrolidine.The relative rates can be understood by single crystal X-Ray analyses of the alkenes and
- Porter, Ned A.,Wu, Wen-Xue,McPhail, Andrew T.
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p. 707 - 710
(2007/10/02)
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- The Reactions of the Cyclic Hydrocarbon Radicals c-C3H5, c-C5H9, and c-C6H11 with O Atoms in the Gas Phase
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The production of the cyclic hydrocarbon radicals c-C3H5, c-C5H9, and c-C6H11 by the reactions of c-C3H6, c-C5H10, and c-C6H12 with F atoms and the reactions of the cyclic hydrocarbon radicals with O atoms were studied at low pressure (around 1 mbar) and low temperature (290-300 K) using the discharge flow method.Stable and labile species were detected by mass spectrometry combined with molecular beam sampling.The rate constants of the reactions c-C3H5 + O (4), c-C5H9 + O (5), and c-C6H11 + O (6) were determined: k4 = 1.4*1014, k5 = 1.7*1014, k6 = 1.4*1014 cm3/mol*s.One main reaction route is interpretated by the formation of a highly excited adduct, decomposing to H + oxygen containing ring products; at high strain energy (reaction (4)) total ring cleavage, at low strain energy (reaction (6)) partial ring cleavage was observed.A second reaction route leads to OH + alkene.The rate constant for the reaction F + c-C6H12 (3) was found as k3 = 5.7*1013 cm3/mol*s.
- Heinemann-Fiedler, P.,Hoyermann, K.,Rohde, G.
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p. 1400 - 1404
(2007/10/02)
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- Kinetics of abstraction reactions of methyl radicals with alkanes in gas phase
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Rate data for the abstraction reactions between methyl radicals and some alkanes viz. 2,2-dimethylpropane, 2,2,3,3-tetramethylbutane, cyclohexane, butane and 2-methylpropane (containing primary, secondary and tertiary C-H bonds) have been determined between 399 and 434 K.The methyl radicals are generated by thermolysis of di-t-butyl peroxide.From the yields of methane and ethane.Arrhenius parameters for the reaction: RH + CH3 -> CH4 + R have been obtained.They are log (A6) = 7.93 +/- 0.13, 7.95 +/- 0.14, 8.07 +/- 0.10, 7.96 +/- 0.10 and 8.13 +/- 0.20, and E6/kJmol-1 = 47.59 +/- 1.56, 46.17 +/- 1.54, 40.14 +/- 1.20, 38.94 +/- 1.20 and 33.60 +/- 2.30, respectively.The data obtained in this work are compared with those obtained by other workers.The experimentally determined activation energies for 2,2-dimethylpropane, cyclohexane, butane and 2-methylpropane are compared favourably with those calculated by a semiempirical method.
- Sway, Mohammad I
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p. 748 - 752
(2007/10/02)
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- KINETIC ISOTOPE EFFECTS FOR HYDROGEN ABSTRACTION FROM VARIOUS SATURATED HYDROCARBONS BY DEUTERIUM ATOMS IN THE GAS PHASE.
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Using deuterium atoms produced in the radiolysis of heavy water vapor, the kinetic isotope effects k//H/k//D were determined by the isotope competitive method for the authors' abstraction reaction. The alkanes RH investigated include isobutane, 2,3,4-trimethylpentane, neopentane, cyclopentane, cyclohexane, cycloheptane, and n-heptane. The results are represented in terms of the Arrhenius-type expression k//H/k//D equals A//H/A//D exp((E//D minus E//H) (kJ mol** minus **1)/RT), over the temperature range 363-473 K. The pre-exponential factor ratios A//H/A//D vary from 0. 36 for neopentane to 0. 70 for 2,3,4-trimethylpentane, the E//D minus E//H values from 6. 4 for 2,3,4-trimethylpentane to 10. 4 kJ mol** minus **1 for neopentane. The variation in the E//D minus E//H values was correlated with the bond strength of the C-H bond being broken.
- Fujisaki,Gaeumann,Ruf
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p. 375 - 382
(2007/10/02)
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- 1-Naphthylcarbene: Spectroscopy, Kinetics, and Mechanisms
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The reactions of 1-naphthylcarbene have been examined by using laser flash photolysis techniques.Generation of the carbene from the diazo precursor in hydrocarbon solvents leads to the formation of 1-naphthylmethyl radicals, which were characterized by their absorption at 363 nm.However, product studies in cyclohexane and cyclohexane-d12 reveal that the main reaction path is carbene insertion into the C-H bond rather than H abstraction. 1-Naphthylcarbene reacts readily with nitriles to yield nitrile ylides which can also be generated from the corresponding azirine.For example, the carbene reacts with acetonitrile with a rate constant of 4.6 X 1E5 M-1s-1 at 300 K to yield an ylide that can be trapped readily with electron-deficient olefins, such as diethyl fumarate.Reaction of the carbene with oxygen (k = 3.5 X 1E9 M-1 s-1) yields the carbonyl oxide (λmax 435 nm).Other reactions examined include halogen abstraction, addition to ketones, and reactions with triethylamine, pyridine, and ethers.The absolute kinetics of the various reactions were examined by monitoring the formation of the products (e.g., ylides, radicals, etc.), since the carbene spectrum was not detected directly.
- Barcus, R. L.,Hadel, L. M.,Johnston, L. J.,Platz, M. S.,Savino, T. G.,Scaiano, J. C.
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p. 3928 - 3937
(2007/10/02)
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- Electronic Effects in the Regiochemistry of the Alkenylsilyl Radical Cyclizations
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The intramolecular reaction of alkenylsilyl radicals has been studied by MINDO/3.Partitioning of the total energy into local contributions is used to analyse the different effects competing in exo-endo cyclization.The change in the regioselectivity of the dimethylpent-4-enylsilyl and but-3-enylsilylmethyl radical with respect to the hex-5-enyl radical has been studied.The reversal in the regioselectivity of the silyl radical is due to an enthalpy factor caused by electronic effects.
- Sarasa, J. Pedro,Igual, Joan,Poblet, Josep M.
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p. 861 - 867
(2007/10/02)
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- The Free Radical Reaction between Alkanes and Carbon Tetrachloride
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Product studies and kinetic electron paramagnetic resonance methods were used to investigate the free radical reaction between alkanes and carbon tetrachloride in solution.Trichloromethyl radicals abstracted hydrogen from simple alkanes with rate constants of ca. 60 M-1 s-1 at 300 K. in good agreement with gas-phase data.However, rate constants for chlorine abstraction by alkyl radicals from carbon tetrachloride were ca.E4 M-1 s-1 and were therefore ca. 2 orders of magnitude higher in solution than in the gas phase.Possibilities for the origin of this effect are discussed.
- Hawari, J. A.,Davis, S.,Engel, P. S.,Gilbert, B. C.,Griller, D.
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p. 4721 - 4724
(2007/10/02)
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- Reactions of Cyclanyl Radicals in the Gas Phase. II. Cyclohexyl Radicals from Mixtures of Azomethane and Cyclohexane
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Cyclohexane-azomethane mixtures in argon as diluent were converted by temperatures between 400 and 480 deg C in a stirred flow system.All products were analytically determined in a comprehesive way.In this way not only isomeric C6H11-radicals, which were already known from former investigations, but also hitherto unknown C6H11-radicals, especially the 3-methylcyclopentyl radical, could be detected as a recombination product with methyl radicals or as the corresponding olefinic compounds.From the results the stationary concentrations of important radical species were estimated and mechanistic consequences discussed.
- Ondruschka, B.,Zimmermann, G.,Anders, G.
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p. 853 - 862
(2007/10/02)
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- Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 42. Some Reactions of the Bis(trifluoromethyl)aminoxyl Radical
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Absolute rate constants have been determined in Freon solvents over a temperature range from ca. 190 to 300 K for H atom abstraction by (CF3)2NO. from 11 substrates and for the addition of this radical to CH2=CCl2.Some values found for log A (M
- Doba, T.,Ingold, K. U.
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p. 3958 - 3963
(2007/10/02)
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- Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 36. Stereoelectronic Effects in Hydrogen Atom Abstraction from Ethers
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Relative rates of hydrogen atom abstraction by photogenerated tert-butoxyl from a variety of cyclic and acyclic ethers, acetals, and orthoformates have been measured at -60 deg C by an EPR spectroscopic technique.There is a pronounced stereoelectronic effect which produces high rates of abstraction from those C-H bonds adjacent to oxygen which have a relatively small dihedral angle (ca. 30 deg) with respect to the p-type orbital(s) on the oxygen(s).For C-H bonds which have a large dihedral angle (ca. 90 deg) abstraction is very much slower.The barrier to inversion of the 2-methoxy-1,3-dioxolan-2-yl radical is 10 kcal/mol.
- Malatesta, V.,Ingold, K. U.
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p. 609 - 614
(2007/10/02)
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