- Halogen Atom Abstraction Dynamics of Fluorine Atoms Reacting with Allyl Bromide and Iodobenzene Molecules
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Reactive scattering of F atoms with C3H5Br and C6H5I molecules leading to Br and I atom abstraction has been studied at an initial translational energy E ca. 40 kJ mol-1 using a supersonic beam of F atoms seeded in He fuffer gas.The center-of-mass angular distributions of BrF and IF scattering show peaking in the forward and backward directions, which is consistent with reaction via persistent C3H5-Br-F and C6H5-I-F complexes with lifetimes greater than two rotational periods.The sharply peacked angular distribution observed for F + C3H5Br is consistent with a microcanonical description, whereby reactive scattering arises from a product transition state which approximates to a strongly prolate symmetric top.The mildly peaked angular distribution observed for F + C6H5I is consistent with a phase space description whereby unconstrained rotation is established between the nascent molecules in the product transition state.The product translational energy distributions are both consistent with randomization of energy over internal modes of the collision complex.The lifetime of the C3H5BrF collision complex greatly exceeds that of the FCH2-.CH-CH2Br free radical intermediate in the Br atom displacement pathway, showing that these radicals are not coupled via a four-membered-ring transition state.Similarly, the rate of coupling to the .CH2-CHF-CH2Br radical must be slower than the rate of dissociation of the C3H5BrF radical, and the rate of migration of the F atom to the ? system of the very long-lived C6H5IF radical must also be slower than its rate of dissociation.
- Wang, J. J.,Zhu, Z. Z.,Smith, D. J.,Grice, R.
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- FORMATION OF H AND D ATOMS IN PYROLYSIS OF BENZENE-d6, CHLOROBENZENE, BROMOBENZENE, AND IODOBENZENE BEHIND SHOCK WAVES
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Highly dilute mixtures of benzene-d6, chlorobenzene, bromobenzene (including the 4-d and 2,4,6-d3 isotopic compounds), and iodobenzene in argon were pyrolyzed behind incident shock waves with a total pressure of 0.4 atm and temperatu
- Rao, V. Subba,Skinner, Gordon B.
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- Velocity-map ion-imaging of the NO fragment from the UV-photodissociation of nitrosobenzene
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The velocity and angular distribution of NO fragments produaed by UV photodissociation of nitrosobenzene have been determined by velocity-map ion-imaging. Excitation of the S2-state by irradiation into the peak of the first UV absorption band at 290.5 nm leads to a completely isotropic velocity distribution with Gaussian shape. The average kinetic energy in both fragments correlates with the rotational energy of the NO fragment and increases from 6% of the excess energy for j = 6.5 to 11% for j = 29.5. A similar isotropic distribution albeit with larger average velocity is observed when the ionization laser at 226 nm is also used for photodissociation, corresponding to excitation into a higher electronic state Sn (n ≥ 3) of nitrosobenzene. It is concluded that photodissociation occurs on a timescale much slower than rotation of the parent molecule, and after redistribution of the excess energy into the vibrational degrees of freedom.
- Obernhuber, Thorsten J.,Kensy, Uwe,Dick, Bernhard
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- Kinetics of phenyl radical reactions with propane, n-butane, n-hexane, and n-octane: Reactivity of C6H5 toward the secondary C-H bond of alkanes
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The kinetics of C6H5 reactions with n-CnH2n+2 (n = 3, 4, 6, 8) have been studied by the pulsed laser photolysis/mass spectrometric method using C6H5COCH3 as the phenyl precursor at temperatures between 494 and 1051 K. The rate constants were determined by kinetic modeling of the absolute yields of C6H6 at each temperature. Another major product C6H5CH3 formed by the recombination of C6H5 and CH3 could also be quantitatively modeled using the known rate constant for the reaction. A weighted least-squares analysis of the four sets of data gave k (C3H8) = (1.96 ± 0.15) × 1011 exp[-(1938 ± 56)/T], and k (n-C4H10) = (2.65 ± 0.23) × 1011 exp[-(1950 ± 55)/T], k (n-C6H14) = (4.56 ± 0.21) × 1011 exp[-(1735 ± 55)/T], and k (n-C8H18) = (4.31 ± 0.39) × 1011 exp[-(1415 ± 65)/T] cm3 mol-1 s-1 for the temperature range studied. For the butane and hexane reactions, we have also applied the CRDS technique to extend our temperature range down to 297 K; the results obtained by the decay of C6H5 with CRDS agree fully with those determined by absolute product yield measurements with PLP/MS. Weighted least-squares analyses of these two sets of data gave rise to k (n-C4H10) = (2.70 ± 0.15) × 1011 exp[-(1880 ± 127)/T] and k (n-C6H14) = (4.81 ± 0.30) × 1011 exp[-(1780 ± 133)/T] cm3 mol-1 s-1 for the temperature range 297-1046 K. From the absolute rate constants for the two larger molecular reactions (C6H5 + n-C6H14 and n-C8H18), we derived the rate constant for H-abstraction from a secondary C-H bond, ks-CH = (4.19 ± 0.24) × 1010 exp[-(1770 ± 48)/T] cm3 mol-1 s-1.
- Park,Wang, Liming,Lin
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- Kinetics and Mechanism of the Reaction of OH with C6H6 over 790-1410 K
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The AeroChem high temperature photochemistry (HTP) techniqur was used to study reaction OH + C6H6 products (1).Direct measuremnts of qverall rate coefficient k1(T) for this reaction were obtained over the temperature range 790-1410 K.The upper temperature of this work exceeds, by some 400 K, the upper temperature of any 100)/T> reported kinetic study on this reaction.The data are fitted by the expression k1(T)=(3,5+/-0.3) X 10-11 exp cm3s-1. mechanistic studies near 1300 K showed that the reaction 1 is dominated by the abstraction channel, OH + C6H6C6H5 + H2O (1a).The effecty of the reverse reaction, C6H5 + H2OOH + C6H6 (-1a), was also observed at suitably chosen experimental conditions, and its rate coefficient, k-1a was measured.The value of k-1a, (10+/-0.3) X 10 -14 cm3s-1, was found to agree with estimates obtained from the measured forward rate coefficient, k1a, and available thermochemical data.The 1300 K work also showed that a nonabstraction channel recently proposed by ther workers on the basis of theoretical calculations, OH + C6H6C6H5OH + H (1b), does not contribute significantly (20percent) to the overall reaction rate.
- Madronich, S.,Felder, W.
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- Elementary Photoprocesses in Benzene Clusters
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The article reports the use of the resonant two-photon ionization technique to selectively excite a molecular ion within a cluster and observe the dynamical outcome.We have excited clusters containing up to 14 benzene molecules to energies of 10.00 or 12.84 eV and measured the probability that an initially formed C6H6+ attacks a neighboring benzene unit of the cluster according to the vapor-phase reaction C6H6+ + C6H6 -> C7H7+ + C5H5, ΔH = 0.63 eV.The C5H5 radical is expelled from the cluster.At either energy excitation proceeds through an X112 vibrational level of benzene (X = 6 or 8) but in the latter case the benzene cation is also produced electronically excited.Accordingly, at low-energy excitation the above pathway is entirely absent, while the 12.84-eV excitation leads to reaction with a probability increasing with cluster size, as predicted by solvation models.This result makes it appear quite likely that >12-eV excitation of condensed benzene will lead to transient tropylium ion centers for conduction electrons, accompained by variable trapping of C5H5.
- Schriver, K. E.,Camarena, A. M.,Hahn, M. Y.,Paguia, A. J.,Whetten, R. L.
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- Crossed-molecular-beam study on the formation of phenylacetylene from phenyl radicals and acetylene
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(Figure Presented) A hot topic! Crossed-molecular-beam experiments showed that phenylacetylene molecules - precursors to polycyclic aromatic hydrocarbons in combustion flames and in carbon-rich planetary nebulae - can be formed through reactions of phenyl radicals with acetylene under single-collision conditions (see picture).
- Gu, Xibin,Zhang, Fangtong,Guo, Ying,Kaiser, Ralf I.
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- Experimental and theoretical studies of the unimolecular decomposition of nitrosobenzene: High-pressure rate constants and the C-N bond strength
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The unimolecular decomposition of nitrosobenzene has been studied at 553-648 K with and without added NO under atmospheric pressure. Kinetic modeling of the measured C6H5NO decay rates by including the rapid reverse reaction and minor secondary processes yielded the high-pressure first-order rate constant for the decomposition C6H5NO → C6H5 + NO (1), k∞1 = (1.42 ± 0.13) × 1017 exp [-(55 060 ± 1080)/RT] s-1, where the activation energy is given in units of cal/mol. With the thermodynamics third-law method, employing the values of k∞1 and those of the reverse rate constant measured in our earlier study by the cavity ring-down technique between 298 and 500 K, we obtained the C-N bond dissociation energy, D○0 (C6H5-NO) = 54.2 kcal/mol at 0 K, with an estimated error of ±0.5 kcal/mol. This new, larger bond dissociation energy is fully consistent with the quantum mechanically predicted value of 53.8-55.4 kcal/mol using a modified Gaussian-2 method. Our high-pressure rate constant was shown to be consistent with those reported recently by Horn et al. (ref 13) for both forward and reverse reactions after proper correction for the pressure falloff effect.
- Park,Dyakov,Mebel,Lin
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- IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon
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Electron bombardment of aniline (PhNH2) in an Ar matrix mainly generated the aniline cation (PhNH2+), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix s
- Chou, Sheng-Lung,Lin, Shu-Yu,Tseng, Chien-Ming,Wu, Yu-Jong
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- Low-Temperature Photolysis of Benzoyl Peroxide
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Abstract: The photolysis of dry benzoyl peroxide (BP) at 77 K in the 480–236 nm range of wavelengths and an ethanol solution is studied via EPR. It is determined that the main photochemical process in irradiating BP at λ = 480–365 nm is the direct photodi
- Kuzina,Bol’shakov,Kulikov,Mikhailov
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p. 189 - 195
(2020/03/31)
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- Phenylsulfinyl Radical: Gas-Phase Generation, Photoisomerization, and Oxidation
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Arylsulfinyl radicals are key intermediates in sulfoxide chemistry. The parent molecule, phenylsulfinyl radical PhSOa€¢, has been generated for the first time in the gas phase through high-vacuum flash pyrolysis of PhS(O)R (R = CF3 and Cl) at a
- Xu, Jian,Wu, Zhuang,Wan, Huabin,Deng, Guohai,Lu, Bo,Eckhardt, André K.,Schreiner, Peter R.,Trabelsi, Tarek,Francisco, Joseph S.,Zeng, Xiaoqing
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p. 9972 - 9978
(2018/07/21)
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- Investigating the mechanisms of aromatic amine-induced protein free radical formation by quantitative structure-activity relationships: Implications for drug-induced agranulocytosis
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Aromatic amine drugs have been associated with agranulocytosis (neutrophil depletion) for which the mechanism is unknown. We have previously shown that the metabolism of two aromatic amine drugs by human myeloperoxidase (MPO) results in phenyl radical metabolite formation and also in protein free radical formation on MPO. Because the concentration of drug required to produce a maximum signal for MPO protein free radical (MPO?) detection was different for each drug, this prompted us to consider that other aromatic amines may also show varying degrees of ability to induce MPO? formation. Immunoassay experiments using the immuno-spin-trapping technique were performed, which evaluated the potency of different aromatic amines containing the aniline substructure to generate the MPO?. Each reaction contained equal amounts of H2O2, 5,5-dimethyl-1-pyrroline- N-oxide, MPO, and variable concentrations of aniline derivatives. Several physicochemical parameters for aniline derivatives were used to derive quantitative structure-activity relationship equations, which showed that the Hammett constant (-) best correlated with the MPO? formation for all aniline derivatives. More statistically robust equations were derived if the anilines were separated into mono- and disubstituted groups. However, some aniline derivatives did not induce MPO? formation. Using electron spin resonance spectroscopy, we evaluated the ability of all aniline derivatives tested to produce phenyl radical metabolites, as previously shown by spin trapping for the aromatic amine drugs. Interestingly, we found that only those aniline derivatives that produced a phenyl radical also formed MPO ?. We propose that the phenyl radical is the reactive free radical metabolite responsible for generating the MPO?+.
- Siraki, Arno G.,Jiang, Jinjie,Mason, Ronald P.
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experimental part
p. 880 - 887
(2011/03/17)
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- Matrix isolation and IR characterization of the benzoyl and benzoylperoxy radicals
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The benzoyl radical 1 was synthesized in argon matrices by the thermal reaction of the phenyl radical 2 with CO. The IR spectrum, with the C=O str. vibration at: 1824.4 cm-1 is in good agreement with DFT calculations. The formation of 1 is reve
- Mardyukov, Artur,Sander, Wolfram
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experimental part
p. 2904 - 2909
(2010/08/05)
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- Photochemistry and reactivity of the phenyl radical-water system: A matrix isolation and computational study
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The reaction of the phenyl radical 1 with water has been investigated by using matrix isolation spectroscopy and quantum chemical calculations. The primary thermal product of the reaction between 1 and water is a weakly bound complex stabilized by an OH...π interaction. This complex is photolabile, and visible-light irradiation (λ > 420 nm) results in hydrogen atom transfer from water to radical 1 and the formation of a highly labile complex between benzene and the OH radical. This complex is stable under the conditions of matrix isolation, however, continuous irradiation with λ>420nm light results in the complete destruction of the aromatic system and formation of an acylic unsaturated ketene. The mechanisms of all reaction steps are discussed in the light of ab initio and DFT calculations.
- Mardyukov, Artur,Crespo-Otero, Rachel,Sanchez-Garcia, Elsa,Sander, Wolfram
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scheme or table
p. 8679 - 8689
(2010/11/03)
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- Matrix isolation and spectroscopic characterization of the phenylperoxy radical and its rearranged products
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The phenylperoxy radical 1 has been synthesized by the reaction of the phenyl radical 2 with 3O2. Radical 1 could be either generated in the gas phase and subsequently trapped in solid argon at 10 K, or directly synthesized in argon
- Mardyukov, Artur,Sander, Wolfram
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body text
p. 1462 - 1467
(2009/09/04)
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- Generation and reactivity of the phenyl cation in cryogenic argon matrices: Monitoring the reactions with nitrogen and carbon monoxide directly by IR spectroscopy
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The phenyl cation 1 has been prepared by co-deposition of iodobenzene 6 or bromobenzene 7 with a microwave-induced argon plasma and characterized by IR spectroscopy in cryogenic argon matrices. The cation can clearly be identified by its strongest absorpt
- Winkler, Michael,Sander, Wolfram
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p. 6357 - 6367
(2007/10/03)
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- Magnetic Field Effects on the Photodissociation Reaction of Triarylphosphine in Nonviscous Homogeneous Solutions
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The magnetic field effects (MFEs) on the photodecomposition reactions of triphenylphosphine and its halogen and methyl derivatives are investigated in fluid solutions. The yield of diarylphosphinyl radicals decreased with increasing magnetic field from 0.1 to 5 T but was stationary below 0.1 T and above 5 T. The MFE becomes larger by the substitution of halogen atoms and the 3- or 4-methyl group. In cyclohexane, the yields of the escaped diarylphosphinyl radicals at 1 T are reduced to 0.69, 0.55, 0.59, and 0.56 of those at 0 T for triphenylphosphine and its tris(4-chloro), tris(3-methyl), and tris(4-methyl) derivatives, respectively. This magnetic field dependence was ascribed to originate from the deactivation process of the excited triplet state, which is a variant of the d-type triplet mechanism originally proposed by Steiner. The interaction between the closely lying nπ* and ππ* states makes their solvent dependence complicated.
- Sakaguchi, Yoshio,Hayashi, Hisaharu
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p. 3421 - 3429
(2007/10/03)
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- Heavy ion radiolysis of liquid benzene
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The mechanisms responsible for the main products in liquid benzene radiolysis (biphenyl, molecular hydrogen, and phenyl radical) are probed with protons, helium ions, and carbon ions of a few to 30 MeV energy. Phenyl radical yields have been examined using iodine scavenging techniques. The results are combined with similar data for γ-rays and suggest that phenyl radicals mainly react with benzene to give a long-lived adduct, which leads to polymer formation. Iodine can react with this adduct to give enhanced yields of biphenyl. Biphenyl is the predominant single hydrocarbon product in the radiolysis of neat benzene with a yield of 0.075 molecule/100 eV. Its yield is nearly independent of radiation type and energy suggesting that its formation in neat benzene is due to a fast ion-molecule process and not due to phenyl radicals. The total yield of 0.7 radicals/100 eV is almost entirely due to phenyl radicals and H atoms. A reexamination of the fluorescence from the singlet excited state of benzene suggests that this state is the precursor for molecular hydrogen and acetylene, whereas the triplet excited state decays to phenyl radicals and H atoms. Most of the excited states formed in the γ-radiolysis of benzene seem to decay to ground without formation of any product.
- LaVerne, Jay A.,Araos
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p. 11408 - 11413
(2007/10/03)
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- Application of a new kinetic method in the investigation of cleavage reactions of haloaromatic radical anions
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A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constants of radical anions over the range of 107 - 5 × 109 s-1 in aprotic solvents. The standard potential for the formation of the radical anions may be extracted from the kinetic analysis as well. The method employs electrochemical steady-state or optical detection techniques and is an extension of the redox catalysis approach described previously in the literature. The applicability of the method is illustrated through a systematic study of the cleavage reactions for a number of short-lived haloaromatic radical anions. Interestingly, the radical anion of iodobenzene is found to be an intermediate in the homogeneous reduction of iodobenzene, even though recent investigations have shown that the corresponding heterogeneous reduction at an electrode surface proceeds by a concerted electron transfer-bond cleavage process. The nature of the cleavage reactions is discussed in terms of the activation driving force plot of the cleavage rate constants versus the relevant Gibbs energies. While the exergonic cleavage reactions follow a simple decay mechanism taking place at the halogen site, the endergonic processes are best described as intra-molecular electron transfers from the substituent to the carbon-halogen bond. Nevertheless, the overall intrinsic barrier is found to be relatively small (27-39 kJ mol-1) and it is suggested that the endergonic reactions may proceed by a stepwise mechanism, in which a σ* radical anion is formed as an intermediate prior to the formation of the dissociated products, the aryl radical and the halide. The above conclusions were supported by semi-empirical PM3 calculations of structures and charge distributions in the radical anions.
- Enemaerke, Rasmus J.,Christensen, Torben B.,Jensen, Henrik,Daasbjerg, Kim
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p. 1620 - 1630
(2007/10/03)
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- Thermal properties of 1,1′-bis(p-substituted-benzoyloxy-substituted-phenyl)cyclohexanes
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DSC thermograms of 1,1′-bis(p-substituted-benzoyloxy-substituted-phenyl)cyclohexanes 3, 8, 14 and 15 showed two endotherms whereas 2, 4-7, 9, 11-13, 16 and 17 showed more than two endotherms indicating formation of new compounds as a result of decomposition of original compound. TG thermograms of 1, 2, 4, 5, 8, 11-13 and 15 involved a single step decomposition whereas 3, 6, 7, 9, 10, 14, 16 and 17 involved two step degradation and involved first order kinetics.
- Rajkotia,Parsania
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- Chromium(ii)-mediated reactions of iodonium tetrafluoroborates with aldehydes: Umpolung of reactivity of diaryl-, alkenyl(aryl)-, and alkynyl(aryl)iodonium tetrafluoroborates
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The method described herein allows us, for the first time, to perform umpolung of reactivity of diaryl-, alkenyl(aryl)-, and alkynyl(aryl)iodonium tetrafluoroborates. The method involves generation of organochromium(III) species via reaction of iodonium salts with anhydrous chromium dichloride, followed by their nucleophilic addition to aldehydes to yield alcohols. In contrast to the reaction of aryl and alkenyl halides with chromium dichloride, these iodonium salts are so active that organochromium(III) could be generated without using a nickel catalyst. Substituent effects of unsymmetrically substituted diaryliodonium salts on the product profiles are in good agreement with the reported mode of decomposition of the intermediate unsymmetrical diaryliodanyl radicals. Alkenyl(mesityl)iodonium tetrafluoroborates undergo exclusive alkenylation of aldehydes with no signs of the formation of an arylation product.
- Chen, Da-Wei,Ochiai, Masahito
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p. 6804 - 6814
(2007/10/03)
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- One-electron redox potential of thiobenzoic acid. Kinetic characteristics of benzoylthiyl radical β-fragmentation
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By means of pulse radiolysis, one-electron oxidation of benzoylthiolate (PhCOS- was achieved by the azide radical (N3*) in aqueous solution. The spectrum of the resulting benzoylthiyl radical (PhCOS*) shows a broad absorption in the wavelength region from 350 to 500 nm. With N3*/N3- as reference couple, the reduction potential E°(PhCOS*/PhCOS-) was measured to be 1.21 V vs NHE. Using the pK(a) of 2.48 for thiobenzoic acid (PhCOSH), we derive the standard reduction potential E°(PhCOS*, H+/PhCOSH) to be 1.36 V vs NHE. This reduction potential implies an S-H bond energy of PhCOSH of 87 kcal/mol, which is very close to the bond energy of an alkylthiol S-H bond (87.4 kcal/mol). At 22 °C, the PhCOS* radicals decay with a rate constant of 8.5 x 103 s-1 to form colorless species. This process is presumed to be the β-fragmentation of the PhCOS* radical, PhCOS* → Ph* + COS. The kinetics of the β-fragmentation of the PhCOS* radical was found to follow the Arrhenius equation, log(k2/s-1) = (12.3 ± 0.1) - (11.4 ± 0.2)/θ, where θ = 2.3RT kcal/mol. Strong evidence for this process was provided byproduct identification by GCMS, where the main products of the γ- irradiation-induced N3* radical oxidation of PhCOS- in 0.1 M NaN3 solution were found to be phenyl azide (PhN3) and aniline (PhNH2). These products were formed via addition of the Ph* radical to N3- to form the PhN3(-*) radical anion, which subsequently disproportionated. In the Discussion, we summarize the reduction potentials and the bond dissociation energies of related thiols and their oxygen couterparts. The β-fragmentation of PhCOS* radicals resembles that of the oxygen counterpart, benzoyloxyl radical (PhCOO*), and their thermochemical properties are also compared.
- Zhao, Rong,Lind, Johan,Merényi, Gábor,Eriksen, Trygve E.
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p. 2811 - 2816
(2007/10/03)
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- Detailed kinetics of cyclopentadiene decomposition studied in a shock tube
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Mixtures of cyclopentadiene diluted with argon were used to investigate its decomposition pattern in a single pulse shock tube. The temperatures ranged from 1080 to 1550 K and pressures behind the shock were between 1.7-9.6 atm. The cyclopentadiene concentrations ranged from 0.5 to 2%. Gas-chromatographic analysis was used to determine the product distribution The main products in order of abundance were acetylene, ethylene, methane, allene, propyne, butadiene, propylene, and benzene. The decomposition of cyclopentadiene was simulated with a kinetic scheme containing 44 species and 144 elementary reactions. This was later reduced to only 36 reactions The ring opening process of the cyclopentadienyl radical was found to be the crucial step in the mechanism. 1997 lohn Wiley and Sons, Inc.
- Burcat, Alexander,Dvinyaninov, Michael
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p. 505 - 514
(2007/10/03)
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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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- Fluorescence from samarium(II) iodide and its electron transfer quenching: Dynamics of the reaction of benzyl radicals with Sm(II)
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The luminescence from SmI2 in THF can be readily quenched by a variety of electron acceptors. In the case of organohalides, the reaction is quite fast; for example, for dichloromethane the rate constant is 2.7 × 108 M-1 s
- Skene,Scaiano,Cozens
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p. 7918 - 7921
(2007/10/03)
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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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- 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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- Addition and Cyclization Reactions in the Thermal Conversion of Hydrocarbons with Enyne Structure, II. Analysis of Radicals and Carbenes from Ethynylbenzene
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The pyrolysis of ethynylbenzene (1) in helium was studied in a tubular flow reactor at 10.7 mbar/1030 deg C and reaction times ranging from 5 to 30 ms.Reactive intermediates such as radicals and carbenes were scavenged with dimethyl disulfide (DMDS).Qualitative and quantitative analysis of the scavenging products and of the stable pyrolysis products were carried out by GC-MS analysis. - The radicals phenyl (2a), o-, m-, and p-ethynylphenyl (1c, d, e), 2- and 1-phenylvinyl (1a, b), 1- and 2-naphthyl (3a, b), methyl, hydrogen atoms, and radicals resulting from the addition of phenyl and ethynylphenyl radicals to the triple bond of ethynylbenzene were detected.Carbenes, such as phenylvinylidene, occur only in small traces and obviously play no role in the overall pyrolysis.Concentration profiles of the radicals and of some important pyrolysis products are presented and discussed.Based on the detected radicals, a reaction scheme for the pyrolysis of ethynylbenzene is proposed.Results of pyrolyses of mixtures of ethynylbenzene and ethyne show that naphthalene (3), acenaphthylene (12), and other compounds are produced by a sequence of ethyne addition and cyclization reactions. - Key Words: Ethynylbenzene, pyrolysis of / Scavenging of radicals / PAH formation
- Guthier, Klaus,Hebgen, Peter,Homann, Klaus-Heinrich,Hofmann, Joerg,Zimmermann, Gerhard
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p. 637 - 644
(2007/10/02)
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- Aktivierung von Fluorkohlenwasserstoffen durch "nackte" und komplexierte Praseodym-Kationen in der Gasphase
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Stichworte: Fluorkohlenwasserstoffe * Lanthanoidverbindungen * Massenspektrometrie
- Heinemann, Christoph,Goldberg, Norman,Tornieporth-Oetting, Inis C.,Klapoetke, Thomas M.,Schwarz, Helmut
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p. 225 - 229
(2007/10/02)
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- Radical Analysis in the Pyrolysis of Hydrocarbons by Scavenging with Dimethyl Disulfide
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Radicals formed in the low pressure pyrolysis of hydrocarbons can be quantitatively analyzed by scavenging them with dimethyl disulfide (DMDS) in the condensed phase the pyrolyses of several hydrocarbons (ethyne, ethene, 1.3-butadiene, and benzene) have been studied in a flow reactor at 1300 K and 10.7 mbar.Samples of the hot gas mixture leaving the reactor were taken continuously by a quartz probe, the resulting supersonic nozzle beam was frozen together with an excess of DMDS on the inner surface of two hollow hemispheres cooled by liquid nitrogen.While melting the scavenging reaction takes place.The mixture of DMDS, stable pyrolysis products, and methylthio compounds was separated and identified with a GC-MS-system.CH3-radical, C2H3-radical, C6H5-radical radicals and H-atoms have been detected.The presence of acetone in ethyne pyrolysis has an important influence on the radical concentrations and the product yields.
- Guthier, K.,Hebgen, P.,Loock, H. P.,Homann, K. H.,Zimmermann, G.,Hofmann, J.
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p. 140 - 142
(2007/10/02)
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- Pathways and kinetic energy disposal in the photodissociation of nitrobenzene
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Vacuum-ultraviolet photoionization molecular-beam mass spectrometry is a means of identifying primary photodissociation products and determining their recoil energies.At several photolysis wavelengths between 220 and 320 nm, we have observed three primary photodissociation pathways for nitrobenzene.Two of the pathways are C6H5NO2 C6H5 + NO2 and C6H5NO2 C6H5NO + O.The third pathway produces NO by one or both of the processes C6H5NO2 C6H5O + NO and C6H5NO2 C5H5 + CO + NO.The relative yield of the pathways producing NO2 and NO varies strongly with the photolysis wavelength.The production of NO2 exceeds that of NO by about 50percent for the 280 nm photolysis, but increases to almost a sixfold excess in 222 nm dissociation.The second pathways has a threshold energy that is about 0.50 eV greater that the thermodynamic limit for the formation of nitrosobenzene (C6H5NO) and an oxygen atom from nitrobenzene, probably reflecting the energy required to produce triplet nitrosobenzene and, perhaps, a barrier to dissociation on the triplet surface.The distribution in arrival times for a fragment provides an estimate of the recoil energy at each photolysis wavelength in these experiments.The channel producing nitric oxide (NO) radicals releases a relatively large amount of kinetic energy.Assuming the channel producing nitric oxide (NO) also produces phenoxy (C6H5O), we calculate a linear increase in kinetic energy from 0.29 eV at 320 nm to 1.1 eV at 220 nm.By contrast, the other two channels release only a small amount of kinetic energy (ca. 0.1 eV) at all wavelengths.An impulsive model does not describe the observed kinetic energy release for these low energy channels, suggesting that the energy release is more nearly statistical.The recoil energy predicted by an impulsive model for the channel producing nitric oxide and phenoxy radicals is closer to the observed kinetic energy release.
- Galloway, Douglas B.,Bartz, Jeffrey A.,Huey, L. Gregory,Crim, F. Fleming
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p. 2107 - 2114
(2007/10/02)
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- Two Fragmentation Pathways of Radical Reduced Transients of Aryltrimethylammonium Cations in Water. Photoelectron Injection Study
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The decay rates of radical reduced transients (RRTs) of aryltrimethylammonium cations (5 * 103 - 2.5 * 108 s-1) and the reduction potentials of RRT decay products E1/2 = -(1.06 - 1.47) V (vs.SCE) have been determined in water using photoelectron injection.
- Konovalov, Valery V.,Bilkis, Isaak I.,Selivanov, Boris A.,Shteingarts, Vitaly D.,Tsvetkov, Yury D.
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p. 1707 - 1710
(2007/10/02)
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- High Temperature Reactions of Phenylacetylene
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Hydrogen atom abstraction from phenylacetylene (C6H5 - C2H) like its reaction with hydrogen atoms have been studied at elevated temperatures behind reflected shocks. - The unimolecular decomposition of very low initial concentrations (3 - 22 ppm) of phenylacetylene was investigated in the temperature range 1600 to 1900 K by monitoring the temporal H-atom production. - For C6H5 - C2H + H, the thermal decomposition of very low concentrations (1 - 3 ppm) of C2H5I served as H-atom source.Atomic resonance absorption spectrometry (ARAS) was used to record simultaneously H-atom and I-atom profiles.The experiments covered the temperature range 1190 to 1530 K. - For both series of experiments, the total pressure was about 2.3 (+/-0.3) bar.For the unimolecular reaction R 1: a rate constant expression of: was found.For the bimolecular reaction R 2: products a rate constant of: was deduced.From the available thermochemical data two product channels have to be discussed: (R 2a) (R 2b) .Detailed evaluation of existing thermodynamic data enables the calculation of an equilibrium constant for reaction R 2a.From this and under the assumption that reaction R 2a is the dominant product pathway at elevated temperatures, a rate constant expression of is deduced for the reaction of phenyl radicals with acetylene, which is an important process in sooting flames: (R - 2a) .This rate constant agrees within a factor of 2 with the data from recent studies executed by Fahr et al. . Keywords: Chemical Kinetics / Complex Compounds / Elementary Reactions / Shock Waves / Thermodynamics
- Herzler, J.,Frank, P.
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p. 1333 - 1338
(2007/10/02)
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- Rate of the OH + C6H6 + He reaction in the fall-off range by discharge flow and OH resonance fluorescence
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The rate constant K1 of the reaction OH + C6H6(+He) -> products(+He) has been investigated by discharge flow resonance fluorescence of OH in the fall-off pressure range.From systematic measurements at five pressures between 0.5 and 9.5 torr, the Troe parameters K0 and kinfinite (with Fc = 0.6 have been derived at two temperatures: room temperature and 353 K.For room temperature (297+/-3)K, these parameters are k0 = (1.7+/-0.5) x 10-29 cm6 molecule-2 s-1, Kinfinite = (10+/-2) x 10-13 cm3 molecule-1 s-1.Our experimental results are consistent with addition as the dominant path, in agreement with flash photolysis investigations and with the existence of a fast reaction with NO2 of the (OH-benzene) radical adduct.A numerical simulation shows that this latter reaction should have a rate constant of (4+/-2) x 10-11 cm3 molecule-1 s-1 at 353 K. Key Words: gas phase kinetics, discharge flow, resonance fluorescence, reaction of OH with benzene, tropospheric chemistry.
- Gourmi, A.,Pauwels, J. F.,Devolder, P.
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p. 1057 - 1064
(2007/10/02)
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- Reactions of the Benzyne Radical Anion in the Gas Phase, the Acidity of the Phenyl Radical, and the Heat of Formation of o-Benzyne
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The thermally equilibrated ion-molecule reactions of the o-benzyne radical anion have been examined in the gas phase with the flowing afterglow technique.By using the bracketing technique between o-C6H4.- and Broensted acids of known acidity, we have established the gas-phase acidity of the phenyl radical as ΔG degacid.> = 371-3+6 kcal mol-1.Combination of our experimental acidity of the phenyl radical with appropriate thermochemical data from the literature yields a variety of substantially improved thermochemical values of C6H4 and C6H5. species, most notably, ΔHfdeg = 105 kcal mol-1.In addition to behaving as a Broensted base, o-benzyne radical anion is found to undergo a number of other reactions, including electron transfer, H/D exchange, H2+ transfer, and direct addition.The reaction between o-C6H4.- and the simple aliphatic alcohols is shown to be a competition between proton transfer and H2+ transfer while that between o-C6H4.- and dioxygen or 1,3-butadiene is found to be exclusively an associative detachment process.One unanticipated, novel observation from these studies is the facile formation of an addition complex between the o-benzyne radical anion and carbon dioxide, leading to a distonic radical anion (benzoate-type anion, phenyl-type radical) that offers a unique opportunity for examining radical chemistry in ion-molecule encounter complexes.
- Guo, Yili,Grabowski, Joseph J.
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p. 5923 - 5931
(2007/10/02)
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- ELECTROCHEMICAL REACTIONS OF PHOSPHORIC ACID DERIVATIVES. I. HOMOGENEOUS REDUCTION OF TRIARYL PHOSPHATES BY MEANS OF ORGANIC ELECTRON CARRIERS
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The mechanism of the reduction of triaryl phosphates at homogeneous conditions by means of electrochemically generated radical anions of organic molecules, acting as electron carriers, has been studied, as well as at heterogeneous conditions.In both cases the reduction process is controlled by a retarded electron transfer to the substrate, followed by rapid bond rupture in the radical anion formed.
- Kargin, Yu. M.,Budnikova, Yu. G.,Yanilkin, V. V.
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p. 1816 - 1820
(2007/10/02)
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- Benzene as a Selective Chemical Ionization Reagent Gas
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Dilute mixtures of C6H6 or C6D6 in He provide abundant +. or +. ions and small amounts of + or + ions as chemical ionization (CI) reagent ions.The C6H6 or C6D6 CI spectra of alkylbenzenes and alkylanilines contain predominantly M+. ions from reactions of +. or +. and small amounts of MH+ or MD+ ions from reactions of + or +.Benzene CI spectra of aliphatic amines contain M+., fragment ions and sample-size dependent MH+ ions from sample ion-sample molecules reactions.The C6D6 CI spectra of substituted pyridines contain M+. and MD+ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample-size-dependent MH+ ions from sample ion-sample molecule reactions.Two mechanisms are observed for the formation of MD+ ions: proton transfer from +. or charge transfer from +. to give M+., followed by deuteron transfer from C6D6 to M+..The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments.Proton transfer from +. or +. is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic.For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.
- Allgood, Charles,Lin, Yi,Ma, Yee-Chung,Munson, Burnaby
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p. 497 - 502
(2007/10/02)
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- Synthesis and spectroscopic and electrochemical characterization of ionic and σ-bonded aluminum(III) porphyrins. Crystal structure of methyl(2,3,7,8,12,13,17,18-octaethylporphinato)aluminum(III), (OEP)Al(CH3)
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The synthesis and characterization of SO different ionic and σ-bonded aluminum(III) porphyrins are reported. These compounds were studied by mass spectrometry and IR, UV-visibte, and 1H NMR spectroscopy as well as by electrochemistry. The spectroscopically investigated compounds are represented by (P)AlCl an (P)Al(R), where P is the dianion of tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP) and R is CH3, n-C4H9, C6K5, or C6F4H. The molecular structure of (OEP)Al(CH3) was determined by X-ray diffraction and provides the first structural data for an aluminum porphyrin complex. The Al(III) atom in (OEP)Al(CH3) is pentacoordinated and is located 0.465 (1) A? from the mean nitrogen plane. The electrochemically investigated compounds are represented by (P)AlCl and (P)Al(R), where P is OEP or TPP and R is CH3, n-C4H9, or C6H5. An overall mechanism for the oxidation and reduction of each derivative is presented, and data for the σ-bonded complexes are compared to results obtained under the same experimental conditions for oxidation and reduction of (P)M(R), where M = Ga, In, or Tl.
- Guilard,Zrineh,Tabard,Endo,Han,Lecomte,Souhassou,Habbou,Ferhat,Kadish
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p. 4476 - 4482
(2008/10/08)
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- The Photochemistry of Nitrosobenzene: Direct Observation of the Phenyl Radical-Nitric Oxide Triplet Radical Pair in Argon at 12 K
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Photolysis of nitrosobenzene monomer in an argon matrix at 12 K efficiently yields the phenyl radical and nitric oxide as a triplet radical pair, which are characterized by IR, UV, and ESR spectroscopy; these species recombine upon warming to 25-40 K to regenerate nitrosobenzene.
- Hatton, William G.,Hacker, Nigel P.,Kasai, Paul H.
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p. 227 - 229
(2007/10/02)
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- Photochemistry of adsorbed phenyl iodide: Desorption and photofragmentation dynamics
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The photodissociation and photodesorption dynamics of iodobenzene adsorbed at 150 K on LiF substrates has been investigated at 222 nm using the time-of-flight/resonance-enhanced multiphoton ionization/mass spectrometry technique.The Maxwell-Boltzmann (MB)
- Villa, E.,Dagata, J. A.,Lin, M. C.
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p. 1407 - 1412
(2007/10/02)
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- Adduct Formation of OH Radicals with Benzene, Toluene, and Phenol and Consecutive Reactions of the Adducts with NOX and O2
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The formation of the adducts of OH with benzene, toluene and phenol and consecutive reactions of the adducts with NO, NO2 and O2 are investigated in the temperature range from ca. 298-374 K employing the flash photolysis/resonance fluorescence technique in Ar at 133 mbar.Biexponential decays of OH can be observed over at least 2 orders of magnitude in the presence of the aromatics, and the three reaction channels: abstraction of H by OH, addition of OH and unimolecular decomposition of the adduct are distinguished in overlapping temperature ranges.Rate constants and preliminary Arrhenius parameters are obtained for these channels for benzene, toluene, and phenol.The reactions of the adducts with NO are very slow and can be neglected (-13 cm3s-1) in all three cases.Values of ca. 3*10-11 cm3s-1 are obtained for the rate constants of the reactions of the adducts with NO2, almost independent of reactant and temperature.Preliminary determinations of the adduct reactivities against O2 ca. 2*10-16 (benzene-OH) and ca. 5*10-16 cm3s-1 (toluene-OH) near room temperature indicate that this path predominates under tropospheric conditions.The results for O2 are confirmed by direct observations of the adduct benzene-OH by the cw UV-laser longpath absorption technique.
- Knispel, Ralf,Koch, Rainald,Siese, Manfred,Zetzsch, Cornelius
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p. 1375 - 1379
(2007/10/02)
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- Kinetic Study for Reactions of Nitrate Radical (NO3.) with Substituted Toluenes in Acetonitrile Solution
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The absolute rate constants for the reactions of the nitrate radical (NO3.) with substituted toluenes in acetonitrile have been determined by the flash photolysis method.From the plots of the rate constants against the ionization energies, it was revealed that the reaction path for toluene derivatives with low ionization energies is different from that for toluene derivatives with high ionization energies.For toluene, a deuterium isotope effect was observed to be ca. 1.6, suggesting the direct hydrogen atom abstraction reaction; in this group, xylenes and p-chlorotoluene belong.For toluene derivatives with electron-withdrawing substit uents, NO3. may add to the phenyl rings followed by successive reactions.For both groups, linear correlations against ionization energies with negative slopes show that NO3. is highly electrophilic and that strong polar effects exist in the transition states of both reactions.For toluenes with methoxy groups, the electron-transfer reaction from methoxytoluene to NO3. is a main initial path, since the transient absorption band due to the cation radical of methoxytoluene was detected.
- Ito, Osamu,Akhido, Seiji,Iino, Masashi
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p. 2436 - 2440
(2007/10/02)
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- Photoreactivity of σ-bonded metalloporphyrins. 2. Germanium porphyrin complexes with σ-bonded alkyl, aryl, or ferrocenyl groups. Intramolecular quenching of porphyrin excited triplet states by linked ferrocene
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Continuous visible-light irradiation of (TPP)Ge(R)2, where TPP is the dianion of meso-tetraphenylporphyrin and R is either C6H5 or CH2C6H5, results in a photocleavage of one σ-bonded R group to give a zwitterionic porphyrin that was characterized by UV-visible, 1H NMR, and ESR techniques. Laser-flash photolysis and ferrocene quenching studies indicate that the triplet states of (TPP)Ge(R)2 are photoreactive. In contrast, (TPP)Ge(C6H5)(Fc) and (TPP)Ge(Fc)2, where Fc is σ-bonded ferrocene, are photostable under the same experimental conditions. An energy transfer from the porphyrin triplet state to the axially bound ferrocene in (TPP)Ge(C6H5)(Fc) or (TPP)Ge(Fc)2 is postulated to be involved in the photostability of these two complexes.
- Maiya,Barbe,Kadish
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p. 2524 - 2527
(2008/10/08)
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- Elementary Reactions of Fluorine Atoms with Benzene, Toluene, p-Xylene and Etylbenzene
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The rate constants of the reactions of fluorine atoms with benzene, toluene, p-xylene and ethylbenzene were studied in a discharge flow reactor at room temperature and low pressure. - The aromatic compounds R were followed under pseudo first order conditions (0 ) by mass spectrometry (MS).The rate constants: F + C6H6 k1(296 K) = (3.0 +/- 0.7) * 1E13 cm3/mol*s (1), F + C6H5CH3 k2(296 K) = (4.0 +/- 0.8) * 1E13 cm3/mol*s (2), F + C6H4(CH3)2 k3(296 K) = (4.2 +/- 0.6) * 1E13 cm3/mol*s (3), F + C6H5C2H5 k4(296 K) = (4.5 +/- 0.6) * 1E13 cm3/mol*s (4) were obtained. - The products of reaction (1) were C6H5F (80percent) and HF ( 8percent) and those of reaction (2) were C7H7F (50percent), C6H5F (20percent) and HF (20percent).For reaction (3) the exchange mechanism is dominant, C7H7F, C8H9F (65percent), and the abstraction contributes 35percent (HF).In reaction (4) the abstraction is dominant (HF (50percent)) with an exchange pathway of 50percent C6H5F and C8H9F. - Keywords: Chemical Kinetics / Elementary Reactions / Mass Spectrometry
- Ebrecht, J.,Hack, W.,Wagner, H. Gg.
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p. 619 - 626
(2007/10/02)
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- Cw laser absorption study of the reactions of phenyl radicals with NO, NO2, O2 and selected organics between 298-404 K
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The applied aspect concerning the reactivity of phenyl relates to its suggested participation in the formation mechanisms of polycyclic aromatic hydrocarbons (PAHs) and of soot. In the present paper we report the first application of a direct detection method of phenyl in the gas phase. It will be shown that by using the 488 nm laser line absorption in the $UNK2A1 - $UNK2B1 transition phenyl can be detected with sufficient sensitivity to enable kinetic investigations of its reactions. To our knowledge this has not been achieved before. Rate constant for the reactions of phenyl (C6H5) radicals with NO, NO2, O2, C2H4, 2-butene, benzene, toluene and CCl4 have been determined using a combined laser photolysis laser absorption technique. Phenyl radicals were generated by 248 nm excimer laser photolysis of C6H5X, where X = Cl, Br and NO. Their temporal behaviour was monitored using cw-laser line absorption in the $UNK2A1 - $UNK2B1 transition at 488 nm. The magnitude of the rate coefficients k1 and k2 and their negative temperature dependence suggest that the reactions between phenyl and NO/NO2 occur as radical recombination and/or (in the case of NO2) as O-atom metathesis reaction. The apparent low reactivity of phenyl with O2 is likely to be caused by insufficient thermal stability of the phenylperoxi radical.
- Preidel,Zellner
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p. 1417 - 1423
(2007/10/02)
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- Rate constants for abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals over the temperature range 650-770 K
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Rate constants for the abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals have been measured relative to the corresponding abstraction reaction from ethylene.The method is based on the effect on the rates of formation of methane and ethane of the addition of small quantities of the reactants to the thermal chain reactions of ethylene in the temperature range 650-770 K.Taking the following values of the rate constants for the reference reactions (R=8.134 Jmol-1K-1) : CH3 + C2H4 -> CH4 + C2H3; log k3(Lmol-1s-1) = 8.7+/-0.2 - (63000+/- 3000)/2.3RT C2H5 + C2H4 -> C2H6 + C2H3; log k4(Lmol-1s-1) = 8.2+/-0.2 - (62200+/-3000)/2.3RTthe following rate constants were measured: CH3 + C6H6 -> CH4 + C6H5; logk7(Lmol-1s-1) = 9.3+/-0.3 -(63000+/-5000)/2.3RT C2H5 + C6H6 -> C2H6 + C6H5; logk8(Lmol-1s-1) = 8.8+/-0.3 - (62200+/-5000)/2.3RT CH3 + C7H8 -> CH4 + C7H7; logk11(Lmol-1s-1) = 7.4+/-0.5 -(29000+/-7000)/2.3RT C2H5 + C7H8 -> C2H6 + C7H7; log k12(Lmol-1s-1) = 7.7+/-0.5 -(37000+/-7000)/2.3RT CH3 + c-C5H10 -> CH4 + c-C5H9; logk15(Lmol-1s-1) = 8.3+/-0.5 +/- (36000+/-7000)/2.3RT C2H5 + c-C5H10 -> C2H6 + c-C5H9; logk16(Lmol-1s-1) = 8.2+/-0.5 - (40000 +/-7000)/2.3RT The values of activation energies are discussed in relation to the dissociation energies of the C-H bonds in the reactants. Key words: kinetics, abstraction, methyl, ethyl, benzene, toluene.
- Zhang, H.-X.,Ahonkhai, S. I.,Back, M. H.
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p. 1541 - 1549
(2007/10/02)
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- CAPTO-DATIVE EFFECTS MEASURED BY AZO DECOMPOSITIONS
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The activation parameters determined for rates of decomposition of compounds 3a-d indicate a substantial difference in free energies of activation, approximately 6 kcal/mol, and can be explained on the basis of capto-dative stabilization of the incipient radicals
- Janousek, Zdenek,Bougeois, Jean-Luc,Merenyi, Robert,Viehe, Heinz G.,Luedtke, Alan E.,et al.
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p. 3379 - 3382
(2007/10/02)
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- Reductive Fragmentation of 9,9-Diarylfluorenes. Concurrent Radical Anion and Dianion Cleavage. Electron Apportionment in Radical Ion Fragmentations
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Both Radical anions and dianions of 9,9-diarylfluorenes cleave an aryl ring after reduction by alkali metals or naphtalenide radical anions in ether solvents.The relative amount of cleavage through each intermediate depends on the alkali metal cation, the solvent, and the presence or absence of 18-crown-6-ether.The tendency for dianion cleavage parallels that for disproportionation of radical anions to dianions and neutral hydrocarbons.Radical anion fragmentation is proposed to proceed via heterolytic cleavage in which electron flow is in the direction which offsets the charge distribution in the radical ion.In the present case, this initially affords 9-arylfluorenyl radical and aryl anion, which subsequently indergo electron exchange to form the more stable 9-arylfluorenyl anion and aryl radical.
- Walsh, Thomas D.
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p. 1511 - 1518
(2007/10/02)
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- High-Temperature Pyrolysis of Toluene
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The thermal decomposition of toluene has been investigated by two independent shock tube techniques: time-of-flight (TOF) mass spectrometry and laser-schlieren densitometry.These studies cover the temperature range 1550-2200 K for pressures 0.2-0.5 atm.C2H2, C4H2, CH4, and C7H7 were identified as major species along with lesser amounts of C2H4, C6H2, and C6H6.The laser-schlieren profiles require a dominance by CC scission to phenyl and methyl with a rate constant log k (s-1) = 12.95-72.6 (kcal)/2.3RT for 1600-2100 K and 0.5 atm..Such dominance is also required to produce the methane seen in the TOF spectra.A mechanism is proposed which provides an excellent description of density gradient and major species concentration profiles over this range.
- Pamidimukkala, K. M.,Kern, R. D.,Patel, M. R.,Wei, H. C.,Kiefer, J. H.
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p. 2148 - 2154
(2007/10/02)
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- Endothermic Proton Transfer Reactions from Three +. Isomers
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A triple quadrupole mass spectrometer was used to establish the proton affininties of phenyl, CH3CCCCCH2., and HCCCH2CH.CCH radicals as 870 +/- 29, 824 +/- 25, and 757 +/- 21 kJ mol-1, respectively, from the kinetic energy of benzene, 2,4-hexadiyne, and 1,5-hexadiyne molecular ions at which the onset of proton transfer to less basic species occurs in the second rod assembly.These values were confirmed by other triple quadrupole experiments involving bracketing of exothermic proton transfers.
- Kinter, Michael T.,Bursey, Maurice M.
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p. 775 - 778
(2007/10/02)
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