- TUNNELING PARAMETERS FOR THE HYDROGEN ATOM ABSTRACTION REACTIONS OF DIPHENYLCARBENE IN A LOW TEMPERATURE TOLUENE MATRIX
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Tunneling reaction rate constants of diphenylcarbene in a toluene matrix can be fit by an asymmetric Eckart barrier.The barrier heights are in good agreement with theory.
- Wright, Bradford B.,Senthilnathan, V. P.,Platz, Matthew S.,McCurdy, C. W.
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- Dissociation Rate Constants of Alkylbenzenes from Hot Molecules Formed by 158-nm (F2 Laser) Irradiation
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Toluene, p-xylene, deuterated p-xylene, and mesitylene under low-pressure conditions as well as p-xylene under high-pressure conditions were studied with a 158-nm laser.Rise curves of the photoproducts were observed by the method of nanosecond laser photolysis.The ground-state alkylbenzenes with an internal energy of 8 eV are formed by internal conversion following the laser excitation.The alkylbenzenes dissociate to the corresponding benzyl-type and phenyl-type radicals via the hot molecule state.The dissociaton rate constants were 9.3 (+/- 0.7) x 107 s-1, 1.5 (+/- 0.3) x 107 s-1, 4.6 (+/- 0.4) x 106 s-1, and 3.0 (+/- 0.9) x 106 s-1, respectively.Dissociation rate constants of the C-H bond in the methyl group of the alkylbenzenes were estimated from these results and compared with previous predictions.
- Shimada, Tetsuya,Ojima, Yuichi,Nakashima, Nobuaki,Izawa, Yasukazu,Yamanaka, Chiyoe
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- Nanosecond laser flash photolysis and steady-state photolysis studies of benzyltrimethylsilane and trimethylsilyldiphenylmethane
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Flourescence measurements and nanosecond laser flash photolysis were carried out to study the photophysical and photochemical processes of benzyltrimethylsilane and trimethylsilyldiphenylmethane at room temperature. A significant solvent effect was observed for the laser flash photolysis of benzyltrimethylene; in methanol, absorption bands due to benzyl-type radicals and the triplet were seen, while only T-T absorption was observed in cyclohexane. No solvent effect was found for the laser flash photolysis of trimethylsilyldiphenylmethane. A transient absorption attributable to the 1,3-trimethylsilyl-shifted intermediate was seen with a lifetime of ca. 27 ms in both solvents, and no absorption band ascribable to diphenylmethyl radicals or the triplet was observed. GC-MS spectra of photoproducts caused by trimethylsilyldiphenylmethane were consistent with the assignment of the transient absorption. Results were explained in terms of the computed molecular structure for the excited singlet states of the molecules using the PM3 method.
- Hiratsuka, Hiroshi,Kobayashi, Satoshi,Minegishi, Takeshi,Hara, Masaki,Okutsu, Tetsuo,Murakami, Shinji
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- Kinetics and thermochemistry of the reaction of benzyl radical with O2: investigations by discharge flow/laser induced fluorescence between 393 and 433 K
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The kinetics of the reaction of the benzyl radical with molecular oxygen has been studied between 393 and 433 K. The Discharge Flow technique with detection of benzyl radicals by Laser Induced Fluorescence in their visible absorption band has been used. A
- Elmaimouni,Minetti,Sawerysyn,Devolder
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- MOLECULAR ORBITAL CORRELATION OF ARYLMETHYL CHLORIDE ATOM ABSTRACTION BY STANNYL RADICALS
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A series of monochloromethylated polycyclic benzenoid hydrocarbons have been treated with triphenyltin hydride and AIBN initiator at 70 deg C.Under these conditions the only observable reaction is reduction of the starting materials to methylarenes.A sign
- Soppe-Mbang, Helene,Gleicher, Gerald Jay
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- Shock tube determination of the overall rate of NH2 + NO → products in the thermal De-NOx temperature window
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The rate coefficient of the reaction NH2 + NO → products (R1) was determined in shock tube experiments using frequency-modulation absorption spectroscopy for detection of NH2. Because of the sensitivity of the diagnostic system, very low reactant concentrations could be employed in order to reduce the influence of secondary reactions on the NH2 profiles. Benzylamine, C6H5CH2NH2, was used as a thermal source of the NH2 radicals in the experiments. To determine the reaction rate, a perturbation strategy was employed that is based on changes in the NH2 profiles when NO is added to the C6H5CH2NH2/Ar mixtures. The measured NH2 profiles were interpreted by detailed kinetic modeling to obtain the overall reaction rate of R1 in the temperature range 1262-1726 K. The lower temperature limit of the present study is in the middle of the Thermal De-NOx temperature window. The present rate measurements are consistent with both our previous determination of the rate at higher temperatures and lower temperature data. A rate expression obtained by combining our higher temperature data and lower temperature data is. k1 = 6.83 × 1015 T-1.203 e106/T(K) cm3 mol-1 s-1 for the temperature range 200-2500 K. The estimated uncertainty of the rate coefficient is ±20%.
- Song,Hanson,Bowman,Golden
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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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- C-C and C-H Bond Splits of Laser-Excitated Aromatic Molecules. 3. UV Multiphoton Excitation Studies
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Parent disappearance and fragment formation yields are measured in the UV multiphoton excitation of cycloheptatriene, toluene, and ethylbenzene.The dependence on laser fluence and bath gas pressure is recorded.The results are interpreted in terms of a com
- Hippler, H.,Riehn, Ch.,Troe, J.,Weitzel, K.-M.
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- The Formation of Gas Phase Benzyl Radicals during the Reaction of Toluene and Nitrous Oxide over Li-MgO and Sr-La2O3 Coupling Catalysts
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Surface-generated gas-phase benzyl radicals have been detected during the reaction of toluene and nitrous oxide over Li-MgO and Sr-La2O3 catalysts.
- Xu, Mingting,Lunsford, Jack H.
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- Photochemical processes of benzyltrimethylsilane at 77 K. Remarkable solvent effects and reaction mechanism
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Photophysical and photochemical processes of benzyltrimethylsilane have been studied in 3-methylpentane and ethanol glass at 77 K. Remarkable solvent effects were observed in the UV photolysis: in ethanol glass, benzyl radicals were produced, while in 3-methylpentane glass α-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations were produced. It was confirmed that the α-trimethylsilylbenzyl radicals and benzyltrimethylsilane radical cations were formed via the lowest triplet state and the benzyl radicals via the excited singlet state or higher triplet state. The molecular structure of the excited-state benzyltrimethylsilane has been studied by MNDO-PM3 calculations. Based on the results, a possible mechanism involving the excited state of radical pair character has been proposed for the photochemical process of benzyltrimethylsilane.
- Hiratsuka, Hiroshi,Kadokura, Yukihiro,Chida, Hiroyuki,Tanaka, Mieko,Kobayashi, Satoshi,Okutsu, Tetsuo,Oba, Makoto,Nishiyama, Kozaburo
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- Pulse radiolysis studies on the fragmentation of arylmethyl quaternary nitrogen mustards by one-electron reduction in aqueous solution
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The radical species formed on the reaction of eaq- with a series of aryl compounds, all containing a N,N-bis(2-chloroethyl)-N-methylammoniomethyl substituent, have been studied in neutral aqueous solutions by pulse radiolysis using optical absorption spectrophotometry. The benzene, 1, and 4-methylsulfonylbenzene, 2, derivatives fragmented immediately to yield different amounts of benzyl radicals, but the 2-nitrobenzene, 3, and 4-nitrobenzene, 4, derivatives were reduced to long-lived radical anions which decayed by bimolecular processes. These nitrobenzene derivatives differed from the corresponding benzyl halides in that they did not fragment to benzyl radicals. Similarly, no evidence was found for the formation of a benzyl-type radical from the radical anion of the 8-nitronaphthalene derivative, 5. However, benzyl-type radicals were produced upon intramolecular electron transfer from the initially formed radical anions of the 4-nitro-5-imidazole, 6, and the 5-nitro-2-pyrrole, 7, derivatives at rate constants of (8.0 ± 1.0) × 103 and (1.0 ± 0.1) × 104 s-1, respectively. The latter heterocyclic analogues provide an approach for release of cytotoxic tertiary amines via enzymatic or radiolytic reduction in hypoxic regions of tumors.
- Anderson, Robert F.,Denny, William A.,Li, Wenjie,Packer, John E.,Tercel, Moana,Wilson, William R.
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- Reaction of Arylmethanes and Heteroarylmethanes with the Nucleophilic Undecyl Radical
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The relative rates of hydrogen atom abstraction from a series of 13 homoaryl- and heteroarylmethanes by the nucleophilic undecyl radical, thermally generated from lauroyl peroxide, were determined at 70 deg C.A reactivity range of 15 was obtained.Substant
- Mahiou, Belaid,Gleicher, Gerald Jay
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- Carbocations generated under stable conditions by ionization of matrix-isolated radicals: The allyl and benzyl cations
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Carbocations are crucial intermediates in many chemical reactions; hence, considerable effort has gone into investigating their structures and properties, for example, in superacids, in salts, or in the gas phase. However, studies of the vibrational structure of carbocations are not abundant, because their infrared spectra are difficult to obtain in superacids or salts (where furthermore the cations may be perturbed by counterions), and the generation of gas-phase carbocations in discharges usually produces several species. We have applied the technique of ionizing neutral compounds by X-irradiation of cryogenic Ar matrices to radicals embedded in such matrices, thus producing closed-shell cations that can be investigated leisurely, and in the absence of counterions or other perturbing effects, by various forms of spectroscopy. This Article describes the first set of results that were obtained by this approach, the IR spectra of the allyl and the benzyl cation. We use the information obtained in this way, together with previously obtained data, to assess the changes in chemical bonding between the allyl and benzyl radicals and cations, respectively.
- Misic, Vladimir,Piech, Krzysztof,Bally, Thomas
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- Liquid Xenon as a Solvent for E.S.R. Studies
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Liquid xenon is a useful inert solvent for e.s.r. studies of reactive free radicals; the g-factors of many types of radical are not the same in xenon as in normal solvents.
- Cook, Malcolm D.,Roberts, Brian P.
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- Effect of medium on the rate constant of decarbonylation of phenylacetyl radical
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The rate constant kCO of decarbonylation of phenylacetyl radicals generated by photolysis of dibenzyl ketone was measured by laser flash photolysis technique in six solvents in a wide temperature range. The pre-exponential factors A and activation energies Ea of decarbonylation were found for all solvents. The kCO value decreases with an increase in the dielectric constant ε of the solvent, whereas an increase in the ability of the solvent for hydrogen bonding increases kCO. The results of quantum-chemical calculations confirm the mutual compensation of the contributions of specific and nonspecific solvations to the activation energy of decarbonylation in alcohols.
- Tsentalovich,Kurnysheva,Gritsan
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- Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. 41. Diethoxyphosphonyl Radicals
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Rate constants for the abstraction of halogen atoms by diethoxyphosphonyl radicals from some organic halides have been determined by EPR spectroscopy using competitive methods.Some typical rate constants (M-1 s-1 units) at ambient te
- Anpo, M.,Sutcliffe, R.,Ingold, K. U.
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- Collisional deactivation of vibrationally highly excited benzyl radicals
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The collisional deactivalion of vibrationally highly excited benzyl radicals in the ground electronic state has been investigated. Vibrationally excited benzyl radicals have been generated using the fast dissociation of vibrationally excited ethylbenzene prepared from UV absorption followed by fast internal conversion. Subsequent to complete collisional deactivation the benzyl radicals have been reexcited by the absorption of a further UV-photon also followed by fast internal conversion. Once again the collisional deactivation of the benzyl radicals has been monitored by time-resolved UV absorption spectroscopy. Average energies transferred per collision have been determined by comparison of absorption-time profiles with different initial vibrational energies. It was found that the rate of collisional deactivation of the open shell benzyl radical is almost identical to the rate of collisional deactivation of the closed shell hydrocarbons of similar size. WILEY-VCH Verlag GmbH, 1997.
- Damm,Deckert,Hippler
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- Pulse Radiolysis of Aqueous Solutions of Benzyltrialkylammonium Cations. Reactions with the Primary Transients from Water Radiolysis
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The primary reactions of hydrogen atoms, hydrated electrons, hydroxyl radicals, and oxide radical ions with benzyltrimethylammonium cations in aqueous solutions have been studied by the technique of pulse radiolysis.Hydrogen atoms react with the rate constant (2.0 +/- 0.1) X 109 M-1 s-1.The cyclohexadienyl-type radical decayed in a second-order process with a rate constant 2k = (4.8 +/- 0.2) X 109 M-1 s-1.Hydrated electrons react with a rate constant (4.3 +/- 0.2) X 109 M-1 s-1 to form benzyl radical.It has been concluded that irradiation of benzyltrimethylammonium cations results in deamination brought about by hydrated electrons.Hydroxyl radicals react with benzyltrimethylammonium cation with a rate constant (5.0 +/- 0.6) X 109 M-1 s-1 to form the cyclohexadienyl-type radical HOC6H5CH2N+(CH3)3 (ca.65percent) and C6H5C.HN+(CH3)3 (ca.35percent) radicals.The cyclohexadienyl radical has a broad absorption maximum at 325 nm (ε ca. 3300 M-1 cm-1).Oxide radical ions react with the rate constant (5.9 +/- 0.5) X 108 M-1 s-1 to form C6H5C.HN+(CH3)3 with absorption maxima at 260 and 305 nm.With the increasing chain length of the alkyl substituent (as in the benzyltri-n-butylammonium cation) the reaction of hydrogen abstraction from the alkyl groups becomes more important.
- Bobrowski, K.
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- Role of Ionic Processes in the Formation of Free Radicals in γ-Irradiated Glassy Toluene
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Spectrophotometric study has been made on the reaction intermediates generated by γ-irradiation in glassy toluene at 77 K.Benzyl and methylcyclohexadienyl radicals are generated roughly in equal amounts, and the total yield (G value) of the radicals is 0.19.The addition of biphenyl results in the formation of biphenyl anion and cation and depletes the radical yield by 40 percent.The results indicate that the excited-state toluene, the precursor of the radicals, is generated by recombination of ions as well as by the direct effect of the radiation on toluene.
- Noda, Shoji,Ohi, Masami,Ershov, B. G.,Yoshida, Hiroshi
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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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- A shock tube study of benzylamine decomposition: Overall rate coefficient and heat of formation of the benzyl radical
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The decomposition rate of benzylamine (C6H5CH2NH2) and the heat of formation of the benzyl radical (C6H5-CH2) were determined in shock tube experiments combined with RRKM calcula
- Song, Soonho,Golden, David M.,Hanson, Ronald K.,Bowman, Craig T.
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- Homolytic Substitution at Furan and Thiophene: Rate Constants for the Formation and Decay of the Radical Intermediates
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Absolute rate constants for radical addition to thiophene and furan have been measured by using kinetic EPR spectroscopy and laser flash photolysis.Additions of phenyl, diethoxyphosphonyl, and triethylsilyl radicals are rapid and have rate constants > 106M-1s-1 at 25 deg C in hydrocarbon solvent.Comparison with data for the analogous reactions with benzene and 1-hexene shows that the heterocyclic compounds are generally less reactive than 1-hexene but are substantially more effective as radical scavengers than benzene.
- Burkey, T. J.,Griller, D.,Lunazzi, L.,Nazran, A. S.
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- Spectroscopic evidence of α-methylbenzyl radical in the gas phase
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We report the observation of the spectroscopic evidence of the α-methylbenzyl radical in a corona excited supersonic expansion using a pinhole-type glass nozzle for the first time. The precursors, toluene, ethylbenzene, and isopropylbenzene, seeded in a l
- Lee, Gi Woo,Ahn, Hyeon Geun,Kim, Tae Kyu,Lee, Sang Kuk
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- Absolute Rate Constants for the Decarbonylation of the Phenylacetyl Radical
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The rate of α scission of the phenylacetyl radical has been determined by laser flash photolysis in isooctane, methanol and tetrahydrofuran.There is no significant solvent effect on the rate.In isooctane, the rate constant for decarbonylation can be represented by log (k/s-1) = (12.0 +/- 0.3) - (6.9 +/- 0.4)/2.3RT kcal/mol.
- Lunazzi, L.,Ingold, K. U.,Scalano, J. C.
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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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- Specific rate constants for the fragmentation of vibrationally excited benzyl radicals
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The competition between collisional deactivation and fragmentation of vibrationally highly excited benzyl radicals in the ground electronic state has been investigated. Vibrationally highly excited benzyl radicals have been prepared from the dissociation of ethylbenzene after UV photon excitation followed by rapid internal conversion. Subsequently, the benzyl radicals are collisionally deactivated in collisions with the bath gas, argon. After selected delay times the benzyl radicals were excited again by absorption of another UV-photon, followed by fast internal conversion. A certain fraction of the reexcited benzyl radicals decomposes while the rest is collisionally stabilized. The total loss of benzyl radicals due to reexcitation has been monitored by UV absorption spectroscopy. Absolute values of the specific rate coefficients for fragmentation have been derived for excitation energies between 47000 and 53000 cm-1. These specific rate constants agree well with a deconvolution of the thermal high pressure rate constant obtained from previous shock wave experiments.
- Damm,Deckert,Hippler,Rink
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- Thermal Decomposition of Toluene: A Comparision of Thermal and Laser-Photochemical Activation Experiments
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The thermal decomposition of toluene has been reinvestigated in shock waves detecting toluene, benzyl radicals, and benzyl fragment concentrations by UV absorption spectroscopy in the range 190-320 nm.The experiments are interpreted in terms of a dominant toluene dissociation into benzyl radicals + H fragments with a rate constant k1 = 1015.45 +/- 0.2 (exp(-(371.9 +/- 10) kJ mol-1/RT) s-1 (at = (2.6 +/- 0.4) * 10-4 mol cm-3) and in terms of a faster benzyl fragmentation which is in agreement with experiments on other benzyl sources.The derived thermally averaged rate constants of toluene dissociation k1 are consistent with specific rate constants k(E,J) from laser excitation experiments.Similarly, the nature of the dominant product channel is consistently identified in thermal and photochemical experiments, ruling out a recently discussed dominance of fragmentation into the phenyl + methyl channel.
- Brouwer, L. D.,Mueller-Markgraf, W.,Troe, J.
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- RELATIVE KINETICS OF THE RADICAL ADDITION OF BENZYL BROMIDE TO UNSATURATED COMPOUNDS
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The relative rate constants of the addition of the C6H5CH2 radical to unsaturated compounds CH2=CHX (X = C4H9, SiMe3, CF3, CO2Me, CN) were determined under the conditions of initiation by the Fe(CO)5 + DMF system or by benzoyl peroxide.Depending on the values of the relative addition rate constants, the monomers can be arranged into the following series (X): CF3 ca.C4H9 SiMe3 CO2Me CN.The relatively nucleophilic character of the benzyl radical has been shown.It was found that under the initiation conditions by the Fe(CO)5 + DMF system, the addition stage proceeds by a free radical mechanism. Keywords: radical addition, relative kinetics, benzyl bromide, benzyl radical, unsaturated compounds, iron pentacarbonyl, dimethylformamide.
- Terent'ev, A. B.,Gapusenko, S. I.,Vasil'eva, T. T.,Vitt, S. V.
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- Bond Homolysis in High-Temperature Fluids
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Rate constants for the homolysis of 1,2-diphenylethane have been determined in tetralin, in dodecahydrotriphenylene, and in the gas phase at temperature above 350 deg C.The least-squares-derived Arrhenius expression for this reaction in the gas phase is k
- Stein, S. E.,Robaugh, D. A.,Alfieri, A. D.,Miller, R. E.
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- Role of Hot Molecules Formed by Internal Conversion in UV Single-Photon and Multiphoton Chemistry
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The UV photochemistry of benzene, alkylbenzene, and olefins in the gas phase is decribed from a new point of view: the hot molecule mechanism.After UV excitation the hot molecule (denoted as S0**) is formed by internal conversion fro
- Nakashima, Nobuaki,Yoshihara, Keitaro
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- Femtosecond time-resolved photoelectron spectroscopy of the benzyl radical
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We present a joint experimental and computational study of the nonradiative deactivation of the benzyl radical, C7H7, after UV excitation. Femtosecond time-resolved photoelectron imaging was applied to investigate the photodynamics of the radical. The experiments were accompanied by excited state dynamics simulations using surface hopping. Benzyl has been excited at 265 nm into the D-band (ππ?) and the dynamics was probed using probe wavelengths of 398 nm or 798 nm. At a probe wavelength of 398 nm a single time constant of around 70-80 fs was observed. When the dynamics was probed at 798 nm, a second time constant τ2 = 1.5 ps was visible, which can be attributed to further non-radiative deactivation to the lower-lying D1/D2 states.
- R?der,Humeniuk,Giegerich,Fischer,Poisson,Mitri?
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p. 12365 - 12374
(2017/07/24)
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- Development of an indicator for the direct visualization of radical intermediates in organic reactions
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A color-indicator based on a derivative of rhodamine amide was developed for the detection of radical intermediates in organic reactions. This derivative showed sensitive color changes for various radical intermediates initiated by chemical reagents or UV
- Yao, Qiuli,Li, Chao-Jun
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p. 11225 - 11228
(2017/10/17)
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- Mesolysis Processes with Benzylic Carbon?Oxygen Bond Cleavage in Radical Anions of Aryl Benzyl Ethers Studied by Electron Pulse Radiolysis in DMF
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Based on transient absorption measurements obtained upon electron-pulse radiolysis of aromatic ethers in N,N-dimethylformamide (DMF), mesolysis processes of the radical anions were investigated. We observed transient absorption spectral change due to the
- Yamaji, Minoru,Tojo, Sachiko,Fujitsuka, Mamoru,Sugimoto, Akira,Majima, Tetsuro
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supporting information
p. 798 - 803
(2016/08/02)
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- Alkyl group dissociation during corona excitation of alkylbenzenes
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Well-resolved vibronic emission spectra were recorded in the visible region from the corona discharge of precursor alkylbenzenes in a technique of corona excited supersonic expansion using a pinhole-type glass nozzle. From the observed spectra, we found t
- Yoon, Young Wook,Lee, Sang Kuk
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experimental part
p. 741 - 745
(2012/01/03)
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- Predicting organic hydrogen atom transfer rate constants using the Marcus cross relation
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Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO ?) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments areshown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2′-6,6′-tetramethylpiperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and α-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
- Warren, Jeffrey J.,Mayer, James M.
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scheme or table
p. 5282 - 5287
(2010/09/10)
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- Reactions of Mn(II) and Mn(III) with alkyl, peroxyalkyl, and peroxyacyl radicals in water and acetic acid
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The kinetics of oxidation of Mn(II) with acylperoxyl and alkylperoxyl radicals were determined by laser flash photolysis utilizing a macrocyclic nickel complex as a kinetic probe. Radicals were generated photochemically from the appropriate ketones in the presence of molecular oxygen. In both acidic aqueous solutions and in 95% acetic acid, Mn(II) reacts with acylperoxyl radicals with k = (0.5-1.6) × 106 M-1 s-1 and somewhat more slowly with alkylperoxyl radicals, k = (0.5-5) x 10 5 M-1 s-1. Mn(III) rapidly oxidizes benzyl radicals, k = 2.3 × 108 M-1 s-1 (glacial acetic acid) and 3.7 × 108 M-1 s-1 (95% acetic acid). The value in 3.0 M aqueous perchloric acid is much smaller, 1× 107 M-1 s-1. The decarbonylation of benzoyl radicals in H2O has k = 1.2 × 106 s -1.
- Jee, Joo-Eun,Bakac, Andreja
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body text
p. 2136 - 2141
(2010/07/05)
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- The role of structural effects on the reactions of alkoxyl radicals with trialkyl and triaryl phosphites. A time-resolved kinetic study
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(Figure presented) A time-resolved kinetic study on the reactions of alkoxyl radicals with trialkyl and triaryl phosphites ((RO)3P: R = Me, Et, i-Pr, t-Bu; (ArO)3P: Ar = C6H5, 2,4-(t-Bu)2C6H3) has been carried out. In the (RO)3P series, the alkoxyl radicals (cumyloxyl (CumO ·) and benzyloxyl (BnO·)) undergo addition to the phosphorus center with formation of intermediate tetraalkoxyphosphoranyl radicals (R′OP·(OR)3: R = Me, Et, i-Pr, t-Bu; R′ = Bn, Cum). The addition rate constants are influenced by steric effects, decreasing on going from R = Me to R = t-Bu and from BnO · to CumO·. Rate constants for β-scission of the phosphoranyl radicals R′OP·(OR) 3 have also been determined, increasing, for a given alkyl group R, in the order R′ = tert-butyl · reacts with triaryl phosphites (ArO) 3P to give phenoxyl radicals, with rate constants that are influenced to a limited extent by substitution of the aromatic rings. The radical scavenging ability of these substrates is briefly discussed.
- Bietti, Massimo,Calcagni, Alessandra,Salamone, Michela
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scheme or table
p. 4514 - 4520
(2010/10/02)
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- Understanding atom transfer radical polymerization: Effect of ligand and initiator structures on the equilibrium constants
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Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22°C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend o
- Tang, Wei,Kwak, Yungwan,Braunecker, Wade,Tsarevsky, Nicolay V.,Coote, Michelle L.,Matyjaszewski, Krzysztof
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scheme or table
p. 10702 - 10713
(2009/02/05)
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- Photolysis of dibenzyl ketones sorbed on MFI zeolites in the presence of spectator molecules: Cage effects, kinetics, and external surface sites characterization
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Over the past two decades, the photolytic reactions of dibenzyl ketones sorbed on zeolites have been investigated. The reported results are consistent with a supramolecular model that takes into account the physical and chemical nature of the structure of
- Moscatelli, Alberto,Liu, Zhiqiang,Lei, Xuegong,Dyer, Joanne,Abrams, Lloyd,Ottaviani, M. Francesca,Turro, Nicholas J.
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experimental part
p. 11344 - 11354
(2009/02/05)
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- FTIR and computational studies of gas-phase hydrogen atom abstraction kinetics by t-butoxy radical
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By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10-16 to 10-14 cm3 molecule-1 s-1 have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic compounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for selected reactions have been measured in the temperature range 299-318 K. Transition states and activation barriers for such reactions have been computed with the help of Gaussian 03 software and found to match very well with the experimental values.
- Li, Shuping,Fan, Wai Yip
-
p. 276 - 280
(2007/10/03)
-
- C-H activation by a mononuclear manganese(III) hydroxide complex: Synthesis and characterization of a manganese-lipoxygenase mimic?
-
Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the p
- Goldsmith, Christian R.,Cole, Adam P.,Stack, T. Daniel P.
-
p. 9904 - 9912
(2007/10/03)
-
- Chemistry of the t-butoxyl radical: Evidence that most hydrogen abstractions from carbon are entropy-controlled
-
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.
-
p. 7578 - 7584
(2007/10/03)
-
- Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols
-
The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO. is 1.36 × 103 L mol -1 cm-1 at λmax 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 ± 0.1 L mol-1 s-1 at 25°C. The reactions of PINO . with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with σ+ (ρ = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with σ (ρ = -0.91). The kinetic isotope effect was also studied and significantly large values of kH/kD were obtained: 25.0 (p-xylene), 27. 1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25°C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d10, the difference of the activation energies, EaD - E aH, was 12.6 ± 0.8 kJ mol-1 and the ratio of preexponential factors, AH/AD, was 0.17 ± 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.
- Koshino, Nobuyoshi,Saha, Basudeb,Espenson, James H.
-
p. 9364 - 9370
(2007/10/03)
-
- Ultrafast Decarboxylation of Carbonyloxy Radicals: Influence of Molecular Structure
-
Experimental and theoretical investigations on the ultrafast photoinduced decomposition of three tert-butyl peroxides of general structure R-C(O)O-O-tert-butyl with R = phenyloxy, benzyl, or naphthyloxy in solution are presented. Photoinduced O-O bond scission occurs within the time resolution (200 fs) of the pump-probe experiment. The subsequent dissociation of photochemically excited carbonyloxy radicals, R-CO2, has been monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R-CO2 is simulated via statistical unimolecular rate theory using molecular energies, geometries, and vibrational frequencies obtained from density functional theory (DFT) calculations. The results are compared with recent data for rerf-butyl peroxybenzoate (R = phenyl). While benzoyloxy radicals exhibit nanosecond to microsecond lifetimes at ambient temperature, insertion of an oxygen atom or a methylene group between the phenyl or naphthyl chromophore and the CO 2 moiety significantly decreases the stability and thus lowers the lifetime of the carbonyloxy radicals in solution to picoseconds. The reasons behind this structural effect on decomposition rate are discussed in terms of barrier heights for decarboxylation on the ground-state potential energy surface and of a fast reaction channel via electronically excited states of carbonyloxy radicals. Arrhenius parameters are reported for thermal rate constants, k(T), of R-CO2 decarboxylation as deduced from modeling of the time-resolved experimental data in conjunction with the DFT calculations.
- Abel, Bernd,Assmann, Jens,Buback, Michael,Grimm, Christian,Kling, Matthias,Schmatz, Stefan,Schroeder, Joerg,Witte, Thomas
-
p. 9499 - 9510
(2007/10/03)
-
- EPR investigation of persistent radicals produced from the photolysis of dibenzyl ketones adsorbed on ZSM-5 zeolites
-
Photolysis of ketones (1, 1-oMe, 2, 2-oMe, 3, and 4) adsorbed on ZSM-5 zeolites produces persistent carbon-centered radicals that can be readily observed by conventional steady-state EPR spectroscopy. The radicals are persistent for time periods of seconds to many hours depending on the supramolecular structure of the initial radical@zeolite complex and the diffusion and reaction dynamics of radicals produced by photolysis. The structures of the persistent radicals responsible for the observed EPR spectra are determined by a combination of alternate methods of generation of the same radical, by deuterium substitution, and by spectral simulation. A clear requirement for persistence is that the radicals produced by photolysis must either separate and diffuse from the external to the internal surface or be generated within the internal surface and separate and diffuse apart. The persistence of radicals located on the internal surface is the result of inhibition of radical-radical reactions. Radicals that are produced on the external surface and whose molecular structure prevents diffusion into the internal surface are transient because radical-radical reactions occur rapidly on the external surface. The reactions of the persistent radicals with oxygen and nitric oxide were directly studied in situ by EPR analysis. In the case of reaction with oxygen, persistent peroxy radicals are formed in high yield. The addition of nitric oxide scavenges persistent radicals and leads initially to a diamagnetic nitroso compound, which is transformed into a persistent nitroxide radical by further photolysis. The influence of variation of radical structure on transience/persistence is discussed and correlated with supramolecular structure and reactivity of the radicals and their parent ketones.
- Turro, Nicholas J.,Lei, Xue-Gong,Jockusch, Steffen,Li, Wei,Liu, Zhiqiang,Abrams, Lloyd,Ottaviani, M. Francesca
-
p. 2606 - 2618
(2007/10/03)
-
- Supramolecular effects on the dynamics of radicals in MFI zeolites: A direct EPR investigation
-
Photolysis of the supramolecular complexes (dibenzyl ketones@ZSM-5) produced supramolecular complexes of benzyl radicals@ZSM-5, which were directly detected by CW-EPR spectroscopy, and provided information on the dynamics of the radicals. The lifetimes of
- Turro, Nicholas J.,Jockusch, Steffen,Lei, Xue-Gong
-
p. 5779 - 5782
(2007/10/03)
-
- Solvent effect on the decarbonylation of acyl radicals
-
The rate constants of the decarbonylation reaction of phenylacetyl and 2-hydroxy-2-methyl-propanoyl radicals were determined in various solvents, e.g., hexane, chloroform, acetic acid, acetonitrile, etc., over a wide temperature range. For the first time, it was shown that the electrostatic interactions with solvent and hydrogen bonding could influence the reaction in different ways: the decarbonylation of phenylacetyl proceeded more slowly in polar solvents while hydrogen bonding decreased the activation barrier and increased the reaction rate constant. The scale of the solvent effect was determined by the change of the radical polarity in the course of the reaction rather than by radical polarity itself. The solvent effect for phenylacetyl was significantly distinct than that for the 2-hydroxy-2-methyl-propanoyl radical of similar polarity. A linear relationship existed between the enthalpy of the reaction of decarbonylation and the barrier to the reaction for most acyl radicals.
- Tsentalovich,Kurnysheva,Gritsan,Kurnysheva,Gritsan
-
p. 3677 - 3682
(2007/10/03)
-
- Absolute rate constants for some reactions of the triethylamineboryl radical and the borane radical anion
-
Laser flash photolysis (LFP) of dj-tert-butyl peroxide or dicumyl peroxide at ambient temperatures in the presence of Et3N→BH3 or BH4- generated the title radicals which were found to have broad, featureless absorptions in the visible region. Rate constants for H-atom abstraction from Et3N→BH3 by cumyloxyl radicals show a small solvent dependence, e.g. 12 × 107 and 2.2 × 107 dm3 mol-1 s-1 in isooctane and acetonitrile, respectively. Rate constants for halogen atom abstraction by Et3N→BH2. and BH3.- from a number of chlorides and bromides were determined by LFP and by competitive kinetics, e.g., for Et3N→BH2. + CCl4/PhCH2Cl/CH3(CH2)2Cl, k = 4.4 × 109/1.1 × 107/5.1 × 105 dm3 mol-1 s-1 and for BH3.- + CCl4/PhCH2Cl, k = 2.0 × 109/3.0 × 107 dm3 mol-1 s-1. Rates of addition of Et3N→BH2. to 1-and 1,1-substituted olefins increase dramatically as the electron affinity of the olefin increases, confirming the nucleophilic character of amine-boryl radicals. A comparison of the present results with literature data for the addition of olefins of four nucleophilic carbon-centered radicals proves that Et3N→BH2. is by far the most nucleophilic radical for which kinetic data are available. A few rate constants for abstraction of hydrogen from electron-deficient carbon by Et3N→BH2. are also reported.
- Sheeller, Brad,Ingold, Keith U.
-
p. 480 - 486
(2007/10/03)
-
- Homolytic dissociation of 1-substituted cyclohexa-2,5-diene-1-carboxylic acids: An EPR spectroscopic study of chain propagation
-
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.
-
p. 1758 - 1764
(2007/10/03)
-
- Entropy control of the cross-reaction between carbon-centered and nitroxide radicals
-
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
-
p. 2849 - 2857
(2007/10/03)
-
- Co-solvent effects on the indirect reduction of alkyl and benzyl halides: Experimental evidence of a link between electron transfer and SN1-like processes
-
The influence of using water as co-solvent in N,N-dimethylformamide on the electron transfer process between electrochemically generated electron donors and alkyl and benzyl halides has been investigated. While the solvent effect in general is modest for
- Jensen, Henrik,Daasbjerg, Kim
-
p. 1251 - 1257
(2007/10/03)
-
- Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes
-
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
-
p. 3635 - 3641
(2007/10/03)
-
- Reactivity of substituted phenols toward alkyl radicals
-
The rate constants for the reaction of primary alkyl radicals with substituted phenolic compounds have been measured in benzene or toluene at room temperature by using the radical clock technique. With three representative phenols, containing in the ortho positions substituents of different size, the kinetics of the hydrogen transfer to alkyl radicals was studied at different temperatures to obtain the corresponding Arrhenius parameters. The kinetic solvent effect on the reaction with α-tocopherol was also investigated in six different solvents behaving as hydrogen bond acceptors, while the reaction with 2,4,6-trimethylphenol and 2,6-di-tert-butylphenol was studied in toluene and γ-valerolactone. For some phenols, the effect of self-aggregation on the kinetic parameters was also studied.
- Franchi, Paola,Lucarini, Marco,Pedulli, Gian Franco,Valgimigli, Luca,Lunelli, Bruno
-
p. 507 - 514
(2007/10/03)
-
- Low-temperature dehalogenation of benzyl halides with atomic magnesium in the 3P state
-
Low-temperature reactions of benzyl halides with magnesium in the 3P state proceed according to the radical mechanism accompanied by abstraction of the halogen atom by the biradical magnesium atom.
- Egorov,Anisimov,Tarakanova
-
p. 147 - 151
(2007/10/03)
-
- Secondary α-Deuterium Kinetic Isotope Effects Signifying a Polar Transition State in the Thermolysis of Ring-Substituted tert-Butyl Phenylperacetates
-
Several ring-substituted tert-butyl phenylperacetates (YC6H4CH2CO3But) and their deuterated versions (YC6H4CD2CO3But) were prepared (Y: p-OCH3, p-CH3, p-H, and p-NO2). Thermolyses at 80°C in CDCl3 showed excellent first-order kinetics. The rates have been measured as kYH × 104 and kYD × 104 s-1: 11.9 and 9.20 (p-OCH3), 2.64 and 2.22 (p-CH3), 1.06 and 0.93 (p-H), 0.164 and 0.156 (p-NO2). Hammett correlations were derived to yield ρYH+ = -1.17 and ρYD+ = -1.12. However, better Hammett plots were obtained with three points (p-OCH3, p-CH3, and p-H) showing ρYH+ = -1.35 and ρYD+ = -1.28. SDKIE was calculated as 1.293 (p-OCH3), 1.189 (p-CH3), 1.140 (p-H), and 1.051 (p-NO2), showing substantial substituent effects. Values of kYH/kYD for p-NO2 showed little temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic entity that is the bond cleavage.
- Kim, Sung Soo,Tuchkin, Alexey
-
p. 3821 - 3824
(2007/10/03)
-
- Structure and reactivity of perfluorinated branched α-ketoradicals
-
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
-
-
- Photoreactions of the decatungstate anion W10O324- with organic substrates in solution studied by EPR and kinetic absorption spectroscopy: An example for the persistent radical effect
-
EPR and kinetic absorption spectroscopy experiments reveal the simultaneous formation of transient radicals and a persistent reduced tungsten radical species during the photoreaction of W10O324- with organic substrates in
- Kothe, Thomas,Martschke, Rainer,Fischer, Hanns
-
p. 503 - 507
(2007/10/03)
-
- Oxidative DNA damage by radicals generated in the thermolysis of hydroxymethyl-substituted 1,2-dioxetanes through the α cleavage of chemiexcited ketones
-
The 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD) highly efficiently damages DNA compared to the merely alkyl-substituted derivative 3,3,4,4-tetramethyl-1,2-dioxetane (TMD). To elucidate this difference in oxidative reactivity, two additional hydroxymethyl-substituted 1,2- dioxetanes, namely cis/trans-3-(hydroxymethyl)-3,4-dimethyl-4-(phenylmethyl)- (lα/1β) and 3-(hydroxymethyl)-4,4-dimethyl-3(phenylmethyl)-1,2-dioxetane (2), were investigated in regard to their photochemical and photobiological properties. The high genotoxic effects of the hydroxymethyl-substituted 1,2- dioxetanes, which are reflected in the significant formation of single- strand breaks in plasmid pBR 322 DNA and the efficient oxidation of guanine in calf thymus DNA and the nucleoside 2'-deoxyguanosine (dGuo), are for the first time understood in terms of radical chemistry. The reactivity order of the dioxetanes 1α/1β > HTMD > 2 >> TMD to damage DNA parallels the propensity of these dioxetanes to generate radicals. These reactive species are formed in the thermolysis of the dioxetanes through α cleavage of the intermediary triplet-excited α-hydroxy- and α-phenyl-substituted carbonyl products. The presence of radicals was confirmed by spin-trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide and by laser-flash photolysis. These carbon-centered radicals are efficiently scavenged by molecular oxygen to produce peroxyl radicals, which are proposed as the active DNA damaging species in the thermal decomposition of the hydroxymethyl-substituted 1,2- dioxetanes HTMD, 1α/1β, and 2.
- Adam, Waldemar,Andler, Simone,Nau, Werner M.,Saha-M?ller, Chantu R.
-
p. 3549 - 3559
(2007/10/03)
-
- Laser photolysis investigation of induced quenching in photoreduction of benzophenone by alkylbenzenes and anisoles
-
The quenching processes of triplet benzophenone (JBP) by alkylbenzenes (AB) and anisole derivatives (AD) in benzene (Bz) and a mixture of acetonitriie (ACT-,) and water (4 :1 v/V; have been studied on the basis of rate constants and efficiencies determined by nanosecond laser flash photolysis a; 355 n m at 295 K. It was found that (1) the deactivation of 3BPby ADs in ACN H2O (4 :1 v/v) was governed by electron transfer (ET) to produce the benzophenone anion (BP'~) and corresponding cation (AD' + ) radicals wiih efficiencies, atj 1 whereas no chemical species were formed in Bz; and 2) photoreduction of 3BPby ABs resulted in benzophenone ketyl radical (BPK) formation by benzylic hydrogen abstraction (HA) with efficiencies XHA 1 in 3z and ACN-H2O (4 :1 v/v). The residual efficiency (a: 1 -ET or ! -aH/1) was attributed to a birnolecular process with no photochemical product, which was named 'induced-quenching (IQf. The quenching rate constants (Jcq) of ;'BPby ADs and ABs were less than the diffusion limits of both Bz and AC1~H2O (4 :1 v/v). The net bimolecular rate constants for the ET, HA and IQ processes were estimated from the k values and efficiencies. The rate constants (%T and k,Q) of ET and IQ with AD versus the oxidation potential (£) of AD followed Rchm-Weller behaviour while logarithmic rate constants {/CHA and ki(j) of HA and IQ by ABs increased linearly with a decrease in the Em of AB. It was suggested, for the deactivation mechanism of 3BPby ABs and ADs (RH), that ;1) the IQ process was intersystem crossing (ISC) enhanced by the partial charge transfer (CT) character of the triplet excipiexes, 3(BP"~- A-RHa + )a,e; (2) radical ion formation by ET might be accomplished in a polar solvent by further CT interaction in the excipiex; (3) the process of BPK formation was inferred to be H-atom transfer in the exciplex, where the more protic H-atom was readily mobile, rather than ET followed by proton transfer and (4) the loss of efficiencies of photochemicalproduct formation was derived not from back ET but from the IQ process, inherent to photoreactions, via triplet excipiexes. The deactivation processes of 3BPby RH are illustrated in Scheme 1. I ET BP'- + RH'(3BP' + RHJcoj -3(BPO- RHg,.-BPK 4 R' BP + RH Scheme 1.
- Oekada, Kafsuji,Yamaji, Minora,Smzuka, Haruo
-
p. 861 - 866
(2007/10/03)
-
- Absolute Rate Constants for Alkoxycarbonyl Radical Reactions
-
PTOC oxalates (anhydrides of an oxalic acid monoester and N-hydroxypyridine-2-thione) were used in laser flash photolysis (LFP) kinetic studies. Irradiation of the PTOC oxalates gave the following alkoxycarbonyl radicals ROC(·)=O: 2a, R = PhCH2; 2b, R = (trans-2-phenylcyclopropyl)methyl; 2c, R = (2,2-diphenylcyclopropyl)methyl. Rate constants for decarboxylation of radical 2a measured by LFP were in agreement with those obtained previously by indirect kinetic methods. Rate constants for decarboxylation of radical 2c were measured by LFP over the temperature range 3-48°C; the reaction is described by log k = 12.2-9.6/2.3RT (kcal/mol). Reactions of Bu3SnH with radical 2c produced from the corresponding phenylseleno carbonate gave inconsistent kinetic results apparently due to the production of small amounts of PhSeH that reacted rapidly with 2c. An approximate rate constant at 2°C for reaction of Bu3SnH with 2c produced from the PTOC oxalate was obtained. Tin hydride reacts with the alkoxycarbonyl radical approximately 1 order of magnitude less rapidly than it reacts with alkyl radicals. Rate constants for 5-exo cychzations of simple alkoxycarbonyl radicals were estimated from previous results and the approximate rate constant for tin hydride trapping; the cyclizations are slightly faster than 5-exo cyclizations of structurally related alkyl radicals.
- Simakov, Pavel A.,Martinez, Felix N.,Horner, John H.,Newcomb, Martin
-
p. 1226 - 1232
(2007/10/03)
-
- Photochemistry of benzyl β-naphthyl sulfoxide and characterization of the β-naphthylsulfinyl radical
-
Photolysis of benzyl β-naphthyl sulfoxide results mainly in α-cleavage. The isomeric sulfenic ester is the major product. Because the triplet energies of both the sulfoxide and sulfenic ester are below that of acetone, in contrast to the previously studied phenyl and tolyl cases, interpretation of sensitization experiments is straightforward. The sulfinyl radical is characterized by transient absorption and a model is proposed to account for its reactivity with nitroxide radicals.
- Guo, Yushen,Darmanyan, Alexandre P.,Jenks, William S.
-
p. 8619 - 8622
(2007/10/03)
-
- Evaluation of dissociation energies of S-H bonds in thiophenols and thioalcohols on the basis of kinetic measurements
-
Kinetic data on the reactions of alkyl and benzyl radicals with thiophenol C6H5SH were analyzed within the framework of the parabolic model of transition state. The values of the parameter that establishes a correlation between the activation energy of a reaction and its enthalpy were calculated for reactions of alkyl and benzyl radicals with the C6H5SH. The equations of the parabolic model were used to calculate the bond dissociation energies for 11 thiophenols and 4 thioalcohols. The activation energies for reactions of 12 thiophenoxy radicals with cumene and of C6H5S? radical with several alkyl-aromatic hydrocarbons were obtained.
- Denisov
-
p. 238 - 241
(2007/10/03)
-
- Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase
-
The rate constants for the termination reaction (2k1) of some resonance stabilized carbon centered radicals (SR.) derived from hydroaromatics (Sr. + SR. -> P) have been determined at 294 +/- 2 K by laser flash photolysis with UV-vis detection.The radicals were generated by hydrogen atom abstraction by t-BuO-radicals from the corresponding hydrocarbon (SRH + t-BuO. -> SR. + t-BuOH. k4).The extinction coefficients (e) of the SR., essential to calculate 2k1, were obtained using a relative kinetic technique.The change in 2k1 for the radicals derived from 1,4-cyclohexadiene, fluorine, 9,10-dihydroanthracene, diphenylmethane, tetralin, indan, indene, and phenol appeared to be modest; a range of 2k1 = 2-10 x 1E9 M-1 s-1 in mixtures of benzene and di-tert-butyl peroxide was observed.Most of the rate constants are near the diffusion controlled limit.In contrast, quenching the radicals with a persistent radical, 2,2,5,5-tetramethylpiperidin-1-oxyl (TEMPO), resulted in a larger variation of -1 s-1.The strength of the N-O bond formed in the latter process may have an important contribution to the observed rate constant.
- Arends, I.W.C.E.,Mulder, P.,Clark, K.B.,Wayner, D.D.M.
-
p. 8182 - 8189
(2007/10/02)
-
- Time-Resolved REMPI Detection of Methyl Radicals Generated in Laser-Induced Unimolecular Reactions
-
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
-
p. 16290 - 16295
(2007/10/02)
-
- Magnetic Field Effects on the Dynamics of Nitroxide-Based Singlet Radical Pairs in Micelles
-
The nitroxide adduct 1-benzyloxy-2,2,6,6-tetramethylpiperidine (3) undergoes rapid photocleavage to yield 2,2,6,6-tetramethylpiperidine N-oxide and benzyl radicals.Laser excitation (248 nm) of 3 in aqueous micellar solution leads to the formation of singlet radical pairs which decay via a combination of separation and geminate processes.The singlet radical pair produced from 3 undergoes moderately slow recombination and as a result allows for the competition between spin evolution and radical escape.Application of an external magnetic field leads to an increase in the rate constant for the geminate radical reaction, reflecting that the magnetic field prevents the interconversion of the singlet radical pair to the unreactive T- and T+ triplet sublevels.In contrast, when the radicals are produced directly on the triplet surface by sensitization, application of a field leads to a slowdown of radical recombination, as normally observed for triplet radical pairs in heterogeneous media.The magnetic field effects are ascribed to the manifestation of the HFI mechanism.The kinetic analyses for the various cases possible are presented in detail.
- Korolenko, E. C.,Cozens, F. L.,Scaiano, J. C.
-
p. 14123 - 14128
(2007/10/02)
-
- Radical-Stabilization-Energy - the MMEVBH Force Field
-
Making use of the VB method of Malrieu et al. a force field has been developed, which allows to calculate heats of formation of hydrocarbons (conjugated and non-conjugated olefins, radicals and diradicals) with high accuracy.With this method radical stabilization energies (RSE) for a great number of delocalized radicals are calculated and compared with experimental values, derived from shock-tube measurements of dissociation energies or from rotational barriers of substituted olefins.A detailed analysis of the RSE with respect to structure, substituents, strain, and aromaticity is presented. - Key Words: Resonance energy / Heats of formation / Single pulse shock tube / Intrisic rotational barrier
- Roth, Wolfgang R.,Staemmler, Volker,Neumann, Martin,Schmuck, Carsten
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p. 1061 - 1118
(2007/10/02)
-