- Laser flash photolysis of 2,2′-dithiobis(pyridine N-oxide): Reactivity of N-oxypyridyl-2-thio radical
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Reaction kinetics of radicals produced by the nanosecond laser flash photolysis of 2,2′-dithiobis(pyridine N-oxide) and related compounds have been studied. The transient absorption band at 360 nm was attributed to the radical in which the unpaired electron mainly localizes on the S atom (N-oxypyridyl-2-thio radical). The reactivities of the radical for conjugated dienes are lower than those of the pyridyl-2-thio radical, suggesting that a considerable unpaired electron density on the S atom delocalizes onto the N-oxypyridine moiety. The addition reaction rate of the radical to the conjugating diene was accelerated with hydrogen-bonding solvents and with addition of the cation, which may stabilize the N+-O- canonical structure, increasing the unpaired electron density on the S atom. By the photolysis of N-hydroxypyridine-2-thione, the N-O bond was predominantly dissociated producing a pyridy1-2-thio radical. By the photolysis of its anion, photoejection took place followed by the N-O bond fission, yielding pyridine-2-thione.
- Alam, Maksudul M.,Watanabe, Akira,Ito, Osamu
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- Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase
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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.
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- Hydrogen atom abstraction by radical cations. The reactions of 9-substituted acridine radical cations with cyclohexa-1,4-diene
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9-Substituted acridine radical cations undergo facile hydrogen atom abstraction reactions with cyclohexa-1,4-diene in dichloromethane-Bu4NPF6 (0.2 M). The kinetics of the reaction were studied by derivative cyclic voltammetry and obs
- Handoo,Cheng,Parker
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- Diffusion controlled hydrogen atom abstraction from tertiary amines by the benzyloxyl radical. the importance of C-H/N hydrogen bonding
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The rate constants for H-atom abstraction (kH) from 1,4-cyclohexadiene (CHD), triethylamine (TEA), triisobutylamine (TIBA), and DABCO by the cumyloxyl (CumO?) and benzyloxyl (BnO ?) radicals were measured. Comparable ksub
- Salamone, Michela,Anastasi, Gloria,Bietti, Massimo,Dilabio, Gino A.
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- Radical-Type Reactivity of the Methylenedimethylsulfonium Ion, (CH3)2S(+)-CH2(.)
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Ab initio molecular orbital calculations at the MP2/6-31G*//6-31G* + ZPVE level of theory suggest that the ion (CH3)2S(+)-CH2(.) is 75 kJ mol-1 higher in energy than ionized ethyl methyl sulfide.However, this distonic ion is stable toward isome
- Smith, Rebecca L.,Chyall, Leonard J.,Stirk, Krista M.,Kenttaemaa, Hilkka I.
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- Energetics of the allyl group
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(Figure Presented) Aiming to improve our understanding of the stability of radicals containing the allylic moiety, carbon-hydrogen bond dissociation enthalpies (BDEs) in propene, isobutene, 1-butene, (E)-2-butene, 3-metylbut-1-ene, (E)-2-pentene, (E)-1,3-pentadiene, 1,4-pentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have been determined by quantum chemistry calculations. The BDEs in cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have also been obtained by time-resolved photoacoustic calorimetry. The theoretical study involved a DFT method as well as ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)). By taking the C(sp3)-H BDE in propene as a reference, we have concluded that one methyl group bonded to C3 in propene (i.e., 1-butene) leads to a decrease of 12 kJ mol-1 and that a second methyl group bonded to C3 (3-methylbut-1-ene) further decreases the BDE by 8 kJ mol-1. When the methyl group is bonded to C2 in propene (isobutene), an increase of 7 kJ mol-1 is observed. Finally, a methyl group bonded to C1 in propene (2-butene) has essentially no effect (-1 kJ mol-1). While this trend can be rationalized in terms of stabilization of the corresponding radical (through hyperconjugation and π-delocalization), the BDE values observed for the dienes can only be understood by considering the thermodynamic stabilities of the parent compounds.
- Agapito, Filipe,Nunes, Paulo M.,Cabral, Benedito J. Costa,Dos Santos, Rui M. Borges,Simoes, Jose A. Martinho
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- Laser flash photolysis study of the photochemistry of 4,5-diaza-9-fluorenone
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The triplet excited state of 4,5-diaza-9-fluorenone (1) shows absorption maxima at 410 and 470 nm and a lifetime of 3 μs, in acetonitrile. Its intersystem crossing quantum yield was determined using 9-fluorenone as a secondary standard and a value of 0.41 ± 0.01 was obtained. The reactivity of the triplet excited state of 1 towards several quenchers, in acetonitrile, was investigated employing the laser flash photolysis technique quenching rate constants ranging from 7.9 × 104 M-1 s-1 (2-propanol) to 1.0 × 1010 M-1 s-1 (triethylamine) were obtained. From the quenching rate constants obtained one can conclude that 4,5-diaza-9-fluorenone has a ππ? triplet excited state. A Hammett plot for the quenching rate constants of triplet 1 by phenols containing polar substituents against σ + gave a reaction constant ρ of -1.54 ± 0.10, which demonstrates the electrophilic character of the 4,5-diaza-9-fluorenone triplet excited state.
- Bertoti, Ada R.,Guimar?es, Alexandre K.,Netto-Ferreira, José Carlos
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- Accurate O-H bond dissociation energy differences of hydroxylamines determined by EPR spectroscopy: Computational insight into stereoelectronic effects on BDEs and EPR spectral parameters
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Differences in O-H bond dissociation enthalpies (δBDEs) between the hydroxylamine of 15N-labeled TEMPONE and 10 N,N-di-tert-alkyl hydroxylamines were determined by EPR. These δBDEs, together with the g and aN values of the derived nitroxide radicals, are discussed in relation to various geometric, intramolecular dipole/dipole, and steric effects and in relation to the results from DFT calculations. We find that dipole/dipole interactions are the dominant factors in dictating aN values and O-H BDEs in all of these structurally similar nitroxides and hydroxylamines, respectively. The importance of including the Boltzmann distribution of conformations for each nitroxide in the aN calculations is emphasized.
- Billone, Paul S.,Johnson, Paul A.,Lin, Shuqiong,Scaiano,Dilabio, Gino A.,Ingold
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experimental part
p. 631 - 636
(2011/03/21)
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- Electronic transitions of protonated benzene and fulvene, and of C 6H7 isomers in neon matrices
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Electronic transitions of protonated benzene (A 1B 2X 1A1, origin at 325 nm) and ±-protonated fulvene (A 1A′X 1A′, at 335 nm) trapped in 6 K neon matrices have been detected. The cations were produced from several different precursors, mass-selected, and co-deposited with neon. After neutralization of the cations, the electronic transitions of cyclohexadienyl (onsets at 549 and 310 nm) and ±-hydrogenated fulvene (532 and 326 nm) radicals were identified. Upon excitation of cyclohexadienyl to the B 2B1 state, photoisomerization to an open-chain structure and ±-hydrogenated fulvene was observed.
- Garkusha, Iryna,Fulara, Jan,Nagy, Adam,Maier, John P.
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experimental part
p. 14979 - 14985
(2011/01/07)
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- Gas-phase reaction of the OH-benzene adduct with O2: Reversibility and secondary formation of HO2
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The reaction of OH radicals with benzene and consecutive reactions of benzene-OH adducts with O2 were studied in the gas phase in N2-O2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 n
- Bohn, Birger,Zetzsch, Cornelius
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p. 5097 - 5107
(2007/10/03)
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- The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
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A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C+OH + R1COR2 → >C=O + R1R2C+OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C+OH + R1CH=CH2 → >C=O + R1C+ HCH3; >R1CH=CH2 + R2C+HCH2R3 → R2C+HCH3 + R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (aldehydes, ketones, and quinones) from the C-H and O-H bonds were compared.
- Denisov
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p. 2110 - 2116
(2007/10/03)
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- ESR study of radical cations from γ-irradiation of bicyclo[3.1.0]hex-2-ene in freon matrices
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Bicyclo[3.1.0]hex-2-ene radical cation, generated by γ-irradiation of the parent compounds in freon matrices at 77 K, undergoes ring opening to the 1,3-cyclohexadiene radical cation, In CF2ClCFCl2 matrix both radical cations also undergo deprotonation to the corresponding neutral radicals.
- Faucitano,Buttafava,Martinotti,Sustmann,Korth
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p. 2223 - 2226
(2007/10/02)
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- Dynamics of H2 elimination from cyclohexadiene
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A comprehensive study of the dynamics of H2 elimination from 1,4 and 1,3 cyclohexadiene is reported.Rotational and vibrational quantum state distributions as well as translation energy distributions for the H2 product are measured.State specific detection of H2 is accomplished with a transform limited vacuum ultraviolet-extreme ultraviolet laser system via (1+1) resonance enhanced multiphoton ionization (REMPI).Rate constants for the H2 elimination and 1,4 to 1,3 isomerization reactions are derived.A (v,J) correlation for H2 with vJ primarily is observed from anisotropy in the Doppler profiles.A clear picture of the transition state configuration of 1,4 cyclohexadiene is provided from the information obtained.
- Cromwell, E.F.,Liu, D.-J.,Vrakking, M.J.J.,Kung, A.H.,Lee, Y.T.
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p. 297 - 307
(2007/10/02)
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- Pulse Radiolysis, Flash Photolysis, and Shock Wave Study of the Recombination H + Benzyl -> Toluene at 300 and 1300-1650 K
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Pulse radiolysis, discharge flash photolysis, and laser flash photolysis have been employed to study the recombination reaction H + benzyl -> toluene at room temperature.Both H atoms and benzyl radicals were monitored.The same reaction was studied directly in shock waves between 1300 and 1650 K.The value of the high-pressure recombination rate constant of (2.5 +/- 0.8) * 1E14 cm3 mol-1 s-1 was found to be independent of the temperature between 300 and 1650 K.It is argued that high pressure rate constants for other recombination reactions should also have verysmall temperature coefficients over wide temperature ranges.Room temperature rate constants for addition of H atoms to toluene, cycloheptatriene, p-xylene, benzene, phenyl, and p-methylbenzyl are also reported.
- Ackermann, L.,Hippler, H.,Pagsberg, P.,Reihs, C.,Troe, J.
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p. 5247 - 5251
(2007/10/02)
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- Dissociation of cyclohexene and 1,4-cyclohexadiene in a molecular beam
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Molecular-beam photofragmentation translational spectroscopy of cyclohexene and 1,4-cyclohexadiene was carried out using 193 nm and IR multiphoton excitation. At 193 nm, both the retro-Diels-Alder reaction of cyclohexene and H2 elimination from both molecules were observed in the ground electronic state, indicating the occurrence of internal conversion from the initially excited electronic states. The retro-Diels-Alder reaction is shown to be concerted up to an internal energy higher than 148 kcal/mol. Hydrogen-atom elimination was also observed for both molecules following 193 nm excitation. The H atom is eliminated from an excited state of cyclohexene and is assigned to be from a carbon adjacent to the double bond, with a corresponding C-H bond energy of 87 ± 3 kcal/mol. It is shown that the peak of the translational energy distribution for concerted dissociation in the ground state is determined mainly by the dynamics of the potential-energy release along the reaction coordinate, and is not sensitive to either the amount of internal energy or the form of excitation.
- Zhao, Xinsheng,Continetti, Robert E.,Yokoyama, Atsushi,Hintsa, Eric J.,Lee, Yuan T.
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p. 4118 - 4127
(2007/10/02)
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- The Reactions of Hydroxyl Radicals with 1,4- and 1,3-Cyclohexadiene in Aqueous Solution. A Pulse Radiolysis and Product Study
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The reactions of radiolytically generated hydroxyl radicals and H atoms with 1,4- and 1,3-cyclohexadiene were studied by pulse radiolysis and product analysis.Hydrogen abstraction from these substrates by the OH radical yields the cyclohexadienyl radical (ε (310 nm) = 4400 dm3 mol-1 cm-1 from the reaction of the H atom with benzene) with an efficiency of 50percent (0.29 μmol J-1) in the case of 1,4-cyclohexadiene and 25percent (0.15 μmol J-1) in the case of 1,3-cyclohexadiene as determined by pulse radiolysis.The remaining OH radicals add to the olefin.In 1,4-cyclohexadiene the yield of the resulting adduct radicals has been determined in a steady-state 60Co-γ-irradiation experiment by reducing it with added 1,4-dithiothreitol (DTT) to 4-hydroxycyclohexene.There are two sites of OH radical attack in the case of 1,3-cyclohexadiene, and only the alkyl radical is reduced quantitatively by DTT (G(3-hydroxycyclohexene) = 0.15 μmol J-1).From material balance considerations it is concluded that the allylic radical must be formed with a G value of 0.28 μmol J-1 but largely escapes reduction by DTT (G(4-hydroxycyclohexene) = 0.03 μmolJ-1).H atom add preferentially to the double bonds of 1,4- and 1,3-cyclohexadiene (78percent and 93percent, respectively), while the O(-) radical (the basic form of the OH radical) undergoes mainly H-abstraction (92percent and 83percent, respectively).The radicals formed in these systems decay bimolecularly (2k = 2.8E9 dm3 mol-1 s-1).In their combination reactions the cyclohexadienyl radicals form the four possible dimers in proportions such that the dienyl radical moiety shows a 2:1 preference to react from its central (1a) rather than from a terminal carbon atom (1b).Cyclohexadienyl radicals and the OH- and H-adduct radicals also cross-terminate by disproportionation and dimerization.Material balance has been obtained for the 1,4-cyclohexadiene system in N2O-saturated solution (E-2 mol dm-3) at a dose rate of 0.14 Gy s-1, the products (G values in μmol J-1) being: benzene (0.085), 4-hydroxycyclohexene (0.25), cyclohexadienyl-dimers (0.144), cyclohexadienyl-OH-adduct-dimers (0.02).Some of the 4-hydroxycyclohexene is formed in an H-abstraction reaction by the OH-adduct radical from 1,4-cyclohexadiene. - Keywords: 1,4-Cyclohexadiene, 1,3-Cyclohexadiene, Hydroxyl Radical, Cyclohexadienyl Radical, Pulse Radiolysis
- Pan, Xian-Ming,Bastian, Eugenie,Sonntag, Clemens von
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p. 1201 - 1205
(2007/10/02)
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- Matrix Absorption Spectra of the Radicals Formed by the Addition of Hydrogen Atoms to Phenylacetylene
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Codeposition of hydrogen atoms from a blind discharge with phenylacetylene in argon at 20 K gave two new products with absorptions at 314.4, 474.7 nm and 346.9, 614.4 nm, respectively.These bands exhibited different photolysis behavior and are grouped acc
- Andrews, Lester,Kelsall, Benuel J.
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p. 1435 - 1440
(2007/10/02)
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- Heats of Formation of Radicals and Molecules by a Photoacoustic Technique
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A photoacoustic technique was used to measure heats of free radical reactions.To illustrate the method, bond dissociation energies were measured for the Sn-H bond in n-Bu3SnH, for the C3-H bond in 1,4-cyclohexadiene, and for the C2-H bond in diethyl ether.Heats of formation of t-BuOPPh3 and t-BuOP(Bu-n)2 were determined, and the method was also used to measure quantum yields for the photolysis of di-tert-butyl peroxide and diphenyl disulfide.
- Burkey, T. J.,Majewski, M.,Griller, D.
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p. 2218 - 2221
(2007/10/02)
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- Formation of D and H Atoms in the Pyrolysis of Benzene-d6 and Chlorobenzene behind Shock Waves
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Dilute mixtures (3-20 ppm) of C6D6 (benzene-d6) were pyrolyzed behind reflected shock waves at temperatures of 1630-1940 K and total pressures of 2-3 atm.Progress of the reaction was followed by analysis for D atoms using resonance absorption spectroscopy.Appearance of D atoms was a first-order process with respect to benzene concentration, and with respect to time during the first part of each experiment.An Arrhenius equation for the formation of D atoms, based on 34 experiments, is kD = 9.7E12 exp(-87100 cal/RT) s-1 with an estimated uncertainty of a factor of 1.5.From measurements of H atoms during pyrolysis of chlorobenzene under similar conditions at 1570-1790 K, the first-order rate constant for the dissociation of chlorobenzene to chlorine atoms and phenyl radicals was found to be k6 = 1.2E14 exp(-90000 cal/RT) s-1, and that for the dissotiation of phenyl radicals to H atoms and other products k3a = 1.2E15 exp(-82000 cal/RT) s-1.With this information, the rate constant for dissociation of benzene-d6 to phenyl-d5 and D atoms was found to be k1D = 4.6E13 exp(-95000 cal/RT) s-1.The rate constant for the exchange reaction H + C6D6 -> C6D5H + D was found to be k4a = 3.2E13 exp(-4200 cal/RT) mol-1 cm3 s-1 over the range 300-1400 K by combining our results with others at lower temperatures.A very simple kinetic model based on a reaction chain with H as carrier can relate our data to other shock-tube work at higher benzene concentrations.
- Rao, V. Subba,Skinner, Gordon B.
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p. 5990 - 5995
(2007/10/02)
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- ELECTRON SPIN RESONANCE AND CHEMICAL STUDIES ON THE 6-(TRIMETHYLSILYL)CYCLOHEXADIENYL AND RELATED RADICALS.
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Various 3,6-disubstituted cyclohexadienyl radicals were generated and investigated by ESR at low temperature. Analysis of the ESR parameters indicated that the significant out-of-plane deformation of the carbon framework of 6-Me//3Si- and 6-Me//3Ge-substituted cyclohexadienyl radicals occurred to gain stabilization due to the effective hyperconjugation between the substituted methylene pseudo- pi orbital and the pi SOMO, while the 6-unsubstituted and 6-t-Bu cyclohexadienyl radicals were essentially planar. Investigation of the fate of the 3,6-bis(trimethylsilyl)cyclohexadienyl radical showed strong evidence for the temperature-dependent reversibility of the silyl radical addition to aromatics. The spontaneous elimination of the trimethylsilyl radical was proved by spin-trapping experiments.
- Kira,Sugiyama,Sakurai
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p. 6436 - 6442
(2007/10/02)
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- Studies on the Spirooctadienyl Radical and the 2-Phenylethyl Rearrangement
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The title radical, 5, has been generated by hydrogen atom abstraction from spiroocta-4,6-diene (4).The radical could not be observed by EPR spectroscopy even at temperatures as low 100 K.Instead, the EPR spectrum of the cyclopropyl ring-opened produc
- Effio, A.,Griller, D.,Ingold, K. U.,Scaiano, J. C.,Sheng, S. J.
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p. 6063 - 6068
(2007/10/02)
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- Rearrangements of free radicals IX : Thermal and photochemical isomerizations of bicyclo(3.2.0)heptenyl radicals
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Bicyclo(3.2.0)heptenyl radicals rearrange in matrix to cyclopentadienyl and cyclohexadienyl radicals.
- Brandes, Dieter,Lange, Fritz,Sustmann, Reiner
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p. 261 - 264
(2007/10/02)
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