- C-C and C-H Bond Splits of Laser-Excitated Aromatic Molecules. 1. Specific and Thermally Averaged Rate Constants
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Toluene, m-,o-,and p-xylene, mesitylene, ethyl-, isopropyl-, and tert-butylbenzene were irradiated by nanosecond laser flashes at 193 nm.After fast internal conversion to the electronic ground state, the molecules dissociate by C-C or C-H bond splits.The
- Brand, U.,Hippler, H.,Lindemann, L.,Troe, J.
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- Isolation and characterization of the triradical 1,3,5-trimethylene-benzene
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The highly symmetrical quartet triradical trimethylenebenzene has been isolated for the first time (see structure). Despite its open-shell character, it was photo-chemically stable and this makes it a promising building block for magnetic materials. (Figure presented).
- Neuhaus, Patrik,Sander, Wolfram
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experimental part
p. 7277 - 7280
(2010/12/19)
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- Kinetics of the reaction of the TEMPO radical with alkylarenes
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The kinetics of the reaction of the stable radical 2,2,6,6- tetramethylpiperidine-N-oxyl (TEMPO) with a series of alkylarenes containing primary and secondary benzyl C-H bonds was studied by ESR, and the reaction rate constants were determined. The scheme
- Opeida,Matvienko,Bakurova,Voloshkin
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p. 900 - 904
(2007/10/03)
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- Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes
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The reactions of the photoexcited aromatic ketones, xanthone and 1-azaxanthone, with polyalkylbenzene donors yields the corresponding ketyl radicals as detected by nanosecond laser flash photolysis. On the basis of formation of these photoreduced products, the quenching of the photoexcited species is expected to occur either by a one-step hydrogen abstraction from the donor, electron transfer followed by proton transfer from the donor, or by formation of a charge-transfer type encounter complex prior to hydrogen atom transfer. The reactions of triplet xanthone and triplet 1-azaxanthone with polyalkylbenzene donors in acetonitrile were investigated to probe the effect of the nature of the triplet state and the redox properties on the relative importance of each quenching pathway. Determination of bimolecular rate constants, as well as analysis of kinetic isotope effects and ketyl radical yields, suggests that for both xanthone and 1-azaxanthone the quenching process is dominated by formation of charge-transfer encounter complexes between excited-state aromatic ketone acceptor and ground-state polyalkylbenzene donor. The reactivites of the xanthone π,π* triplet and 1-azaxanthone n,π* triplet toward these donors is shown to be governed by their reduction potentials, with their electronic configuration being unimportant to the kinetics of encounter complex formation. The only exception to this is found when sterically encumbered polyalkylbenzene donors are employed. Results with these compounds suggest that π,π* and n,π* states form encounter complexes of different structure which affects their ability to react with hindered donors. Additionally, product yields with all of the donors are controlled by both the extent of charge transfer within encounter complexes and the encounter complex structure.
- Coenjarts,Scaiano
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p. 3635 - 3641
(2007/10/03)
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- Laser photolysis investigation of induced quenching in photoreduction of benzophenone by alkylbenzenes and anisoles
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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
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p. 861 - 866
(2007/10/03)
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- The mechanism of formation of m-xylylene type biradicals produced by photolysis of polymethyl benzenes or dihalomethyl benzenes
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The mechanism of formation of the mesitylylene biradical (3) produced by short-wavelength photolysis of matrix-isolated mesitylene (1) has been investigated. The data rule out a mechanism involving the sequential formation of the biradical 3 via photolysi
- Haider,Migirdicyan,Platz,Soundararajan,Despres
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p. 733 - 738
(2007/10/02)
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- Pyrolysis of Alkyl Benzenes. Relative Stabilities of Methyl-Substituted Benzyl Radicals
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Decomposition rates for β C-C bond scission in several methyl-substituted ethylbenzenes were measured in a very low pressure pyroly (VLPP) system from 1050 to 1200 K, and relative stabilities of the resulting substituted benzyl radicals were derived.Relative to ethylbenzene, rates were highest for o-methyl-substituted ethylbenzenes, and little affected by meta and para substitution.Activation energy differences were approximately separable into ortho and nonortho contributions, amounting to a lowering of the activation energy, relative to ethylbenzene, of 1.3-1.7 kJ/mol per m- or p-CH3, and 5.0-6.3 kJ/mol per o-CH3.Rate differences were explained in terms of a partial relaxation of steric interaction in the activated complex.Methyl inductive effects were found to be very small, amounting to a decrease of 1.7 kJ/mol per CH3 in the Gibbs energy of formation of the radicals produced.
- Barton, Barrie D.,Stein, Stephen E.
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p. 2141 - 2145
(2007/10/02)
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