Showing papers on "Radical ion published in 1991"
TL;DR: In this article, the rate of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radical ion pairs, depending upon the reaction exothermicity in a manner consistent with the Marcus inverted region.
Abstract: The two primary intermediates that play a major role in determining the efficiencies of bimolecular photoinduced electron-transfer reactions are the contact (A{sup {sm bullet}{minus}}D{sup {sm bullet}+}) and the solvent-separated (A{sup {sm bullet}{minus}}(S)D{sup {sm bullet}+}) radical ion pairs, CRIP and SSRIP, respectively. These two species are distinguished by differences in electronic coupling, which is much smaller for the SSRIP compared to the CRIP, and solvation, which is much larger for the SSRIP compared to the CRIP. The present work addresses the quantitative aspects of these and other factors that influence the rates of energy-wasting return electron transfer within the ion-pair intermediates. The electron acceptor tetracyanoanthracene (TCA) forms ground-state charge-transfer complexes with alkyl-substituted benzene donors. By a change of the excitation wavelength and/or donor concentration, either the free TCA or the CT complex can be excited. Quenching of free {sup 1}TCA{sup *} by the alkylbenzene donors that have low oxidation potentials, such as pentamethylbenzene and hexamethylbenzene, in acetonitrile solution leads to the direct formation of geminate SSRIP. Excitation of the corresponding charge-transfer complexes leads to the formation of geminate CRIP. Rates of return electron transfer within the two types of ion pair are determined from quantum yields for formation of free radicalmore » ions pairs depend upon the reaction exothermicity in a manner consistent with the Marcus inverted region.« less
293 citations
TL;DR: In this article, a good Hammett correlation between the first anodic potential of only p-substituted triarylamines and the σ/σ values has been established.
Abstract: 21 triarylamines (1n – 1z, 1za, 1zb) and triarylamine analogs (2a, 2b, 3a, 3b, 4a, 4b) with substituents in at least all three p positions and some of their cation-radical hexachloroantimonates have been synthesized. The electrochemical behavior has been studied by cyclic voltammetry. Most of the compounds show chemically and electrochemically reversible first oxidation waves in the formation of the cation radicals. With the exception of 4a and 4b, the second wave for the formation of the dication is chemically irreversible. The UV spectra of the triarylamine cation radicals have been obtained in the presence of a slight excess of SbCl5. A good Hammett correlation between the first anodic potential of only p-substituted triarylamines and the σ/σ values has been established. Some redox-catalytic properties of triarylamine cation radicals are described.
144 citations
TL;DR: In this paper, the ultraviolet, visible, and near-infrared absorption spectra of naphthalene (C10H8) and its radical ion, formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K.
Abstract: The ultraviolet, visible, and near-infrared absorption spectra of naphthalene (C10H8) and its radical ion (C10H8/+/), formed by vacuum ultraviolet irradiation, were measured in argon and neon matrices at 4.2 K. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing ion production in the solid phase. The absorption coefficients were calculated for the ion and found lower than previous values, presumably due to the low polarizability of the neon matrix.
118 citations
TL;DR: The hydroxyl radical was found to be the direct active oxygen species that can generate lipid radicals as a result of .OH-mediated hydrogen atom abstraction in the presence of superoxide dismutase or the metal-chelating agent.
105 citations
TL;DR: In this article, cyclic voltammetry was used to investigate the mechanism of the initial step of the electropolymerization of diphenylamine in acetonitrile.
100 citations
TL;DR: In this article, photo-induced catalyzed electron transfer reaction using catalytic amounts of triarylpyrylium tetrafluoroborates as sensitizers and an acid chloride as a trapping agent.
Abstract: Diels-Alder reactions between indole (3) and susbtituted cyclohexa-1,3-dienes (2) can be effected by a photoinduced catalyzed electron transfer reaction using catalytic amounts of triarylpyrylium tetrafluoroborates (1) as sensitizers and an acid chloride as a trapping agent. Irradiation generates N-acyl-1,4,4a,9a-tetrahydro-1,4-ethanocarbazoles in one step. The products are formed with nearly total regioselectivity, such that a substituent in the 1-position of the cyclohexa-1,3-diene is always found in the 1-position of the tetrahydrocarbazole, and a substituent in the 2-position of the diene always appears in the 3-position of the product
90 citations
TL;DR: The electrochemical oxidation of 1,1′-bis(diphenylphosphino)ferrocene (dppf) at a platinum electrode in 1,2-dichloroethane has been studied by cyclic voltammetry, controlled potential electrolysis, and 31P NMR spectroscopy as mentioned in this paper.
88 citations
TL;DR: Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported.
Abstract: In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.
88 citations
TL;DR: In this paper, the reaction efficiency of homo-and heterodisubstituted dihalobenzene radical cations with NH3 was investigated by FT-ICR spectrometry.
Abstract: The reactions of homo- and heterodisubstituted dihalobenzene radical cations with NH3 were investigated by FT-ICR spectrometry. A halogen atom X (X = Cl, Br, I) is substituted in a gas-phase nucleophilic ipso substitution, yielding haloanilinium ions. The reaction efficiency, i.e., the percentage of reactive ion-molecule collisions, ranges from < 0.006% for 1-chloro-4-iodobenzene radical cations to 18% for 1-bromo-2-chlorobenzene radical cations. The reactivity of the halogenated benzene radical cations was found to be governed by two structural parameters. First, the radical cations with a low ionization energy, i.e., iodobenzene and its derivatives, react especially slowly regardless of the reaction exothermicity. Second, the reactivity of all dihalobenzene radical cations is strongly influenced by the substitution pattern. The reactivity of isomeric radical cations is always highest for the 1,2-isomer and lowest for the 1,4-isomer. These results show that the nucleophilic substitution of the halo- and dihalobenzene radical cations by NH3 proceeds by a multistep reaction mechanism with a double-well potential energy surface. The rate-determining step is the addition of NH3 to the aromatic radical cation in the collision complex. The structural parameters influencing this reaction step can be analyzed by the reactivity model of polar reactions of Shaik and Pross. From this the most important feature of the substitution reaction of mono- and dihalobenzene radical cations with NH3 is the different charge localization in reactants and products. This results in a strong influence of the difference of the ionization energies of the halogenated benzene and NH3 and of the dipole moment of the halogenated benzene, i.e., the precursor of the ionic reactant, on the activation energy of the addition step.
67 citations
65 citations
TL;DR: Results show that manganese peroxidase, or physiological concentrations of Mn3+, oxidize only the lower potential congeners, which indicates a possible role for Mn3+ in lignin degradation, as neutral dienyl radicals are proposed to be products of lignIn peroxIDase action.
TL;DR: In this paper, a review of photo-induced electron transfer (PET) processes is presented, showing that contact ion pairs and solvent-separated ion pairs (SSIPs) can be differentiated by various experimental methods and their controlled formation facilitates the control of chemical reactions.
Abstract: Radical ion pairs play an important role in photoinduced electron transfer (PET) processes. Several types of them may exist and it will be shown in this article that contact ion pairs (CIPs) and solvent-separated ion pairs (SSIPs) can be differentiated by various experimental methods. Moreover, their controlled formation facilitates the control of chemical reactions. Although this review is by no means exhaustive, it is hoped that interest in the fundamental mechanistic aspects of PET processes in homogeneous media as well as in their synthetic applications will arise.
TL;DR: In this article, the magnetic field effect (MFE) was monitored with optical methods: both absorption and emission spectroscopy, and discussed with the help of qualitative diagrams that provide an integrated picture of the combination of spin dynamics and molecular dynamics.
Abstract: Experimentally observed temperature-dependent shifts in magnetic-field-dependent reaction yield spectra of polymethylene-linked radical ion pairs are correlated with motional shifts in calculated spin-exchange spectra. The compounds investigated are pyrene-(CH{sub 2}){sub n}-N,N-dimethylaniline, n {le} 16, in the polar solvent acetonitrile. Radical ion pairs have been generated by photoinduced intramolecular electron transfer. The magnetic field effect (MFE) was monitored with optical methods: both absorption and emission spectroscopy. The MFE is explained and discussed with the help of qualitative diagrams that provide an integrated picture of the combination of spin dynamics and molecular dynamics. A through-space (solvent) exchange interaction is assumed in our discussions.
TL;DR: In this article, it was shown that organophosphines (1-4) form charge transfer stabilized exciplex with excited singlet 1 DCN*, and one electron oxidation of phosphines to corresponding radical cation via phosphine was reported.
TL;DR: In this article, a series of extended model thiophene oligomers (nT) have been oxidized in dilute solution by stoichiometric amounts of FeCl 3.
TL;DR: In this paper, the direct and triplet-sensitized photochemistry of diphenylchloronium and bromonium was studied, and the effect of halogen and arene substituent substituents on these interconversions was discussed.
Abstract: The direct and triplet-sensitized photochemistry of diphenylchloronium, diphenylbromonium, bis(4-methylphenyl) chloronium, and bis(4-methylphenyl) bromonium salts was studied. Direct photolysis of the diarylhalonium salts gave haloarene 2-, 3-, and 4-halobiaryls, and acetanilide, predominantly from a heterolytic cleavage mechanism via the aryl cation-haloarene pair, via in-cage recombination and cage-escape reactions. However, triplet-sensitized photolysis gave haloarene, 2-, 3-, and 4-halobiaryl, and reduced arene mainly from homolytic cleavage to the arene radical-haloarene radical cation pair, which also gave in-cage recombination and cage-escape products. There is evidence for singlet-triplet interconversion of the excited states by spin-orbit coupling and interconversion of the singlet-triplet radical pairs by spin-spin coupling. The effect of halogen and arene substituent on these interconversions is discussed
TL;DR: In this paper, it was shown that α,β-amino alcohols undergo a clean C−C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors.
Abstract: The studies presented in this paper show that α,β-amino alcohols undergo a very clean C−C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different α,β-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (>90%), yet with low to medium quantum efficiencies (0.0001−0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of α,β-amino alcohols in the presence of electron acceptors including thioindigo (TI), 9,10Dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN)
TL;DR: In this paper, photoinduced electron transfer/bond cleavage and radical coupling was attributed to the concerted nature of the bond cleavage, close proximity of thearyl π system to the sulfonium moiety, as well as to the favorable thermodynamics for electron transfer.
Abstract: Intramolecular photoinduced electron-transfer concerted bond cleavage has been observed in both a phenylanthracene and a phenylnaphthacene sulfonium salt derivative to produce a singlet phenylaryl cation radical/radical pair. Subsequent, cation radical/radical coupling produces photorearrangement, which appears to be a kinetic process due to the correlation of the product distribution with spin densities in the phenylaryl cation radical. In the phenylanthracene derivative, amide and sulfide products are also formed, presumably from electron transfer from the radical to the cation radical within the solvent cage. The efficacy of the photoinduced electron transfer/bond cleavage and radical coupling is attributed to the concerted nature of the bond cleavage, close proximity of thearyl π system to the sulfonium moiety, as well as to the favorable thermodynamics for electron transfer
Book•
01 Jan 1991
TL;DR: In this article, the authors studied the properties of radical cations in rare gas matrices by Electron Spin Resonance Spectroscopy (ESR) and time-resolved magnetic resonance (MRI).
Abstract: I Cation Radicals.- I.1 Electronic Structure, Spectroscopy, and Photochemistry of Organic Radical Cations.- I.2 Infrared and Optical Absorption Spectroscopy of Molecular Ions in Solid Argon.- I.3 Generation and Study of Inorganic Cations in Rare Gas Matrices by Electron Spin Resonance.- I.4 ESR Studies on Cation Radicals of Saturated Hydrocarbons: Structure, Orbital Degeneracy, Dynamics, and Reactions.- I.5 ESR Studies of Radical Cations Cycloalkanes and Saturated Heterocycles.- I.6 Deuterium Labelling Studies of Cation Radicals.- I.7 Radical Cations of Aliphatic Ethers: A Case Study by ESR and Theoretical Analyses.- I.8 Study of Radical Cations by Time-Resolved Magnetic Resonance.- I.9 Radical Cations in Pulse Radiolysis.- I.10 Ion Pairs in Liquids.- II Anion Radicals and Trapped Electrons.- II.1 Radical Anions in Disordered Matrices.- II.2 Trapped Anions in Organic Crystals.- II.3 Anion Radicals in Inorganic Crystals.- II.4 Trapped Electrons in Disordered Matrices.- II.5 Trapped Electrons in Crystals.- II.6 The Electron as a Reducing Agent: A Topic of a Novel Anion.- III Complex Systems.- III.1 Radical Ions and Their Reactions in DNA and its Constituents: Contribution of Electron Spin Resonance Spectroscopy.- III.2 Conducting Polymers.
TL;DR: Generation of the diquat radical cation by hepatocytes is not affected by the inhibition of cytochrome P-450 by carbon monoxide or metyrapone, suggesting that this enzyme is probably not involved in the reduction of diqu at as had been proposed previously.
TL;DR: In this article, the formation of quinone radical ions was observed at room temperature using pulse radiolysis and the absorption spectra of the radical ions were found in the near UV-VIS range.
Abstract: The formation of quinone radical ions was observed at room temperature using pulse radiolysis. The following quinones were investigated: p-benzoquinone, p-naphthoquinone, 5,8-dihydroxy-1,4-naphthoquinone, anthraquinone and 1,4-dihydroxyanthraquinone. The radical cations were produced in freon-113 solutions and the radical anions were produced in methanol solutions. The absorption spectra of the radical ions were found in the near-UV–VIS range. The spectra of the radical cations were blue shifted compared with the spectra of the corresponding radical anions.
TL;DR: In this article, the quenching rate of the excited triplet state of TPP+ by the alkene is 400 times faster than by dissolved oxygen under the photooxygenation conditions.
Abstract: Triphenylpyrylium tetrafluoroborate (TPP+BF4–) has been found to sensitize oxygenation of adamantylideneadamantane to its dioxetane in good yield. Product studies, measurements of quantum yields for dioxetane formation, laser flash photolysis, and quenching experiments of TPP+ fluorescence by various electron donors have been carried out to elucidate the mechanism for dioxetane formation. Cyclic voltammetry was also used to examine the reactivity of pyryl radicals (TPP˙) with molecular oxygen. The results are consistent with a mechanism involving electron transfer from the alkene to the excited states of TPP+ to give radical pairs consisting of the alkene radical cations and TPP˙. The alkene radical cation reacts with molecular oxygen to afford the dioxetane. A singlet oxygen mechanism is not likely since the quenching rate of the excited triplet state of TPP+ by the alkene is 400 times faster than by dissolved oxygen under the photooxygenation conditions.
TL;DR: Linoleic Acid-derived radicals, which are formed in the reaction of linoleic acid with soybean lipoxygenase, were trapped with nitrosobenzene and the resulting radical adducts were analysed and suggested that the 12,13-epoxylinolesic acid radical is a C-9-centred radical.
Abstract: Linoleic acid-derived radicals, which are formed in the reaction of linoleic acid with soybean lipoxygenase, were trapped with nitrosobenzene and the resulting radical adducts were analysed by h.p.l.c.-e.p.r. and liquid chromatography-thermospray-m.s. Three nitrosobenzene radical adducts (peaks I, II and III) were detected; these gave the following parent ion masses: 402 for peak I, 402 for peak II, and 386 for peak III. The masses of peaks I and II correspond to the linoleic acid radicals with one more oxygen atom [L(O).]. The radicals are probably carbon-centred, because the use of 17O2 did not result in an additional hyperfine splitting. Computer simulation of the peak I radical adduct e.p.r. spectrum also suggested that the radical is carbon-centred. The peak I radical was also detected in the reaction of 13-hydroperoxylinoleic acid with FeSO4. From the above results, peak I is probably the 12,13-epoxylinoleic acid radical. An h.p.l.c.-e.p.r. experiment using [9,10,12,13-2H4]linoleic acid suggested that the 12,13-epoxylinoleic acid radical is a C-9-centred radical. Peak II is possibly an isomer of peak I. Peak III, which was observed in the reaction mixture without soybean lipoxygenase, corresponds to a linoleic acid radical (L.). The 12,13-epoxylinoleic acid radical, 12,13-epoxylinolenic acid radical and 14,15-epoxyarachidonic acid radical were also detected in the reactions of linoleic acid, linolenic acid and arachidonic acid respectively, with soybean lipoxygenase using nitrosobenzene and 2-methyl-2-nitrosopropane as spin-trapping agents.
TL;DR: In this paper, the charge-resonance band of dimer radical cations for three isomers of dinaphthylpropanes and poly(vinylnaphthalene)s was measured by laser photolysis.
Abstract: The charge-resonance band of dimer radical cations for three isomers of dinaphthylpropanes and poly(vinylnaphthalene)s was measured by laser photolysis. The results show that the partially overlapped dimer radical cation is more stable than the fully overlapped one in dinaphtylpropanes. The radical cation in poly(vinylnaphthalene)s gives the same transient absorption spectrum as that of the dimeric model compound. This was assigned to the partially overlapped dimer radical cation. The radical cation was not stabilized by the charge resonance among more than two chromophores
TL;DR: In this paper, an EPR spectrum was recorded at 77 K which can be unequivocally assigned to the radical cation of 1,2,3,4,5,6,7,8,9-octalin.
Abstract: Following adsorption of cyclopentene onto H-mordenite at low temperatures an EPR spectrum was recorded at 77 K which can be unequivocally assigned to the radical cation of 1,2,3,4,5,6,7,8-octahydronaphthalene (9-octalin), since it is identical with the spectrum recorded from an authentic sample of 9-octalin following γ-irradiation in a CFCl3 matrix. This radical shows an interesting dynamic behaviour in terms of a concerted inversion of both rings that modulates the pseudoaxial and pseudoequatorial coupling. On initial adsorption at 195 K the activation energy associated with this process is 14.4 kJ mol–1(actually less than that measured in solution by ODMR) but which surprisingly increases to 28.3 kJ mol–1 on warming the sample to 295 K. This is interpreted in terms of radicals located at differing adsorption sites.A similar adsorption study of dimethylacetylene on this zeolite led, at 150 K, to the detection of a spectrum with an odd number of broadish lines and a spacing of ca. 10 G which we attribute to the formation of the tetramethylcyclobutadiene radical cation. On heating to 350 K, the spectrum of the hexamethylbenzene radical cation was observed.A comparative study of these compounds in freon matrices at varying concentrations was also made in order to probe the chemical properties of their radical cations by an authentic and independent method.