Showing papers on "Radical ion published in 1989"
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01 Jan 1989
353 citations
TL;DR: In this article, an experimental study of the distance dependence of intramolecular, long-distance hole transfer in organic radical cations has been performed, where the aromatic donor and acceptor groups, biphenyl and naphthalene (D and A), were held at fixed distances by five rigid saturated hydrocarbon spacer groups.
Abstract: An experimental study of the distance dependence of intramolecular, long-distance hole transfer in organic radical cations has been performed. The aromatic donor and acceptor groups, biphenyl and naphthalene (D and A), were held at fixed distances by five rigid saturated hydrocarbon spacer groups. The kinetics of the transfer reactions (D{sup +}SA {yields} DSA{sup +}) were monitored by transient absorption of the hole donor, 4-biphenylyl radical cation. Intramolecular hole-transfer rate constants were determined for this process and compared with previous results for electron transfer in radical anions of the identical molecules. A comparison of the hole- and electron-transfer results reveals that the attenuation of the rate constants, and therefore the electronic coupling, as a function of distance is the same for both within the limits of error of the experiment.
144 citations
TL;DR: In this paper, a simple scheme of interconversion of the four unit structures, of which radical cation is related to electrical conduction, is useful to understand the complex interconverting of colors and other physicochemical properties of the polyaniline forms.
129 citations
TL;DR: In this article, the first binuclear semiquinone complexes were obtained as redoxactive yet stable species, which are situated at the borderline between anion radical complexes and metal-centered mixed-valent dimers.
Abstract: Paramagnetic binuclear complexes ((Ru(bpy)/sub 2/)/sub 2/(/mu/-L))/sup 3+/ with L being N,O;N',O'-coordinating 4,7-phenanthroline-5,6-semidione and P,O;P',O'-coordinating 2,5-bis(diphenylphosphino)-p-benzosemiquinone were obtained as redoxactive yet stable species. UV-vis/near infrared spectroscopic results have been analyzed for the three chemically and electrochemically available oxidation states (2+/3+/4+), and the EPR data of the (3+) intermediates show that these first binuclear semiquinone complexes are situated at the borderline between anion radical complexes and metal-centered mixed-valent dimers.
125 citations
TL;DR: The rates of dimethylaniline oxidation were found to correlate with the substrate oxidation-reduction potential within each series of substrates supported by a particular oxygen activation protocol; the kcat for each substrate studied was approximately 20-fold faster in the iodosylbenzene-supported system relative to the NADPH-P-450 reductase supported system.
Abstract: Rates of N-demethylation of N,N-dimethylaniline and of eight meta- or para-substituted N,N-dimethylanilines by rat liver cytochrome P-450PB-B (P-450) were determined under conditions where oxidation was supported by iodosylbenzene or NADPH-P-450 reductase. The rates of dimethylaniline oxidation were found to correlate with the substrate oxidation-reduction potential within each series of substrates supported by a particular oxygen activation protocol; the kcat for each substrate studied was approximately 20-fold faster in the iodosylbenzene-supported system relative to the NADPH-P-450 reductase supported system. Since the N-demethylation of amines is believed to proceed via an initial electron-transfer step, a kinetic scheme for P-450 was proposed that enabled evaluation of the data according to theoretical treatments that correlate rates of electron transfer with extrakinetic parameters. In these analyses, the data could be fitted to the Rehm-Weller and Agmon-Levine equations, providing lambda values (for the energetics of enzyme-substrate reorganization) of 22-26 kcal mol-1 and apparent E1/2 (oxidation-reduction potentials) of 1.7-2.0 V (vs saturated calomel) for the oxidized enzyme. The apparent E1/2 for the enzyme is composed of contributions from the intrinsic potential of the active prosthetic core of the enzyme, the Fe = O - porphyrin species, and a coulombic factor that is a function of the charge-separated radical anion/radical cation pair produced upon electron transfer.(ABSTRACT TRUNCATED AT 250 WORDS)
115 citations
TL;DR: In this article, the ir and Raman spectra of monovalent bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) based charge transfer salts were investigated.
107 citations
81 citations
TL;DR: It is found that ferrous ions can substitute for Fraction b and that the diferric center of metB2 is reduced during anaerobic incubation of the system with reduced flavin and ferrous ion.
73 citations
TL;DR: In this paper, selective desilylation was used to generate α-amineradicals from α-alkyl silylamine radical cation and their intramolecular cyclisation.
68 citations
TL;DR: In this article, the solvent viscosity and polarity dependence of the magnetic field effect in polymethylene-linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH2)n-N,Ndimethylaniline, has been studied.
Abstract: The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene‐linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N‐dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance‐dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.
64 citations
TL;DR: It is found that the intramolecular electron transfer rates and their activation parameters observed for the two azurins correlate well with the distances between the reactive sites, their redox potential, and the nature of the separating medium.
Abstract: The Cu(II) sites of azurins, the blue single copper proteins, isolated from Pseudomonas aeruginosa and Alcaligenes spp (Iwasaki) are reduced by CO2- radicals, produced by pulse radiolysis, in two distinct reaction steps: (i) a fast bimolecular phase, at the rates (50 +/- 08) x 10(8) M-1s-1 (P aeruginosa) and (60 +/- 10) x 10(8) M-1s-1 (Alcaligenes); (ii) a slow unimolecular phase with specific rates of 44 +/- 7 s-1 in the former and 85 +/- 15 s-1 for the latter (all at 298 K, 01 M ionic strength) Concomitant with the fast reduction of Cu(II), the single disulfide bridge linking cysteine-3 to -26 in these proteins is reduced to the RSSR- radical ion as evidenced by its characteristic absorption band centered at 410 nm This radical ion decays in a unimolecular process with a rate identical to that of the slow Cu(II) reduction phase in the respective protein, thus clearly suggesting that a long-range intramolecular electron transfer occurs between the RSSR- radicals and the Cu(II) site The temperature dependence of the internal electron transfer process in both proteins was measured over the 4 degrees C to 42 degrees C range The activation parameters derived are delta H* = 475 +/- 40 and 167 +/- 15 kJmol-1; and delta S not equal to = -565 +/- 70 and -171 +/- 18 JK-1mol-1, respectively Using the Marcus theory, we found that the intramolecular electron transfer rates and their activation parameters observed for the two azurins correlate well with the distances between the reactive sites, their redox potential, and the nature of the separating medium Thus, azurins with distinct structural and reactivity characteristics isolated from different bacteria or modified by site-directed mutagenesis can be used in comparing long-range electron transfer process between their conserved disulfide bridge and the Cu(II) sites
TL;DR: In this article, a detailed discussion of the resonance Raman activity and enhancement is given based on simple molecular orbital considerations, which confirms the planar, quinoidal character of the triplet and radical cation structures.
Abstract: The time‐resolved Raman spectra of the first triplet state—in resonance with the Tn←T1 absorption at 370 nm—and of the radical cation transient—in resonance with the R+.° ←R+.* absorptions at 380 and 690 nm—are reported for biphenyl, fully deuterated biphenyl, p,p’‐bitolyl (4,4’‐dimethyl biphenyl), and m,m’‐bitolyl (3,3’‐dimethyl biphenyl). Complete, reliable vibrational assignments are proposed and a detailed discussion of the resonance Raman activity and enhancement is given based on simple molecular orbital considerations. This investigation confirms the planar, quinoidal character of the triplet and radical cation structures. The structural information available from the analysis of the resonance effects is consistent with the assignment of the 370 nm Tn ←T1 absorption and 690 nm R+.* ←R+. absorption to a φ6←φ3 transition (Dewar’s notation), which causes a decrease of the quinoidal distortion and, particularly, a reduction of the interring CC bond strength. The R+.* ←R+. absorption at 380 nm is ascrib...
TL;DR: The radical cation time-resolved resonance Raman spectra of various isotopic derivatives of N, N-dimethylaniline (DMA), N, D-dimethylp-toluidine (4MDMA) and 3, 5, N, T-tetramethyl-morphine (3,5DMDMA), were reported in the 300-1800 cm−1 range as mentioned in this paper.
Abstract: The radical cation time‐resolved resonance Raman spectra of various isotopic derivatives of N, N‐dimethylaniline (DMA), N, N‐diethylaniline (DEA), N, N‐dimethyl‐p‐toluidine (4MDMA) and 3, 5, N, N‐tetramethylaniline (3,5DMDMA) are reported in the 300–1800 cm−1 range. Excitation was in the weak radical cation absorption around 480 nm. Complete vibrational assignments are proposed. The band activity and the changes in frequency with respect to the neutral molecules are consistent with a quinoidal‐type conformation of the framework close to planarity. Stabilization of this conformation is observed when the phenyl ring contains methyl substituents. The analysis of the Raman enhancements suggests that the quinoidal character of the radical structure is significantly lowered in the resonant excited state. An obvious analogy is found between the spectra of DMA+ ⋅ and of the biphenyl radical cation, which clearly indicates that (i) a nearly common chromophore structure characterizes these two radical cations and (...
TL;DR: In this paper, the ground state, radical cation and the phenoxy radical were used to predict isomer ratios for 3-substituted phenols, based on electron spin density calculations.
TL;DR: In this article, a pronounced solvent viscosity and polarity dependence of the magnetic field effect was found for polymethylene-linked radical ion pairs generated by photoinduced intramolecular electron transfer in compounds of the type pyrene-(CH 2 ) n -N,N-dimethylaniline, with n = 7-16.
TL;DR: The superoxide O 2 radical ion has been generated at the surface of MgO by two different methods, both involving the parallel formation of surface hydroxyls as mentioned in this paper.
Abstract: The superoxide O–2 radical ion has been generated at the surface of MgO by two different methods both involving the parallel formation of surface hydroxyls. EPR spectra at 77 K reveal the presence of only one species with a hyperfine structure due to weak dipole–dipole interaction with a nearby hydroxyl (Bx= 0.37 mT, By= 0.21 mT, Bz= 0.10 mT). The set of B data is satisfactorily reproduced by a simple model for the dipolar interaction envisaging a T-shaped O–2⋯H species. Calculated geometrical features are in excellent agreement with a plausible structure for the superoxide species sitting on low-coordinate Mg2+ and interacting with the hydrogen of an adjacent hydroxyl.
TL;DR: In this article, a beam of vinylidene anions, [CH2C]−•, by xenon gives rise to a stable C2H2 species, having a lifetime ⩾ 0.4 μs.
TL;DR: In this paper, the radical-cation from α-terthienyl, αT, has been characterized using laser flash photolysis techniques, and αT has λmax at 535 nm with e = 29 000 M−1 cm−1.
Abstract: The radical-cation from α-terthienyl, αT⨥, has been characterized using laser flash photolysis techniques. In methanol αT⨥ λmax at 535 nm with e = 29 000 M−1 cm−1. In acetonitrile αT⨥ has λmax at 530 nm, which should be compared with λmax = 445 nm for the triplet state. The triplet is readily quenched by oxygen, but at most 1% of the quenching events lead to radical-cation generation; the reaction is dominated by singlet oxygen formation, which is believed to be responsible for the phototoxicity of αT.
TL;DR: In this article, a statistical mechanics theory is used to describe the time-dependent dynamics of the system and to extract the forward and back-transfer parameters from the data, which enable a detailed description of the electron transfer and recombination dynamics to be given.
Abstract: Electron transfer from an optically excited donor (rubrene) to randomly distributed acceptors (duroquinone) followed by electron back transfer in a rigid solution (sucrose octaacetate) has been studied experimentally. The forward electron-transfer process was observed by time-dependent fluorescence quenching measurements, while the electron back transfer from the radical anion to the radical cation was monitored by use of the picosecond transient grating technique. A statistical mechanics theory is used to describe the time-dependent dynamics of the system and to extract the forward- and back-transfer parameters from the data. The theory includes donor-acceptor and acceptor-acceptor excluded volume. It is found that the inclusion of excluded volume is necessary to obtain accurate transfer parameters. These parameters enable a detailed description of the electron transfer and recombination dynamics to be given. The agreement between theory and experiment is excellent. A variety of time-dependent properties of the system are calculated. These include the time-dependent ion populations and the probability that the ith acceptor is an ion as a function of time and distance. In addition, (R(t)) and ({tau}(t)), which are the average ion separation as a function of time and the average ion existence time as a function of ion separation, respectively, are calculated.
TL;DR: One electron oxidizing agents have been employed to achieve the isomerization of epoxides to ketones as discussed by the authors, which most likely proceed via a radical cation chain electron-transfer mechanism.
TL;DR: In this paper, the relative yields of the cluster ions (NH3)n−1NH+2 and (NH 3)nH+2 produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy, were measured.
Abstract: We have measured the relative yields of the cluster ions (NH3)n−1NH+2 and (NH3)nH+2 produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy. Our observation of a magic number at n=7 in the distribution of (NH3)n−1NH+2 cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH+2 daughter ion and an adjacent NH3 solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n=5 and n=8 in the (NH3)nH+2 cluster ion distribution is consistent with the production of the N2H+8 Rydberg radical cation via an intracluster ion–molecule reaction. Our experimental results are consistent with recent theoretical predictions concerning the structure and stability of N2H+8 ions.
TL;DR: In this article, the decay kinetics of triplet exciplexes (TE) with complete charge transfer (contact radical ion pair type) formed by electron transfer from aromatic amines to the triplet excited states of quinones in benzene, chloroform and 1,1,2,2-tetrachloroethane was studied by laser photolysis.
TL;DR: The photooxidative fragmentation of 1,2-ditertiary amines has been studied in this paper. But it is not shown to be acceptor-dependent and consistent with a mechanism in which the acceptor anion radical acts as a base to promote the donor cation radical in a reaction closely analogous to the two-electron Grob fragmentation.
Abstract: The electron-transfer quenching of excited acceptors by amines and subsequent chemical reactions have been topics of extensive study. Among the various reactions of the photogenerated radical ion pairs, fragmentation via C-C bond cleavage has been shown to be prominent and often a chemically clean path for aminoalcohols, aminoketones, and other systems, recent experiments have shown that the dehydrofragmentation of aminoalcohols is strongly acceptor-dependent and consistent with a mechanism in which the acceptor anion radical acts as a base to promote cleavage of the donor cation radical in a reaction closely analogous to the two-electron Grob fragmentation. The authors report here studies of the photooxidative fragmentation of 1,2-ditertiary amines, a reaction which is probably also quite general and which shows both similarities and yet remarkable contrasts to the aminoalcohol cleavage.
TL;DR: In this article, a beam experiment with unsaturated cyclic cyclic compounds perfluorobenzene (C6F6), perffluorotoluene (c6F5CF3) and perfluorbenzene(c-C5F8) was performed in the energy range 0-20 eV.
IBM1
TL;DR: In this paper, an electron transfer reaction between naphthalene and trifluoromethanesulphonate triflate triflorescence initiates electron transfer reactions, which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide.
Abstract: Irradiation of solutions of naphthalene and triphenylsulphonium trifluoromethanesulphonate initiates an electron transfer reaction which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide. The phenylnaphthalenes are formed by an in-cage reaction between phenyl radical and naphthalene radical cation, whereas diphenyl sulphide is an escape product. A second cage process, oxidation of diphenylsulphide by naphthalene radical cation, generates diphenylsulphinyl radical cation; reaction of this intermediate with phenyl radical gives the phenylthiobiphenyl isomers. However, irradiation of anthracene and triphenylsulphonium triflate yields only phenylated anthracenes and diphenylsulphide, because anthracene radical cation cannot oxidize diphenylsulphide.
TL;DR: In this article, a two-color version of the resonanceenhanced multiphoton dissociation spectroscopy (REMPDS) was used to obtain the first vibrationally resolved spectrum of the dipole-forbidden B (B2)← X (B1) transition of the fluorobenzene radical cation.
TL;DR: In this paper, the authors investigated the kinetics of formation of the radical cation, ABTS.+ at 417 nm and showed that the rate of formation was second-order, first-order each in [ABTS] and [peroxide], except for H2O2 which reacted through Michaelis-Menten kinetics.
Abstract: The kinetics of oxidation of 2,2′-azinobis-(3-ethylbenzothiazole-6-sulphonate) (ABTS) by the inorganic peroxides, peroxomonosulphate, peroxodisulphate, peroxodiphosphate, and hydrogen peroxide were investigated in aqueous solution. The kinetics of formation of the radical cation, ABTS.+, on one-electron abstraction by these peroxides and the further reaction of ABTS.+ with higher concentrations of these peroxides at longer time scale were studied by following the growth and decay of the radical cation, ABTS.+ at 417 nm. The rate of formation of ABTS.+ was found to obey a total second-order, first-order each in [ABTS] and [peroxide], except for H2O2, which reacted through Michaelis-Menten kinetics. All the peroxides investigated were found to react with ABTS.+; however peroxodisulphate alone oxidized ABTS.+ to the dication (ABTS++), the other peroxides reacted via ionic mechanism, probably forming sulphoxide and sulphone as products. The kinetics of decay of the radical cation, ABTS.+, was also found to follow a total second-order, first-order each in [ABTS.+] and [peroxide], except peroxodiphosphate the reaction of which obeyed Michaelis-Menten kinetics. The effect of pH and temperature were also investigated in all the systems and the kinetic and thermodynamic parameters were evaluated and discussed with suitable reaction mechanisms.