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Radical Anion (radical + anion)
Selected AbstractsVoltammetric Study of Nitro Radical Anion Generated from Some Nitrofuran Compounds of Pharmacological SignificanceELECTROANALYSIS, Issue 1 2003S. Bollo Abstract The electrochemical behavior of 2-(5-amino- 1,3,4-oxadiazolyl)-5-nitrofuran (NF359) and its comparison with well-known drugs such as nifurtimox (NFX) and nitrofurazone (NFZ) in protic, mixed and aprotic media by cyclic voltammetry, tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in all media being the reduction of the nitrofuran group the main voltammetric signal. The one-electron reduction couple due to the nitro radical anion formation was visualized in mixed (for NF359 and NFZ) and aprotic media (for all compounds). By applying a cyclic voltammetric methodology we have calculated the decay constants (k2) of the corresponding nitro radical anions in mixed and aprotic media. In mixed medium data fit well with a disproportionation reaction of the nitro radical anion but in aprotic medium fit better with a dimerization reaction. Also, considering cyclic voltammetric measurements in aprotic media we have estimated the reduction potential of the RNO2/RNO2., couple in aqueous medium, pH 7 (E17 values) finding very good correlation with E17 values obtained by pulse radiolysis. Furthermore we have calculated the equilibrium constants from the electron transfer from nitro radical anion to oxygen (kO2) finding that nitro radical anion from NF359 is thermodynamically favored to react with oxygen in respect to both NFZ and NFX. [source] Exchange Interactions at the Supramolecular Level , Synthesis, Crystal Structure, Magnetic Properties, and EPR Spectra of [Mn(MAC)(TCNQ)2] (MAC = Pentaaza Macrocyclic Ligand; TCNQ·, = Radical Anion of 7,7,8,8-Tetracyano- p -quinodimethane)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2003Augustin M. Madalan Abstract The reaction between [Mn(MAC)(H2O)2]Cl2·4H2O and LiTCNQ (MAC = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) affords a complex with the formula [Mn(MAC)(TCNQ)2] (1), whose crystal structure has been determined. Its structure consists of neutral mononuclear entities. The manganese(II) ion is heptacoordinated, with a pentagonal bipyramidal geometry. The apical positions are occupied by the TCNQ·, radicals, while the macrocyclic ligand is coordinated at the equatorial positions. The seven Mn,N distances range from 2.273(3) to 2.301(6) Å. The strong intermolecular ,,, stacking interactions between the TCNQ radicals (3.2 Å) leads to weave-like infinite chains, which propagate along the crystallographic c axis. The cryomagnetic investigation of 1 revealed a weak intermolecular antiferromagnetic coupling of the Mn2+ ions (J = ,0.18 cm,1), which is mediated by the diamagnetic (TCNQ)22, pairs resulting from the stacking interactions in the crystal. The intermolecular exchange interaction between the Mn2+ ions was further confirmed by variable temperature EPR spectroscopic measurements [|J| = 0.15(5) cm,1], which have been carried out in both the X and Q bands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] A Bowl-Shaped ortho -Semiquinone Radical Anion: Quantitative Evaluation of the Dynamic Behavior of Structural and Electronic Features,ANGEWANDTE CHEMIE, Issue 36 2010Akira Ueda Dr. Elegant aufgetischt: Das erste ,schüsselförmige" o -Semichinonradikal (siehe Bild; mit einem Na+ -Ion) wurde synthetisiert und bezüglich seiner elektronischen Eigenschaften charakterisiert. Das dynamische Verhalten (konkav,konvex) wurde mit EPR- und ENDOR/TRIPLE-Messungen sowie DFT-Rechnungen ebenfalls quantitativ untersucht. [source] ChemInform Abstract: Outer-Sphere Oxidation of the Superoxide Radical Anion.CHEMINFORM, Issue 13 2008Ira A. Weinstock Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] The Reduction of Aryl Diethyl Phosphate Esters with Lithium Di-tert-butylbiphenylide Radical Anion: Aromatic Hydrocarbons via the Deoxygenation of Phenols.CHEMINFORM, Issue 31 2005Michael J. Lusch No abstract is available for this article. [source] Dynamics of the Delocalized Charges of a Radical Anion in A,T DNA DuplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 45 2009Ryuhei Yamagami Dr. Gone in the blink of any eye: The transient spectrum of the electron adducts of A,T pairs initially obtained shows that the unpaired electron is localized to the A site (see picture). Subsequently charge transfer from transients (A,T)., to T., occurs over the time range of microseconds. [source] Ring Opening of the Cyclobutane in a Thymine Dimer Radical AnionCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Chryssostomos Chatgilialoglu Dr. Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source] A Radical-Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical AnionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008David Abstract The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N -dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the OO bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be ,0.55±0.15,V and 20±3,kcal,mol,1, respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the OO bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene- cis -1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane- cis -1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a ,-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for ,-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. [source] On the Dissociation of Aromatic Radical Anions in SolutionCHEMPHYSCHEM, Issue 1 2003Damien Laage Abstract A new theoretical formulation is given for the reaction rate and path for the important reaction class of aromatic radical anion dissociation in solution [ArX],.,Ar.+X,, and is illustrated for the case of the cyanochlorobenzene radical anion [CN,Cl],. in dimethylformamide. Among the theory's novel features is the inclusion of the conical intersection aspect of this ground electronic state problem, which is key in allowing the reaction to occur and which has a significant impact on the reaction barrier height. Reasonable agreement with the experimental rate is found. [source] Electrochemical Approach to the Radical Anion Formation from 2,-Hydroxy Chalcone DerivativesELECTROANALYSIS, Issue 5 2006P. Quintana-Espinoza Abstract Three 2,-hydroxy chalcone derivatives were electrochemically reduced to the radical anion by a reversible one-electron transfer followed by a chemical dimerization reaction. Under suitable conditions of the medium, the one-electron reduction produces very well resolved cyclic voltammograms due to the formation of the radical anion. By using appropriately the wide versatility of the cyclic voltammetric technique, was possible to study the generation of the radical anion and its stability. [source] Voltammetric Study of Nitro Radical Anion Generated from Some Nitrofuran Compounds of Pharmacological SignificanceELECTROANALYSIS, Issue 1 2003S. Bollo Abstract The electrochemical behavior of 2-(5-amino- 1,3,4-oxadiazolyl)-5-nitrofuran (NF359) and its comparison with well-known drugs such as nifurtimox (NFX) and nitrofurazone (NFZ) in protic, mixed and aprotic media by cyclic voltammetry, tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in all media being the reduction of the nitrofuran group the main voltammetric signal. The one-electron reduction couple due to the nitro radical anion formation was visualized in mixed (for NF359 and NFZ) and aprotic media (for all compounds). By applying a cyclic voltammetric methodology we have calculated the decay constants (k2) of the corresponding nitro radical anions in mixed and aprotic media. In mixed medium data fit well with a disproportionation reaction of the nitro radical anion but in aprotic medium fit better with a dimerization reaction. Also, considering cyclic voltammetric measurements in aprotic media we have estimated the reduction potential of the RNO2/RNO2., couple in aqueous medium, pH 7 (E17 values) finding very good correlation with E17 values obtained by pulse radiolysis. Furthermore we have calculated the equilibrium constants from the electron transfer from nitro radical anion to oxygen (kO2) finding that nitro radical anion from NF359 is thermodynamically favored to react with oxygen in respect to both NFZ and NFX. [source] Solvent-Mediated Redox Transformations of Ytterbium Bis(indenyl)diazabutadiene ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2005Alexander A. Trifonov Abstract The reactions of diamagnetic [(C9H7)2Yb(THF)2] (2) and [rac -(CH2 -1-C9H6)2Yb(THF)2] (3) with tBuN=CH,CH=NtBu (DAD) in toluene result in the formation of the paramagnetic complexes [(C9H7)2Yb(DAD)] (4) and [rac- (CH2 -1-C9H6)2Yb(DAD)] (5), respectively. The IR, UV/Vis, and 1H NMR spectroscopic data, the magnetic properties, and the single-crystal X-ray diffraction studies of 4 and 5 indicate that in the solid state and in noncoordinating media both complexes are ytterbium(III) derivatives containing the DAD radical-anion, whereas the 1H NMR and UV/Vis spectra of solutions of 4 and 5 in the coordinating solvent THF give evidence for divalent ytterbium. Recrystallization of 4 and 5 from THF/hexane results in the recovery of the starting ytterbium complexes 2 and 3 due to an unusual redox substitution of the radical anion of diazabutadiene by THF in the coordination sphere of ytterbium. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The Electronic Structure of (Diiminopyridine)cobalt(I) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2004Quinten Knijnenburg Abstract DFT calculations show that square-planar LCoIR complexes of a diiminopyridine ligand are best regarded as containing low-spin CoII antiferromagnetically coupled to a ligand radical anion. The lowest triplet state, corresponding to a 3d,,* excitation, is calculated to be only a few kcal/mol above the ground state, and is thermally accessible. The anomalous 1H NMR chemical shifts of the LCoR complexes are suggested to be due to thermal population of the triplet state at room temperature. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Synthesis and Photophysical Properties of a Pyrazolino[60]fullerene with Dimethylaniline Connected by an Acetylene LinkageEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2006Andreas Gouloumis Abstract A new triad based on pyrazolino[60]fullerene and a conjugated dimethylaniline group has been synthesized by a copper-free Sonogashira cross-coupling reaction using microwave irradiation as the source of energy. The electrochemical and photophysical properties of the triad were systematically investigated by techniques such as time-resolved fluorescence and transient absorption spectroscopy. Charge separation via the excited singlet state of the C60 moiety was confirmed in polar and nonpolar solvents and competes with triplet formation of the C60 moiety. The charge-separated state persisted for 91 ns. Such long lifetimes are characteristic of long distances between the radical anion of the pyrazolino[60]fullerene derivative and the radical cation of the dimethylaniline moiety. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Efficient Photosensitized Splitting of Thymine Dimer by a Covalently Linked Tryptophan in Solvents of High PolarityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2005Qin-Hua Song Abstract Tryptophan-thymine dimer model compounds used to mimic the repair reaction of DNA photolyase have been synthesized. The photosensitized cleavage of the dimer by the covalently linked tryptophan is strongly solvent-dependent with the reaction rates increasing in increasingly polar solvents, for example, the quantum yield , = 0.004 in THF/hexane (5:95) and 0.093 in water. The fluorescence of the tryptophan residue is quenched by the dimer moiety by electron transfer from the excited tryptophan to the dimer. Fluorescence-quenching studies indicated that the electron transfer was efficient in polar solvents. The splitting efficiency of the dimer radical anion within the tryptophan·+,dimer·, species is also remarkably solvent-dependent and increases with the polarity of the solvents. The back-electron-transfer reaction in the charge-separated species, which competes with cleavage, was suppressed in polar solvents. These results are in contrast to those of earlier solvent-dependent studies of indole-dimer systems, but they can be rationalized in terms of the differences in the distances between the chromophore unit and the attached dimer. The pH-dependent measurements of the splitting reaction and the deuterium isotope effect showed that the tryptophan radical cation within the charge-separated species does not deprotonate prior to the cleavage of the dimer radical anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Electrochemical, ESR and quantum chemical study of 1-substituted naphthalenes and their radical anions,,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2008N. V. Vasilieva Abstract Electrochemical reduction and oxidation of a series of 1-substituted naphthalenes (1-X-naphthalenes) have been studied by the method of cyclic voltammetry (CV). The first reduction peak of the majority of these compounds corresponds to a one-electron transfer to form the relatively stable radical anion (RA). For these species, ESR spectra have been registered and interpreted, the life time has been estimated. The first oxidation peaks of 1-X-naphthalenes are irreversible and correspond to a transfer of two or more electrons. Copyright © 2007 John Wiley & Sons, Ltd. [source] Radical clocks and electron transfer.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2005Comparison of crown ether effects on the reactivity of potassium, magnesium towards 1-bromo-2-(3-butenyl)benzene. Abstract The reaction of the title precursor of the aryl radical clock 1-bromo-2-(3-butenyl)benzene, 1Br, towards potassium and magnesium in THF was studied in the presence and absence of various additives, at ambient and low temperatures. The additives were cis -dicyclohexano-18-crown-6 or tert -butyl alcohol; the first one to render soluble potassium by forming its alkalide, the second to distinguish carbanionic from radical cyclization. The addition of 1Br to a THF stirred suspension of potassium pieces yields remarkably low amounts of products resulting from radical cyclization, in contrast to the amounts reported by Bunnett and Beckwith's group for the reaction in 67% ammonia,33% tert -butyl alcohol medium. The amount of cyclized products obtained with potassium pieces in THF is in the same range as that observed in the reaction of magnesium with 1Br in THF. This similarity allows us to discard the earlier triad hypothesis that we proposed to account for the unexpectedly low amounts of cyclized products of aryl halides radical clocks in Grignard reagent formation. The addition of crown ethers to the THF reaction medium induces contrasting effects for potassium and magnesium. A distinctive increase in the radical cyclization is observed for potassium, whereas the addition of crown inhibits the formation of Grignard reagent more efficiently when the solvent is diethyl ether than when it is THF. The observed effects are explained by putting in perspective the metal reactive dissolution with elementary steps occurring in the vicinity of a cathode. The reaction of potassium pieces or magnesium turnings is comparable to the heterogeneous electron transfer occurring at a cathode whereas the reaction of potassium in the presence of crown ether is comparable to homogeneous conditions of electron transfer obtained in redox catalysis. A discussion of the dianion hypothesis for the Grignard reaction of aryl halides is provided and the importance of considering the reactivity of reactive metal dissolution (or organic corrosion) in the framework of recent progress made in the modelling of electrode reactivity is emphasized. This paper shows that caution should be taken when radical clocks are used to study reactions at the metal,solution interface. More specifically, the non-observation of rearranged products from the radical clock (even for the very rapid ones) under these conditions does not necessary imply that there is no radical intermediate along the dominant reaction channel. This pattern of reactivity strongly contrasts with that usually observed when radical clocks are used in homogeneous media. The leading parameters in the rearranged/unrearranged products ratio seem to be the time that the reactive species (radical anions) created by the first electron transfer spend in the close vicinity of this surface, the rate constant of rearrangement of the radical formed by the cleavage of the radical anion and the redox properties of this radical. Copyright © 2005 John Wiley & Sons, Ltd. [source] Polymerization of acrylamide photoinitiated by tris(2,2,-bipyridine)ruthenium(II),amine in aqueous solution: Effect of the amine structureJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2001Claudia R. Rivarola Abstract The photopolymerization of acrylamide (AA) initiated by the metallic complex tris(2,2,-bipyridine)ruthenium(II) [Ru(bpy)3+2] in the presence of aliphatic and aromatic amines as co-initiators was investigated in aqueous solution. Aromatic amines, which are good quenchers of the emission of the metal-to-ligand-charge-transfer excited state of the complex, are more effective co-initiators than those that do not quench the luminescence of Ru(bpy)3+2, such as aliphatic amines and aniline. Laser-flash photolysis experiments show the presence of the reduced form of the complex, Ru(bpy)3+1, for all the amines investigated. For aliphatic amines, the yield of Ru(bpy)3+1 increases with temperature, and on the basis of these experiments, a metal-centered excited state is proposed as the reactive intermediate in the reaction with these amines. The decay of the transient Ru(bpy)3+1 is faster in the presence of AA. This may be understood by an electron-transfer process from Ru(bpy)3+1 to AA, regenerating Ru(bpy)3+2 and producing the radical anion of AA. It is proposed that this radical anion protonates in a fast process to give the neutral AA radical, initiating in this way the polymerization chain. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4265,4273, 2001 [source] Extended Car,Parrinello molecular dynamics and electronic g -tensors study of benzosemiquinone radical anion,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005James R. Asher Abstract Car-Parrinello molecular dynamics simulations of benzoquinone and benzosemiquinone radical anion in both aqueous solution and the gas phase have been carried out at ambient conditions. Hydrogen bonding is considerably more extensive to the anionic than to the neutral aqueous system. In addition to the conventional hydrogen bonding to the carbonyl oxygen atoms, T-stacked hydrogen bonding to the , -system is statistically and energetically significant for the semiquinone anion but not for the neutral quinone. EPR g -tensors have been calculated at DFT level for snapshots taken at regular intervals from the gas-phase and solution semiquinone anion trajectories. Different criteria for extraction of semiquinone/water clusters from the solution trajectory give insight into the effects of different interactions on the g -tensor, as does correlation of the g -tensor with statistically significant hydrogen-bond configurations identified along the trajectories. Comparison of gas-phase and solution results indicates opposite directions of direct electronic and indirect structural influences of hydrogen bonding on g -tensors. Short-time oscillations in gx along the trajectory are due mainly to CO bond vibrations. Copyright © 2005 John Wiley & Sons, Ltd. [source] Vitamin B2 -sensitized Photo-oxidation of DopaminePHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2008Walter A. Massad Kinetics and mechanism of the photo-oxidation of the natural catecholamine-type neurotransmitter dopamine (DA) has been studied in aqueous solution, under aerobic conditions, in the presence of riboflavin (Rf, vitamin B2) as a photosensitizer. Results indicate the formation of a weak dark complex Rf,DA, with a mean apparent association constant Kass = 30 m,1, only detectable at DA concentrations much higher than those employed in photochemical experiments. An intricate mechanism of competitive reactions operates upon photoirradiation. DA quenches excited singlet and triplet states of Rf, with rate constants of 4.2 × 109 and 2.2 × 109 m,1 s,1, respectively. With the catecholamine in a concentration similar to that of dissolved molecular oxygen in air-saturated water, DA and oxygen competitively quench the triplet excited state of Rf, generating superoxide radical anion (O2,,) and singlet molecular oxygen (O2(1,g)) by processes initiated by electron and energy-transfer mechanisms, respectively. Rate constants values of 1.9 × 108 and 6.6 × 106 m,1 s,1 have been obtained for the overall and reactive (chemical) interaction of DA with O2(1,g). The presence of superoxide dismutase increases both the observed rates of aerobic DA photo-oxidation and oxygen uptake, due to its known catalytic scavenging of O2,,, a species that could revert the overall photo-oxidation effect, according to the proposed reaction mechanism. As in most of the catecholamine oxidative processes described in the literature, aminochrome is the DA oxidation product upon visible light irradiation in the presence of Rf. It is generated with a quantum yield of 0.05. [source] The first structure of a cold-adapted superoxide dismutase (SOD): biochemical and structural characterization of iron SOD from Aliivibrio salmonicidaACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 2 2009Hege Lynum Pedersen Superoxide dismutases (SODs) are metalloenzymes that catalyse the dismutation of the superoxide radical anion into O2 and H2O2 in a two-step reaction. The crystal structure of the iron superoxide dismutase from the cold-adapted and fish-pathogenic bacterium Aliivibrio salmonicida (asFeSOD) has been determined and refined to 1.7,Å resolution. The protein has been characterized and compared with the closely related homologous iron superoxide dismutase from the mesophilic Escherichia coli (ecFeSOD) in an attempt to rationalize its environmental adaptation. ecFeSOD shares 75% identity with asFeSOD. Compared with the mesophilic FeSOD, the psychrophilic FeSOD has distinct temperature differences in residual activity and thermostability that do not seem to be related to structural differences such as intramolecular or intermolecular ion bonds, hydrogen bonds or cavity sizes. However, an increased net negative charge on the surface of asFeSOD may explain its lower thermostability compared with ecFeSOD. Activity measurements and differential scanning calorimetry measurements revealed that the psychrophilic asFeSOD had a thermostability that was significantly higher than the optimal growth temperature of the host organism. [source] Synthesis and Photoinduced Electron-Transfer Properties of Phthalocyanine,[60]Fullerene ConjugatesCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2008Maurizio Quintiliani Dr. Abstract A series of three novel ZnPc,C60 conjugates (Pc=phthalocyanine) 1,a,c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc,C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy, towards the preparation of 1,a,c, involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1,a,c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc.+ radical cation and the C60., radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1,a). [source] A Radical-Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical AnionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008David Abstract The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N -dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the OO bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be ,0.55±0.15,V and 20±3,kcal,mol,1, respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the OO bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene- cis -1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane- cis -1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a ,-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for ,-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. [source] Ring Opening of the Cyclobutane in a Thymine Dimer Radical AnionCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Chryssostomos Chatgilialoglu Dr. Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source] Ytterbocenes as One- and Two-Electron Reductants in their Reactions with Diazadienes: YbIII Mixed-Ligand Bent-Sandwich Complexes Containing a Dianion of DiazabutadieneCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2007Alexander Abstract Ytterbocene [Yb(C5MeH4)2(thf)2] reacts with diazabutadiene 2,6- iPr2C6H3NCHCHNC6H3iPr2 -2,6 (DAD) as a one-electron reductant to afford a bis(cyclopentadienyl) YbIII derivative containing a DAD radical anion [Yb(C5MeH4)2(dad,.)]. However, ytterbocenes [YbCp*2(thf)2] (Cp*=C5Me5, C5Me4H) coordinated by sterically demanding cyclopentadienyl ligands act as two-electron reductants in their reactions with DAD. These reactions occur by abstraction of one Cp* ring and result in the formation of novel YbIII mixed-ligand bent-sandwich complexes, [YbCp*(dad)(thf)], in which the dianion of DAD has an uncommon terminal ,4 -coordination to the ytterbium atom. The variable-temperature magnetic measurements of complex [Yb(C5Me5)(dad)(thf)] suggest the existence of redox tautomerism for this compound. [source] Carbon Dioxide Activation by Surface Excess Electrons: An EPR Study of the CO2, Radical Ion Adsorbed on the Surface of MgOCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2007Mario Chiesa Dr. Abstract The CO2, radical anion has been generated at the surface of MgO by direct electron transfer from surface trapped excess electrons and characterized by electron paramagnetic resonance spectroscopy. Both 13C and 17O hyperfine structures have been resolved for the first time, leading to a detailed mapping of the unpaired electron spin density distribution over the entire radical anion. The magnetic equivalence of the two O nuclei has been ascertained allowing a side-on adsorption structure at low-coordinate Mg2+ ions to be proposed for the surface stabilized radical. [source] In Situ Spectroelectrochemical Studies on Ladder-Type Oligomers in Solution and the Solid StateCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2006Peter Rapta Dr. Abstract A series of thermally stable fluoranthenopyracylene oligomers with extended , conjugation were studied by in situ ESR-UV/Vis/NIR spectroelectrochemistry with respect to their application in devices such as organic light-emitting diodes and field-effect transistors. The oligomers are both soluble in o -dichlorobenzene and form thin films by evaporation in the temperature range of 300,500,°C in vacuum. Their electrochemical behavior was studied in reduction (n doping) and oxidation (p doping) under standard voltammetric and thin-layer conditions. The HOMO and LUMO energies and the band gaps of all compounds under study were estimated from both electrochemical and UV/Vis/NIR spectroscopic data. The fluorene-type oligomers A2,A6 and B2 bearing flexible alkyl chains exhibit both reversible multistep reductions and oxidations. The spectroelectrochemistry indicates substantial differences in delocalization of the positive and negative charges in these ladder-type oligomers. The formation of doubly charged , dimers was observed for the first time for both the radical anion and radical cation of the same molecule (B1). The redox behavior of the oligomers was studied in the solid state and in solution. [source] Comprehensive Analysis of DNA Strand Breaks at the Guanosine Site Induced by Low-Energy Electron AttachmentCHEMPHYSCHEM, Issue 1 2010Jiande Gu Prof. Dr. Abstract To elucidate the role of guanosine in DNA strand breaks caused by low-energy electrons (LEEs), theoretical investigations of the LEE attachment-induced CO ,-bonds and N-glycosidic bond breaking of 2,-deoxyguanosine-3,,5,-diphosphate (3,,5,-dGMP) were performed using the B3LYP/DZP++ approach. The results reveal possible reaction pathways in the gas phase and in aqueous solutions. In the gas phase LEEs could attach to the phosphate group adjacent to the guanosine to form a radical anion. However, the small vertical detachment energy (VDE) of the radical anion of guanosine 3,,5,-diphosphate in the gas phase excludes either CO bond cleavage or N-glycosidic bond breaking. In the presence of the polarizable surroundings, the solvent effects dramatically increase the electron affinities of the 3,,5,-dGDP and the VDE of 3,,5,-dGDP,. Furthermore, the solvent,solute interactions greatly reduce the activation barriers of the CO bond cleavage to 1.06,3.56 kcal,mol,1. These low-energy barriers ensure that either C5,O5, or C3,O3, bond rupture takes place at the guanosine site in DNA single strands. On the other hand, the comparatively high energy barrier of the N-glycosidic bond rupture implies that this reaction pathway is inferior to CO bond cleavage. Qualitative agreement was found between the theoretical sequence of the bond breaking reaction pathways in the PCM model and the ratio for the corresponding bond breaks observed in the experiment of LEE-induced damage in oligonucleotide tetramer CGTA. This concord suggests that the influence of the surroundings in the thin solid film on the LEE-induced DNA damage resembles that of the solvent. [source] Ionization-Induced Proton Transfer in Model DNA Base Pairs: A Theoretical Study of the Radical Ions of the 7-Azaindole DimerCHEMPHYSCHEM, Issue 12 2004Hsing-Yin Chen Dr. Abstract Proton-transfer reactions of the radical anion and cation of the 7-Azaindole (7AI) dimer were investigated by means of density functional theory (DFT). The calculated results for the dimer anion and cation were very similar. Three equilibrium structures, which correspond to the non-proton-transferred (normal), the single-proton-transferred (SPT) and the double-proton-transferred (tautomeric) forms, were found. The transition states for proton-transfer reactions were also located. The calculations showed that the first proton-transfer reaction (normal,SPT) is exothermic and almost barrier-free; therefore, it should occur spontaneously in the period of a vibration. In contrast, the second proton-transfer reaction (SPT,tautomer) was found to be far less-probable in terms of reaction energy and barrier. Hence, it was concluded that both (7AI)2and (7AI)2exist in the SPT form. The conclusion was further confirmed by the calculated electron vertical detachment energy (VDE) of the SPT form of (7AI)2, 1.33 eV, which is very close to the experimental measurement of 1.35 eV. The calculated VDEs of the normal and tautomeric (7AI)2forms were too small compared to the experimental value. The proton transfer process was found to be multidimensional in nature involving not only proton motion but also intermolecular rocking motion. In addition, IR spectra were calculated and reported. The spectra of the three structures showed very different features and, therefore, can be considered as fingerprints for future experimental identifications. The implications of these results to biology and spectroscopy are also briefly discussed. [source] Radical Ions from 3,3,,,,3,,,,-Tris(butylsulfanyl)-2,2,:5,,2,:5,,2,,,,5,,,,2,,,,:5,,,,,2,,,,-sexithiophene: An Experimental and Theoretical Study of the p - and n -Doped OligomerCHEMPHYSCHEM, Issue 11 2003Angelo Alberti Dr. Abstract The 3,3,,,,3,,,,-tris(butylsulfanyl)-2,2,:5,,2,:5,,2,,,,5,,,,2,,,,:5,,,,,2,,,,-sexithiophene 1 was investigated through spectroscopic (NMR, EPR, UV/Vis-NIR), electrochemical, spectroelectrochemical and theoretical (DFT) studies. The charged species obtained upon its oxidation and reduction were characterised, showing that 1 can exist in at least five different oxidation states, that is, a neutral species, a radical cation, a dication, a radical anion, and a dianion. The long term stability of the radical cation 1+. was evidenced by the 1H NMR study in the presence of small quantities of trifluoroacetic acid (TFA). This approach allowed a comparison of the relative broadening of proton signals of 1, induced by the electron exchange process with traces of radical cation 1+., and the hfc (hyperfine coupling) constants obtained from the EPR study and DFT calculations. In the radical cation, all of the heterocyclic sulphur atoms are not significantly involved in the delocalisation of the unpaired electron, whereas the opposite holds for the radical anion. Time-dependent DFT calculations reproduced well the wavelengths of the optical transitions observed in the spectroelectrochemical experiments for all the five oxidation states and support the formation of the dianion 12,. [source] | ||||||||||||||||||||||||||||