Showing papers on "Radical ion published in 1999"

Long-distance charge transport in duplex DNA: The phonon-assisted polaron-like hopping mechanism

Abstract: An anthraquinone-linked duplex DNA oligomer containing 60 base pairs was synthesized by PCR. The strand complementary to the quinone-containing strand has four isolated GG steps, which serve as traps for a migrating radical cation. Irradiation of the quinone leads to electron transfer from the DNA to the quinone forming the anthraquinone radical anion and a base radical cation. The radical cation migrates through the DNA, causing reaction at GG steps revealed as strand breaks. The efficiency of strand cleavage falls off exponentially with distance from the quinone (slope = −0.02 Å−1). This finding necessitates reinterpretation of mechanisms proposed for radical cation migration in DNA. We propose that radical cations form self-trapped polarons that migrate by thermally activated hopping.

Magnetic properties of organic materials

Abstract: Genesis of the concept: intermolecular ferromagnetic spin exchange structural determinants of the chemical and magnetic properties of non-kekule molecules antiaromatic triplet ground state molecules - building blocks for organic magnets. Theory of exchange in organic systems: resonating valence bond theory and magnetic properties qualitative and quantitative predictions and measurements of singlet-triplet splittings in non-Kekule hydrocarbon diradicals and heteroatom derivatives. Exchange in small organic open-shell systems: conformational exchange modulation in trimethylenemethane-type biradicals use of dinitrenes as models for intramolecular exchange radical ion building blocks for organic magnetic materials - computational and experimental approaches. Exchange in polymeric organic systems: design and experimental investigation of high-spin organic systems synthesis and properties of organic conjugated polyradicals pendant and conjugated organic polyradicals. Theory of organic magnetism in condensed phases: band theory of exchange effects in organic open shell systems orbital interactions determining the exchange effects in organic molecular crystals molecular orbital theoretical elucidation of spin alignments in organic magnetic crystals. Magnetism in organic condensed phases: exchange effects in three dimensions - real materials bulk magnetic properties of organic verdazyl radical crystals neutron diffraction studies of spin densities in magnetic molecular materials heat capacity studies of organic radical crystals. Summation: an integrated approach to organic-based molecular magnetic materials.

Ground State of the (H2O)2+ Radical Cation: DFT versus Post-Hartree−Fock Methods

Abstract: Correlated calculations show the proton-transferred OH−H3O+ isomer to be the ground-state structure of the (H2O)2+ dimer ion, with the C2h hemibond structure being ca. 8 kcal/mol less stable. Modern density functionals however favor the hemibond structure, overestimating the strength of the three-electron bond by ca. 17 kcal/mol. The wrong prediction of the relative stability of the two isomers is attributed to overestimation by the exchange functionals of the self-interaction part of the exchange energy in the hemibond ion due to its delocalized electron hole. It is cautioned that this erroneous behavior of the density functionals for exchange, if unrecognized, may lead to wrong predictions for ground-state structures of systems with a three-electron bond.

Time-dependent density functional study on the electronic excitation energies of polycyclic aromatic hydrocarbon radical cations of naphthalene, anthracene, pyrene, and perylene

Abstract: Time-dependent density functional theory (TDDFT) and its modification, the Tamm–Dancoff approximation to TDDFT, are employed to calculate the electronic excitation energies and oscillator strengths for a series of polycyclic aromatic hydrocarbon radical cations. For the radical cations of naphthalene and anthracene, TDDFT using the Becke–Lee–Yang–Parr functional and the 6-31G** basis set provides the excitation energies that are roughly within 0.3 eV of the experimental data. The assignments of the electron transitions proposed by TDDFT accord with the previous assignments made by accurate ab initio calculations, except that TDDFT indicates the existence of a few additional transitions of π*←σ character among the several low-lying transitions. The calculated energies for these π*←σ transitions are found to be consistent with the onset of a σ electron ionization manifold in the photoelectron spectra. For the pyrene radical cation, TDDFT supports the previous assignments made by semiempirical calculations, whereas for the perylene radical cation, TDDFT suggests the energy ordering of the three lowest-lying excited states be changed from those of the semiempirical results.

Studies on radical intermediates in the early stage of the nonenzymatic browning reaction of carbohydrates and amino acids.

Abstract: Determination of the color intensity of heated mixtures of L-alanine and carbohydrate degradation products revealed furan-2-carboxaldehyde and glycolaldehyde as by far the most effective color precursors. EPR studies demonstrated that furan-2-carboxaldehyde generated colored compounds exclusively via ionic mechanisms, whereas glycolaldehyde led to color development accompanied by intense radical formation. In agreement with literature data, these radicals were also detected in heated mixtures of L-alanine and pentoses or hexoses, respectively, and were identified as 1,4-dialkylpyrazinium radical cations by EPR as well as LC/MS measurements. Studies on the mechanisms of radical formation revealed that under the reaction conditions applied, glyoxal is formed as an early product in hexose/L-alanine mixtures prior to radical formation. Reductones then initiate radical formation upon reduction of glyoxal and/or glyoxal imines, formed upon reaction with the amino acid, into glycolaldehyde, which was found as the most effective radical precursor. LC/MS measurements gave evidence that these pyrazinium radicals cations are not stable but are easily transformed into hydroxylated 1,4-dialkyl-1, 4-dihydropyrazines upon oxidation and hydrolysis of intermediate diquarternary pyrazinium ions. Besides other types of color precursors, these intermediates might be involved in the formation of colored compounds in the Maillard reaction.

Radical Cations in the OH-Radical-Induced Oxidation of Thiourea and Tetramethylthiourea in Aqueous Solution

Abstract: Hydroxyl radicals were generated radiolytically in N(2)O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN(3) as competitors to be 1.2 x 10(10) dm(3) mol(-1) s(-1) (8.0 x 10(9) dm(3) mol(-1) s(-1)). A transient appears after a short induction period and shows a well-defined absorption spectrum with lambda(max) = 400 nm (epsilon = 7400 dm(3) mol(-1) cm(-1)); that of tetramethylthiourea has lambda(max) = 450 nm (epsilon = 6560 dm(3) mol(-1) cm(-1)). Using conductometric detection, it has been shown that, in both cases, OH(-) and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with lambda(max) = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 x 10(5) dm(3) mol(-1) for thiourea (7.6 x 10(4) dm(3) mol(-1) for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 x 10(8) dm(3) mol(-1) s(-1); tetramethylthiourea, 1.3 x 10(9) dm(3) mol(-1) s(-1)), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 x 10(7) dm(3) mol(-1) s(-1)) to a new intermediate with a broad maximum at 510 nm (epsilon = 750 dm(3) mol(-1) cm(-1)). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 x 10(9) dm(3) mol(-1) s(-1)) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH(-) with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 x 10(9) dm(3) mol(-1) s(-1) for thiourea and 3.8 x 10(9) dm(3) mol(-1) s(-1) for tetramethylthiourea), phenolate ion (3 x 10(8) dm(3) mol(-1) s(-1) for tetramethylthiourea), and even azide ion (4 x 10(6) dm(3) mol(-1) s(-1) for thiourea and similar to 10(6) dm(3) mol(-1) s(-1) for tetramethylthiourea). With Oar the dimeric radical cation of thiourea reacts relatively slowly (1.2 x 107 dm(3) mol(-1) s(-1)) and reversibly (2 x 10(3) s(-1)).

Generation, Characterization, and Deprotonation of Phenol Radical Cations1

Abstract: A variety of methoxy- and methyl-substituted phenol radical cations have been generated and characterized by laser flash photolysis in solution under ambient conditions. The radical cations were generated by either photosensitized electron transfer using 1,4-dicyanonaphthalene with biphenyl as a cosensitizer in acetonitrile or by direct excitation of the phenol at 266 nm. The phenol radical cations have absorption maxima between 410 and 460 nm, with the exception of the 3,5-dimethoxyphenol which absorbs at 580 nm. The assignment of the observed transients to phenol radical cations is based on their spectral similarity to matrix spectra for the same species and to the corresponding methoxybenzene radical cations, as well as their characteristic reactivity. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only observed transient in the photosensitized electron transfer. For those phenols for which authentic phenoxyl spectra are not available, the...

Generation and photosensitization properties of the oxidized radical of riboflavin: a laser flash photolysis study

Abstract: Using 248 nm laser flash photolysis, it has been demonstrated that the neutral oxidized radical of riboflavin (RF(–H) ) resulting from rapid deprotonation of the radical cation (RF + ) can be generated via monophotonic ionization when it is directly excited with a 248 nm laser and via oxidation by the SO 4 − radical in aqueous solution. The RF(–H) or RF + reacts with both purine nucleotides and pyrimidine nucleotides via electron transfer to give the radical cations of nucleotides. These results suggest that the reactions of oxidized riboflavin radical might be involved in DNA photosensitization in vivo and in vitro.

Absolute Rate Constants for the β-Scission and Hydrogen Abstraction Reactions of the tert-Butoxyl Radical and for Several Radical Rearrangements: Evaluating Delayed Radical Formations by Time-Resolved Electron Spin Resonance

Abstract: An analysis is presented for the determination of absolute rate constants for radical transformation reactions in complex reaction systems by time-resolved ESR during intermittent photochemical radical production. The technique is applied to obtain absolute rate constants in large temperature ranges for the β-scission of the tert-butoxyl radical in various solvents and its hydrogen abstraction from cyclohexane, cyclopentane, tert-butylbenzene, and anisole. Kinetic studies of the neophyl and the 2-methyloxiran-2-yl rearrangements and the decarboxylation of the tert-butoxycarbonyl radical also demonstrate the versatility of the method. Further, rate data are given for the addition of the methyl radical to benzene and fluorobenzene, as well as for the hydrogen abstraction of methyl from di-tert-butyl peroxide.

Photoinduced electron transfer in a phenothiazine-riboflavin dyad assembled by zinc-imide coordination in water

Abstract: The known electron acceptor systems whereby the redox centers are linked by reversible noncovalent interactions are in most cases restricted to organic solvents. A kinetically labile coordinative bond has been designed for self-assembly of an electron donor (phenothiazine) and a photoinducible electron acceptor (riboflavin) in water at neutral pH. The pH dependent formation of the donor−acceptor complex in water was investigated by potentiometric titrations showing a binding constant of log K = 5.9. The strong binding constant supports the observed large fluorescence deactivation of the riboflavin emission by the phenothiazine zinc complex. The riboflavin fluorescence lifetime was found to be constant (τ = 4.7 ns) whatever the quencher concentration, clear evidence for a static quenching mechanism. A strong thermodynamical driving force and the observation of the riboflavin radical anion and phenothiazine radical cation by transient spectroscopy provide evidence for intramolecular electron transfer as the...

Sulfur Radical Cations

Abstract: Sulfur radical cations are novel reaction intermediates that have attracted considerable attention recently. This in part is due to the renewed interest in the chemistry of radical cations in general, as well as four other factors. The first is the basis for understanding the structure and reactions of radical cations — can their inherent nature be best depicted by analogy with radicals, with cations, or is a new algorithm required? As is exemplified in this review, sulfur radical cations show novel behavior. Second, some reactions of sulfur radical cations are attracting interest for their application in organic synthesis. Third, biological electron-transfer may be mediated by sulfur and, indeed, sulfur radical cations may be intermediates in biological redox processes. Finally, materials based on tetrathiafulvalene, polythiophene, and related sulfur compounds have high electrical conductivity which may be understood in terms of p-delocalized sulfur radical cations or the related dications. Consequently, a comprehensive and critical review of this field appeared timely.

Raman and Density Functional Study of the S0 State of Phenothiazine and the Radical Cation of Phenothiazine

Abstract: Raman spectra of phenothiazine and its radical cation were obtained. Hartree−Fock (HF) and density functional theory (DFT) ab initio calculations were performed to find optimized structures and computed vibrational frequencies. The ab initio structures are compared to previously reported X-ray diffraction experimental results, and the vibrational frequencies are compared to Raman spectra given here as well as previously reported Raman spectra. We have made vibrational assignments for the observed Raman bands, based on the computed potential energy distribution (PED) and isotopic shifts. Our results indicate that the neutral phenothiazine molecule has a nonplanar structure with ∼153° dihedral angle and the radical cation has a planar (or near planar) structure with ∼180° dihedral angle.

Evaluation of free radical scavenging activities of antioxidants with an H2O2/NaOH/DMSO system by electron spin resonance

Abstract: An H(2)O(2)/NaOH/DMSO system has been developed for the formation of three free radicals, and the application of the system was examined with the antioxidants ascorbic acid and tocopherol. Superoxide anion, hydroxyl radical,and methyl radical are simultaneously generated in this system. The scavenging activity of ascorbic acid and tocopherol for these radicals was estimated by 5, 5'-dimethyl-1-pyrroline-N-oxide spin trapping electron spin resonance. Both water-soluble and oil-soluble antioxidants could be evaluated by using this system. Ascorbic acid specifically inhibited the superoxide anion and hydroxyl radical, whereas tocopherol suppressed the methyl radical.

Surface radical analysis on plasma-treated polymers

Abstract: The transfer of energy from plasma particles or from photons during the plasma treatment of polymers causes bond scission and the formation of radicals. Information about radical formation rates and radical types is crucial for a better understanding of plasma-initiated processes. In this paper, a method of surface radical identification and determination is presented that uses labelling with iodine and nitric oxide, respectively. The radical formation in PE and PP is investigated for irradiation with the emissions from a hydrogen plasma and for a hydrogen plasma treatment with the same photon flux.

β-Phosphatoxyalkyl Radical Reactions: Competing Phosphate Migration and Phosphoric Acid Elimination from a Radical Cation−Phosphate Anion Pair Formed by Heterolytic Fragmentation

Abstract: β-Phosphatoxyalkyl radical reactions were studied experimentally and computationally. The 1,1-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical (1) reacted to give the migration product 2-benzyl-2-(diphenylphosphatoxy)-1,3-diphenylpropyl radical (2) and the elimination product 2-benzyl-1,3-diphenylallyl radical (3) in a variety of solvents. A modest kinetic solvent effect for reactions of 1 was found. Variable temperature studies in THF and acetonitrile gave Arrhenius functions with similar log A terms; the entropies of activation are ∼−5 eu. A deuterated analogue of radical 1 reacted in THF and acetonitrile with rate constants indistinguishable from those of 1, but the ratio of products 2:3 increased for the deuterated radical requiring kinetic isotope effects (KIEs) in reactions following the rate-limiting step. In aqueous acetonitrile solutions, the β,β-dibenzylstyrene radical cation (4) was detected as a short-lived intermediate, and the rate constants for formation of 3 and 4 indicated that both...

Role of configurational gating in intracomplex electron transfer from cytochrome c to the radical cation in cytochrome c peroxidase.

Abstract: Electron transfer within complexes of cytochrome c (Cc) and cytochrome c peroxidase (CcP) was studied to determine whether the reactions are gated by fluctuations in configuration. Electron transfer in the physiological complex of yeast Cc (yCc) and CcP was studied using the Ru-39-Cc derivative, in which the H39C/C102T variant of yeast iso-1-cytochrome c is labeled at the single cysteine residue on the back surface with trisbipyridylruthenium(II). Laser excitation of the 1:1 Ru-39-Cc−CcP compound I complex at low ionic strength results in rapid electron transfer from RuII* to heme c FeIII, followed by electron transfer from heme c FeII to the Trp-191 indolyl radical cation with a rate constant keta of 2 × 106 s-1 at 20 °C. keta is not changed by increasing the viscosity up to 40 cP with glycerol and is independent of temperature. These results suggest that this reaction is not gated by fluctuations in the configuration of the complex, but may represent the elementary electron transfer step. The value of k...

Reactions of Cations Derived from Naphthalene with Molecules and Atoms of Interstellar Interest

Abstract: The chemistry of naphthalene radical cation and its derivatives (C10Hn+, n = 6,7,8,9) has been studied with molecules and atoms of interstellar interest in a selected ion flow tube. The radical cation C10H8+ is unreactive with H2, CO, H2O, and NH3 but reacts with H, O, and N atoms. Adduct formation is the only channel detected in the case of reaction with H atoms, but additional channels contribute in reactions between C10H8+ and O and N atoms; these latter reactions proceed through novel reaction pathways via C and CH abstraction, leading to the formation of the stable compounds CO and HCN, respectively. The closed-shell naphthylium cation C10H7+ is essentially unreactive with atoms but associates via nucleophilic addition with most of the molecules studied. The reaction kinetics are close to saturation in the accessible helium pressure range. The naphthyne radical cation C10H6+ does not react with H2, CO, or H2O but forms an adduct with NH3 at a moderate rate. Reactions with atoms were found to be very ...

Ab initio studies of the radical cation diels-alder reaction

Abstract: The radical cation Diels−Alder reaction of the 1,3-butadiene radical cation with ethylene, yielding the cyclohexene radical cation, was studied by B3LYP hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis set. The intermediates and transition states involved in three different mechanisms, a concerted Cs-symmetric and a stepwise unsymmetric anti [4 + 2] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyclobutane, have been considered. The synchronous Cs-symmetric pathway is prevented by a pseudo-Jahn−Teller distortion and is 19 kcal/mol higher in energy than the stepwise pathways. The stepwise anti pathway was found to be the lowest-energy pathway with an activation energy of 0.3 kcal/mol relative to the initially formed ion−molecule complex. The gauche-out pathway, leading to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pathway, leading to cyclohexene. In contrast to earlier calculations by Bauld at the MP2/6-31G*//3-21G level of theory, an i...

Relative basicities of the oxygen atoms of the Linquist polyoxometalate [Mo6O19]2– and their recognition by hydroxyl groups in radical cation salts based on functionalized tetrathiafulvalene π donors

Abstract: Electrocrystallization of three hydroxylated donor molecules derived from tetrathiafulvalene (TTF) or ethylenedithiotetrathiafulvalene (EDT-TTF), i.e. Me3TTF-CH2OH, EDT-TTF(CH2OH)2 and TTF(CH2OH)4, in the presence of [n-Bu4N+]2[Mo6O19]2– afforded 2∶1 cation radical salts, [donor+˙]2[Mo6O19]2–, whose crystal structures have been solved by X-ray diffraction. In the three different salts complex hydrogen bond networks develop in the solid state where the oxygen atoms of both the hydroxyl groups and the [Mo6O19]2– anions act as hydrogen bond acceptors. The observed hydrogen bonding directed toward one surface, bridging oxygen atom of [Mo6O19]2– is rationalized by an analysis of ab initio calculations of the distribution of electrostatic potentials.

Free radical induced oxidation of the azo dye Acid Yellow 9

Abstract: The kinetics and mechanism of the free radical oxidation of the azo dye, Acid Yellow 9, by sulfate radical anions and hydroxyl radicals have been studied using pulse radiolysis and product analysis. Sulfate radicals react via one electron oxidation, generating the dye radical cation, which has an absorption maximum centered on 470 nm. In basic solutions, the radical cation mainly undergoes deprotonation to yield the aminyl radical which has a strong absorption with a maximum centered on 370 nm, and the pKa for the radical cation is 5.5. Formation of a sulfated product, 1, is indicative of a coupling reaction between the radical cation and sulfate radical anions. Studies also indicate that the hydroxyl radicals react with the dye by both electron transfer as well as by adduct formation.