Showing papers on "Radical ion published in 2005"

Oxidation of methanol to formaldehyde on supported vanadium oxide catalysts compared to gas phase molecules.

Abstract: The oxidation of methanol to formaldehyde on silica supported vanadium oxide is studied by density functional theory. For isolated vanadium oxide species silsesquioxane-type models are adopted. The first step is dissociative adsorption of methanol yielding CH3O(O)V(O)2 surface complexes. This makes the OV(OCH3)3 molecule a suited model system. The rate-limiting oxidation step involves hydrogen transfer from the methoxy group to the vanadyl oxygen atom. The transition state is biradicaloid and needs to be treated by the broken-symmetry approach. The activation energies for OV(OCH3)3 and the silsesquioxane surface model are 147 and 154 kJ/mol. In addition, the (OV(OCH3)3)2 dimer (a model for polymeric vanadium oxide species) and the OV(OCH3)3•+ radical cation are studied. For the latter the barrier is only 80 kJ/mol, indicating a strong effect of the charge on the energy profile of the reaction and questioning the significance of gas-phase cluster studies for understanding the activity of supported oxide ca...

On the mechanism of TiO2-photocatalyzed degradation of aniline derivatives

Abstract: Interaction between aromatic amines and TiO 2 takes place preferentially through the amino group when pH > p K a (BH + ) and, as a minor but mechanistically relevant mode, via a π-interaction if pH K a (BH + ). No significant direct photodegradation of aniline or N , N -dimethyl-aniline is detected in acidic medium using λ > 290 nm, but it is enhanced in alkaline medium. 2-Aminophenol and benzoquinone are the main photoproducts of direct irradiation of aniline. The main photoproducts of photocatalytic degradation of aniline at the pH of maximum adsorption are 2-aminophenol and phenol. Scavenging HO • with t -BuOH shows that adsorbed aniline is oxidized by positive holes (h + ), with participation of the anilinium radical cation. In the case of N , N -dimethyl-aniline at the pH of maximum adsorption, N -methyl-aniline is the main photoproduct, formed also via the dimethyl anilinium radical cation. Photocatalytic degradation in acid medium is inhibited due to electrostatic repulsion between the positively charged surface and the protonated amines. Aniline is mainly transformed into phenol and 2-aminophenol, and N , N -dimethyl-aniline into aniline, that undergoes hydroxylation to phenol. In alkaline medium the main photoproduct of degradation of aniline is nitrobenzene, formed with involvement of the anilinyl radical.

Isolation and Characterization of Phenyl Viologen as a Radical Cation and Neutral Molecule

Abstract: The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV2+2Cl-·2H2O and PV•+PF6-·pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV•+ to a twist of 36° between the two central rings in PV2+. The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak π-dimerization in the solid state and reversible π-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.

Electronic coupling in tetraanisylarylenediamine mixed-valence systems: the interplay between bridge energy and geometric factors.

Abstract: We have investigated three organic mixed-valence systems that possess nearly identical inter-redox site distances and differ by the nature of the bridging units benzene, naphthalene, and anthracene: the N,N,N‘,N‘-tetra(4-methoxyphenyl)-1,4-phenylenene-diamine radical cation (1+), the 1,4-bis(N,N-di(4-methoxyphenyl)-amino)naphthalene radical cation (2+), and the 9,10-bis(N,N-di(4-methoxyphenyl)amino)anthracene radical cation (3+). The electronic interactions in these systems have been studied by means of gas-phase ultraviolet photoelectron spectroscopy, vis/NIR spectroscopy, and electronic-structure calculations. The experimental and theoretical results concur to indicate that the strength of electronic interaction decreases in the following order of bridging units: benzene > naphthalene > anthracene. This finding contradicts the usual expectation that anthracene is superior to benzene as a driving force for electronic communication. We explain these results in terms of a super-exchange mechanism and its...

Photochemical generation of a highly reactive iron-oxo intermediate. A true iron(V)-oxo species?

Abstract: Laser flash photolysis of 5,10,15-tris(pentafluorophenyl)corrole−iron(IV) chlorate or nitrate, prepared from the corresponding chloride, gave a highly reactive iron−oxo transient identified as an iron(V)−oxo species on the basis of its UV−visible spectrum and high reactivity as well as by analogy to photochemical ligand cleavage reactions of related manganese species. The transient was shown to be an oxo transfer agent in a preparative reaction with cis-cyclooctene. Representative rate constants for oxidation reactions by the new transient at ambient temperature were k = 5900 M-1 s-1 for cyclooctene and k = 570 M-1 s-1 for ethylbenzene. The new transient is more than 6 orders of magnitude more reactive with typical organic reductants than expected for an iron(IV)−oxo corrole radical cation and 100 times more reactive than an analogous positively charged iron(IV)−oxo porphyrin radical cation. Slow electron transfer isomerization of ligand iron(V)−oxo species to iron(IV)−oxo ligand radical cations might be ...

Regioselective Oxygenative Tetraamination of [60]Fullerene. Fullerene-mediated Reduction of Molecular Oxygen by Amine via Ground State Single Electron Transfer in Dimethyl Sulfoxide

Abstract: The reaction of [60]fullerene with a variety of a secondary aliphatic amines in 20% v/v dimethyl sulfoxide in chlorobenzene under an atmospheric pressure of molecular oxygen allows regioselective introduction of four amino groups and one epoxide group around one pentagon of the fullerene molecule in good to high yield. This new synthesis of tetraaminofullerene expoxide can be carried out with a simple procedure on a multigram scale at room temperature and affords a variety of functionalized fullerene derivatives. Near-infrared analysis of a mixture of [60]fullerene and piperidine in a deaerated dimethyl sulfoxide/chlorobenzene mixture indicated equilibrium formation of [60]fullerene radical anion (C60•-) that persists at least for 2 weeks at room temperature but reacts immediately with molecular oxygen to give the tetraaminofullerene expoxide. The Benesi−Hildebrand analysis of the concentration dependency of the near-infrared absorption indicated that a [C60•- piperidine•+] radical ion pair is formed with...

On the electrophilicity of hydroxyl radical : A laser flash photolysis and computational study

Abstract: The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the tran...

Effect of axial ligation or π-π-type interactions on photochemical charge stabilization in two-point bound supramolecular porphyrin-fullerene conjugates

Abstract: Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine.

Contrastive photoreduction pathways of benzophenones governed by regiospecific deprotonation of imidazoline radical cations and additive effects

Abstract: In the photoreaction of benzophenones with 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI), benzhydrols were major products. Addition of H2O accelerated the reaction with no change in the product distribution, while AcOH, PhOH, and metal salts such as LiClO4 and Mg(ClO4)2 were effective additives to produce benzpinacols. In contrast, benzpinacols were exclusively formed regardless of the solvent and the additive in the reactions with 2-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (o-HPDMBI). These observations are consistent with the hypothesis that DMPBI•+ donates a proton at the C2 position to the benzophenone ketyl radicals while o-HPDMBI•+ donates a phenol proton.

Syntheses, Structures, Spectroscopic Properties, and π-Dimeric Interactions of [n.n]Quinquethiophenophanes

Abstract: A cyclophane-type of dimeric quinquethiophenes (4a−e) with the bridge chains consecutively varying from two to six methylenes has been synthesized and studied as ideal π-dimer models. The double-decker structures of these compounds are verified by upfield shifts for the proton NMR signals of the inside thiophenes, as compared to those of monomeric dimethylquinquethiophene (3). The electronic absorption and emission spectra of 4a−e are perturbed by through-space π-electronic interactions involving exciton−exciton coupling between the two overlapped quinquethiophenes, which become marked with shortening of the bridged alkylene chains. One-electron oxidation of 4a−e with FeCl3 in dichloromethane results in the appearance of specific polaronic bands in the near-infrared region of the electronic absorption spectra, due to the generation of a radical cation species (polaron) on one of the quinquethiophenes, which electronically interacts with the remaining neutral species. Two-electron oxidation of 4a−e introdu...

UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

Abstract: This work presents evidence that photo-excitation of guanine radical cations results in high yields of deoxyribose sugar radicals in DNA, guanine deoxyribonucleosides and deoxyribonucleotides. In dsDNA at low temperatures, formation of C1'* is observed from photo-excitation of G*+ in the 310-480 nm range with no C1'* formation observed > or =520 nm. Illumination of guanine radical cations in 2'dG, 3'-dGMP and 5'-dGMP in aqueous LiCl glasses at 143 K is found to result in remarkably high yields (approximately 85-95%) of sugar radicals, namely C1'*, C3'* and C5'*. The amount of each of the sugar radicals formed varies dramatically with compound structure and temperature of illumination. Radical assignments were confirmed using selective deuteration at C5' or C3' in 2'-dG and at C8 in all the guanine nucleosides/tides. Studies of the effect of temperature, pH, and wavelength of excitation provide important information about the mechanism of formation of these sugar radicals. Time-dependent density functional theory calculations verify that specific excited states in G*+ show considerable hole delocalization into the sugar structure, in accord with our proposed mechanism of action, namely deprotonation from the sugar moiety of the excited molecular radical cation.

Superexchange and sequential mechanisms in charge transfer with a mediating state between the donor and acceptor.

Abstract: The rate of intramolecular charge transfer from biphenyl to naphthalene was determined for the radical anions and radical cations of molecules with the general structure: (2-naphthyl)-(steroid spacer)-(4-biphenylyl). Varied degrees of unsaturation (one double bond, NSenB; two double bonds, NSen2B; and the b-ring completely aromatized, NSarB) were incorporated into the steroid spacer to examine the effect it would have on the charge transfer rate. The charge transfer rate, as inferred from the decay of the biphenyl radical ion absorption, increased in all cases relative to the completely saturated 3-(2-naphthyl)-16-(4-biphenylyl)-5α-androstane (NSB) reference molecule. For the anion charge transfer, the decay rates increased by factors of 1.4, 4.2, and 5.1, respectively, and for the cation, the decay rates increased by factors of 5, 276, and 470. To explain the results, the charge-transfer process was viewed as a combination of two independent mechanisms: a single-step, superexchange mechanism, and a two...

Structure and fragmentation of glycine, alanine, serine and cysteine radical cations. A theoretical study

Abstract: The structure of glycine, alanine, serine and cysteine radical cations, as well as their Ca‐R fragmentations have been studied at the B3LYP/6-31CCG(d,p) level of theory. For all systems the loss of COOH % is found to be the most favourable process. However, when R1 size increases, fragmentations that lead to R1 or R C 1 start become competitive. The energy of a Ca‐R fragmentation can be related to the ionization potential of the two generated fragments, in such a way that the preferred process is the one that leaves the positive charge in the fragment with a lower ionization potential. q 2005 Elsevier B.V. All rights reserved.

Electrogenerated chemiluminescence. 81. Influence of donor and acceptor substituents on the ECL of a spirobifluorene-bridged bipolar system.

Abstract: The electrochemistry and radical ion annihilation electrogenerated chemiluminescence (ECL) of 9,9'-spirobifluorene-bridged bipolar systems containing 1,3,4-oxadiazole-conjugated oligoaryl and triarylamine substituents were investigated. The stability of the oxidized spirobifluorenes was improved by functionalization with triarylamine centers. These donor-acceptor (DA) compounds exhibited a good fluorescence efficiency with an emission maximum that correlated with the potential difference between radical anion and cation formation, suggesting a charge transfer (CT) emission band. An ECL mechanism based on the formation of the CT excited state by radical ion annihilation or production of the triplet state followed by triplet-triplet annihilation, with perhaps some excimer contribution, is proposed.

Charge transfer processes in conjugated triarylamine-oligothiophene-perylenemonoimide dendrimers.

Abstract: The synthesis and charge transfer properties of triarylamine−oligothiophene−perylenemonoimide dendrimers, TPA(T2-PMI)3 and TPA(T4-PMI)3, are described. The fluorescence quantum yields indicate strong emission quenching by electron transfer [ΦTHF = 0.004 for TPA(T2-PMI)3, ΦTHF = 0.003 for TPA(T4-PMI)3, and ΦTHF = 0.8 for PMI]. Moreover, with the increase of the solvent polarity, the quantum yields decrease indicating that the A+•D-• (acceptor/donor) couple is more stabilized. The femtosecond transient absorption spectra show a very fast charge separation process (≈2 ps; kcs ≈ 5 × 1011 s-1) and a charge recombination of more than 1 order of magnitude slower (≈50 ps; kcr ≈ 2 × 1010 s-1), as observed from the rise time and decay of the radical anion and radical cation absorption bands. The analysis of the transient absorption spectroscopy and of the energetics of the process using Marcus theory indicates that in the electron transfer process the thiophene unit is the first electron donor. The triarylamine is ...

Comparative Study of the Oxidation of Fluorene and 9,9-Disubstituted Fluorenes and Their Related 2,7‘-Dimers and Trimer

Abstract: Electroactive polyfluorene films were obtained by anodic coupling of fluorene and a series of 9,9-disubstituted fluorenes and their related 2,7‘-dimers and trimer. The polymerization mechanism is discussed in light of cyclic voltammetry investigations in organic media using the classical millimetric electrode and ultramicroelectrodes, DFT theoretical calculations, and laser flash photolysis experiments. The first step of electropolymerization involves the formation of the carbon−carbon bond through the coupling between two fluorene radical cations. However, the radical cation of the produced dimer is not reactive enough to repeat the coupling reaction as in the classical electropolymerization mechanism. To continue the polymerization, formation of a higher oxidation state is required. This behavior is supported by theoretical expectations.

Efficient unimolecular deprotonation of aniline radical cations

Abstract: Deprotonation of the radical cations of aromatic amines, such as anilines, generally occurs much more slowly than other fragmentation reactions. Here we report a stereoelectronic effect involving twisting of the anilino group out of the plane of the benzene ring that results in a significantly increased rate of reactivity toward deprotonation. Quantitative studies of the rate constants for deprotonation as a function of aniline radical cation pKa (Bronsted plots) demonstrate that the effect is not simply due to a change in the reaction thermodynamics. By combining this stereoelectronic effect with covalent attachment of carboxylate as a base, aniline radical cations that undergo unimolecular deprotonation with rate constants as high as 108 s-1, even in unfavorable protic media, are described.

Radical Addition to “Cation Pool”. Reverse Process of Radical Cation Fragmentation

Abstract: The reaction of an N-acyliminium ion with an alkyl iodide and hexabutyldistannane took place to give the alkylation product. A mechanism involving the addition of an alkyl radical to an N-acyliminium ion to produce the corresponding radical cation has been suggested.

Charge localization in DNA fibers.

Abstract: We study by first-principles molecular dynamics the mechanism of electron hole (positive charge) localization in a laboratory realizable radical cation Z DNA crystal. We find that at room temperature structural deformation does not provide an efficient localization mechanism. Instead, we find evidence for the importance of changes in the protonation state for stabilizing the radical defect.

Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen.

Abstract: The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state (1DCA*) reacts with O2 to eventually afford the phosphine oxide.

Are Radical Cation States Delocalized over GG and GGG Hole Traps in DNA

Abstract: Recently, Kurnikov et al. (J. Phys. Chem. B 2002, 106, 7) have shown that solvation of DNA duplexes destabilizes holes of sizes larger than three base pairs. In this paper, we consider the effects of solvation and internal reorganization on the hole charge distribution in sequences 5'-X-GG-Y-3'. Radical cation states in DNA are found to be localized to a single guanine site independent of the nature of adjacent base pairs.

Can radical cations of the constituents of nucleic acids be formed in the gas phase using ternary transition metal complexes

Abstract: Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) of ternary transition metal complexes of [M(L(3))(N)](2+) (where M = copper(II) or platinum(II); L(3) = diethylenetriamine (dien) or 2,2':6',2''-terpyridine (tpy); N = the nucleobases: adenine, guanine, thymine and cytosine; the nucleosides: 2'deoxyadenosine, 2'deoxyguanosine, 2'deoxythymine, 2'deoxycytidine; the nucleotides: 2'deoxyadenosine 5'-monophosphate, 2'deoxyguanosine 5'-monophosphate, 2'deoxythymine 5'-monophosphate, 2'deoxycytidine 5'-monophosphate) was examined as a means of forming radical cations of the constituents of nucleic acids in the gas phase. In general, sufficient quantities of the ternary complexes [M(L(3))(N)](2+) could be formed for MS/MS studies by subjecting methanolic solutions of mixtures of a metal salt [M(L(3))X(2)] (where M = Cu(II) or Pt(II); L(3) = dien or tpy; X = Cl or NO(3)) and N to ESI. The only exceptions were thymine and its derivatives, which failed to form sufficient abundances of [M(L(3))(N)](2+) ions when: (a) M = Pt(II) and L(3) = dien or tpy; (b) M = Cu(II) and L(3) = dien. In some instances higher oligomeric complexes were formed; e.g., [Pt(tpy)(dG)(n)](2+) (n = 1-13). Each of the ternary complexes [M(L(3))(N)](2+) was mass-selected and then subjected to collision-induced dissociation (CID) in a quadrupole ion trap. The types of fragmentation reactions observed for these complexes depend on the nature of all three components (metal, auxiliary ligand and nucleic acid constituent) and can be classified into: (i) a redox reaction which results in the formation of the radical cation of the nucleic acid constituent, N(+.); (ii) loss of the nucleic acid constituent in its protonated form; and (iii) fragmentation of the nucleic acid constituent. Only the copper complexes yielded radical cations of the nucleic acid constituent, with [Cu(tpy)(N)](2+) being the preferred complex due to suppression, in this case, of the loss of the nucleobase in its protonated form. The yields of the radical cations of the nucleobases from the copper complexes follow the order of their ionization potentials (IPs): G (lowest IP) > A > C > T (highest IP). Sufficient yields of the radical cations of each of the nucleobases allowed their CID reactions (in MS(3) experiments) to be compared to their even-electron counterparts.

Photoinduced processes in protonated tryptamine

Abstract: The electronic excited state dynamics of protonated tryptamine ions generated by an electrospray source have been studied by means of photoinduced dissociation technique on the femtosecond time scale. The result is that the initially excited state decays very quickly within 250fs. The photoinduced dissociation channels observed can be sorted in two groups of fragments coming from two competing primary processes on the singlet electronic surface. The first one corresponds to a hydrogen-atom loss channel that creates a tryptamine radical cation. The radical cation subsequently fragments to smaller ions. The second process is internal conversion due to the H-atom recombination on the electronic ground state. Time-dependent density functional theory calculations show that an excited πσ* state dissociative along the protonated amino N–H stretch crosses both the locally excited ππ* state and the electronic ground state S0 and thus triggers the photofragmentation reactions. The two processes have equivalent quan...

Chiral organic radical cation and dication. A reversible chiroptical redox switch based on stepwise transformation of optically active tetrakis(p-alkoxyphenyl)ethylenes to radical cations and dications.

Abstract: Optically active tetrakis(p-alkoxyphenyl)ethylenes were found to function as reversible chiroptical switches upon redox transformations. Successive one-electron oxidations of chirally modified tetraarylethylene to the corresponding radical cation and then to the dication led to dramatic changes in the electronic absorption and circular dichroism (CD) spectra. The neutral species showed no color or CD in the visible region, while the radical ion was blue in color and exhibited a weak Cotton effect, with the dication green and giving an intense Cotton effect and a sign opposite that observed for the radical cation, at a longer wavelength. Molecular orbital calculations and X-ray crystallographic studies clearly indicate that the olefinic C=C bond is significantly twisted in the dication to minimize the electrostatic and steric repulsions. By lowering the temperature of the dication, the twist around the double bond is more firmly fixed in either P or M chirality to give a stronger Cotton effect and a larger anisotropy (g) factor. Since the spectral changes are completely reversible and reproducible for multiple redox cycles, this chiral redox system can be used in novel redox-driven chiroptical applications, such as molecular switches and memory devices, in which the information is written/read chiroptically in the ternary mode, giving zero CD signal in the neutral form, positive CD for the radical cation, and negative CD for the dication at a given wavelength.