Electron Transfer In Aqueous Solution Research Articles

Confinement Effect on Heterogeneous Electron Transfer in Aqueous Solutions inside Conducting Nanotubes

AbstractAn Fe3+/2+ redox couple in conducting single wall carbon nanotubes filled with water molecules is investigated in the framework of the electron transfer theory and with classical molecular dynamics simulations. The diameter of the nanotubes ranges from 0.8 nm to 3.5 nm. It can be concluded, qualitatively, that the electron transfer rate significantly increases with decreasing nanotube diameter. This effect is explained basically in terms of the solvent reorganization energy. Other factors that can affect the reaction rate are discussed as well.

Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds.

The excited nπ* and ππ* triplets of two benzophenone (BP) and two anthraquinone (AQ) derivatives have been observed in acetonitrile, isopropanol, and mixed aqueous solutions using time-resolved resonance Raman spectroscopic and nanosecond transient absorption experiments. These experimental results, combined with results from density functional theory calculations, reveal the effects of solvent and substituents on the properties, relative energies, and chemical reactivities of the nπ* and ππ* triplets. The triplet nπ* configuration was found to act as the reactive species for a subsequent hydrogen atom transfer reaction to produce a ketyl radical intermediate in the isopropanol solvent, while the triplet ππ* undergoes a proton-coupled electron transfer (PCET) in aqueous solutions to produce a ketyl radical intermediate. This PCET reaction, which occurs via a concerted proton transfer (to the excited carbonyl group) and electron transfer (to the excited phenyl ring), can account for the experimental observation by several different research groups over the past 40 years of the formation of ketyl radicals after photolysis of a number of BP and AQ derivatives in aqueous solutions, although water is considered to be a relatively "inert" hydrogen-donor solvent.

A theoretical exploration of the photoinduced reductive repair mechanisms of thymidine glycol

Abstract The potential energy surface for the reductive repair of thymidine glycol (dTg) via electron transfer in aqueous solution has been mapped out in the present study employing a hybrid function of BH&HLYP in conjunction with the 6-311+G(d,p) basis set. Two types of repair mechanisms have been explored, including the reductive modification of the base portion and the excision of the base. The results indicate that thymidine (dT) is the most energetically feasible reductive product of dTg and also is the feasible degradation product of 5-hydroxy-5,6-dihydrothymidin-6-yl radical. The latter, however, has not been characterized in the previous experimental investigation. The insights explored in this paper help shed light on the interaction between low-energy electron and DNA base.

The oxidation of tyrosine and tryptophan studied by a molecular dynamics normal hydrogen electrode

The thermochemical constants for the oxidation of tyrosine and tryptophan through proton coupled electron transfer in aqueous solution have been computed applying a recently developed density functional theory (DFT) based molecular dynamics method for reversible elimination of protons and electrons. This method enables us to estimate the solvation free energy of a proton (H(+)) in a periodic model system from the free energy for the deprotonation of an aqueous hydronium ion (H(3)O(+)). Using the computed solvation free energy of H(+) as reference, the deprotonation and oxidation free energies of an aqueous species can be converted to pK(a) and normal hydrogen electrode (NHE) potentials. This conversion requires certain thermochemical corrections which were first presented in a similar study of the oxidation of hydrobenzoquinone [J. Cheng, M. Sulpizi, and M. Sprik, J. Chem. Phys. 131, 154504 (2009)]. Taking a different view of the thermodynamic status of the hydronium ion, these thermochemical corrections are revised in the present work. The key difference with the previous scheme is that the hydronium is now treated as an intermediate in the transfer of the proton from solution to the gas-phase. The accuracy of the method is assessed by a detailed comparison of the computed pK(a), NHE potentials and dehydrogenation free energies to experiment. As a further application of the technique, we have analyzed the role of the solvent in the oxidation of tyrosine by the tryptophan radical. The free energy change computed for this hydrogen atom transfer reaction is very similar to the gas-phase value, in agreement with experiment. The molecular dynamics results however, show that the minimal solvent effect on the reaction free energy is accompanied by a significant reorganization of the solvent.

Molecular dynamics investigation of ferrous-ferric electron transfer in a hydrolyzing aqueous solution: calculation of the pH dependence of the diabatic transfer barrier and the potential of mean force.

We present a molecular model for ferrous-ferric electron transfer in an aqueous solution that accounts for electronic polarizability and exhibits spontaneous cation hydrolysis. An extended Lagrangian technique is introduced for carrying out calculations of electron-transfer barriers in polarizable systems. The model predicts that the diabatic barrier to electron transfer increases with increasing pH, due to stabilization of the Fe3+ by fluctuations in the number of hydroxide ions in its first coordination sphere, in much the same way as the barrier would increase with increasing dielectric constant in the Marcus theory. We have also calculated the effect of pH on the potential of mean force between two hydrolyzing ions in aqueous solution. As expected, increasing pH reduces the potential of mean force between the ferrous and ferric ions in the model system. The magnitudes of the predicted increase in diabatic transfer barrier and the predicted decrease in the potential of mean force nearly cancel each other at the canonical transfer distance of 0.55 nm. Even though hydrolysis is allowed in our calculations, the distribution of reorganization energies has only one maximum and is Gaussian to an excellent approximation, giving a harmonic free energy surface in the reorganization energy F(DeltaE) with a single minimum. There is thus a surprising amount of overlap in electron-transfer reorganization energies for Fe(2+)-Fe(H2O)6(3+), Fe(2+)-Fe(OH)(H2O)5(2+), and Fe(2+)-Fe(OH)2(H2O)+ couples, indicating that fluctuations in hydrolysis state can be viewed on a continuum with other solvent contributions to the reorganization energy. There appears to be little justification for thinking of the transfer rate as arising from the contributions of different hydrolysis states. Electronic structure calculations indicate that Fe(H2O)6(2+)-Fe(OH)n(H2O)(6-n)(3-n)+ complexes interacting through H3O2- bridges do not have large electronic couplings.

Outer-sphere electron transfer in aqueous solutions of lithium hexacyanoferrates

Electron transfer between the hexacyanoferrate anions in the presence of the Li+ cations in aqueous solutions was studied by spectrophotometry. The results obtained were compared with the data for the K+- and Me4N+-containing systems. The proposed explanation of the observed cationic catalysis effect of the thermal electron transfer in homogeneous and heterogeneous systems is based on the analysis of electron-transfer distances (viz., contact distances between the anions) in the activated complexes.

Fourier transform EPR study and quantum chemical calculations of dihydrofuran radicals formed by triplet sensitized electron transfer in aqueous solution

The radicals formed by electron transfer from 2,3-dihydrofuran, 2-methyl-4,5-dihydrofuran and 2,3-dimethyl-4,5-dihydrofuran to the laser induced triplet state of anthraquinone-2,6-disulfonic acid were studied in aqueous solution by Fourier transform electron paramagnetic resonance (FT EPR) in the nanosecond time-scale. With 2,3-dimethyl-4,5-dihydrofuran and 2-methyl-4,5-dihydrofuran as electron donor the radical cations were observed directly, whereas with 2,3-dihydrofuran as electron donors the radicals observed are successor radicals of the radical cations. The assignment of the radical structures was done by simulation of the experimental EPR spectra and was supported by quantum chemical density functional theory (DFT) calculations. In the experiments with 2-methyl-4,5-dihydrofuran only one OH−-adduct could be determined although other so far unknown radicals contribute to the measured spectra. With 2,3-dihydrofuran two OH−-adduct radicals and the deprotonated radical cation were detected together with contributions from a dimer radical. The rate constants of the electron transfer to the anthraquinone-2,6-disulfonate triplet and of the formation of the successor radicals from the primary 2,3-dihydrofuran radical cation were determined, respectively. The hyperfine coupling constants calculated by quantum chemical DFT method agree quite well with the experimental one.

Neutral peptide biradicals formed by dissociative electron transfer.

[reaction: see text] A new method for the generation of transient neutral biradicals in liquid solution is reported. Photoinduced electron transfer in aqueous solution of the structures shown above leads to neutral biradicals with peptide spacers. Exchange interactions were measured using time-resolved electron paramagnetic resonance (TREPR) and found to be large for the biradicals possessing two and three glycine residue spacers. These findings are compared to previous results from alkyl-spaced biradicals of similar chain length.

Chiral induction in anion–anion electron transfer in aqueous solution catalyzed by a chiral cation

The oxidation of cobalt(II) chelate complexes [CoY] 2−, where Y=edta (ethylenediaminetetraacetate), pdta (1,2-propanediaminetetraacetate) and cydta ( trans-1,2-cyclohexanediaminetetraacetate), by [Co(mal) 3] 3− (mal=malonate) was catalyzed by chiral [Co(en) 3] 3+ to yield optically active products with the enantiomeric excesses which increase with increasing concentration of chiral [Co(en) 3] 3+. Substituents on the ethylenediamine backbone of [CoY] 2− had significant effects on the sign and the magnitude in the stereoselectivity. The results are interpreted in terms of a chiral induction mechanism in which the ion pair between [CoY] 2− and chiral [Co(en) 3] 3+ is stereoselectively oxidized by [Co(mal) 3] 3−. A model is proposed for the relative orientation of oxidant, reductant, and catalysis complexes.

Cation Catalysis of Anion−Anion Electron Transfer in Aqueous Solution: Self-Exchange Reaction Kinetics of Some Hexa- and Octacyanometalate Couples at Variable Pressure

Rate constants kex for the Os(CN)63-/4-, Mo(CN)83-/4-, and W(CN)83-/4- self-exchange reactions in aqueous solution have been measured by 13C NMR as functions of pressure, temperature, and added electrolyte concentrations. Detailed interpretation of kex values in the absence of added electrolytes is complicated by anion−cation association, but kex is strongly influenced by the nature of added cations (Li+ < Na+ < K+ < Rb+ < Et4N+ < Cs+ ≤ Me4N+) and is linearly dependent on their concentrations. For K+ as the counterion, ΔVex⧧ for Os(CN)63-/4-, Mo(CN)83-/4-, and W(CN)83-/4- is +19.0 ± 0.9, +14.7 ± 0.6, and +22.5 ± 1.1 cm3 mol-1, respectively; these values are completely inconsistent with a simple two-sphere Marcus-type model of the electron-transfer process, which predicts ΔVex⧧ on the order of −6 cm3 mol-1. With Et4N+ as the counterion, however, ΔVex⧧ for Mo(CN)83-/4- is −8.2 ± 0.6 cm3 mol-1, and for W(CN)83-/4- with Me4N+ ΔVex⧧ is −7.4 ± 0.5 cm3 mol-1. The results are interpreted in terms of a mechanism which is outer-sphere as far as the cyanometalate ions are concerned but in which a partially desolvated cation bridges the reacting anions. In the case of alkali metal cations, removal of some (probably two) of the coordinated water molecules precedes electron transfer, whereas the tetraalkylammonium ions have no coordinated water. Anion−anion electron transfer is much more sensitive to cation catalysis than cation−cation exchange is to anion effects, probably because a positively charged bridge provides a much more favorable tunneling path for the negatively charged electron than would an anionic mediator.

The effect of a laser field on electron transfer in metal complexes: Quantum degrees of freedom

The effect of a strong cw laser field on Fe(OH2)6+2/Fe(OH2)6+3 electron transfer in aqueous solution is considered. On the basis of a kinetic master equation for the time-dependent population, an analytical solution for the forward and backward reaction rates is obtained. The presence of a strong ion–ligand vibration at ℏωq=432cm−1 qualitatively changes the intensity dependence of the sum of forward and backward rate constants (denoted as the ‘‘total rate constant’’). It is shown that the total rate constant is strongly dependent on the laser frequency. For the symmetric exchange reaction this rate constant exhibits resonances as a function of the cw field amplitude. For the activationless reaction (achievable by application of an additional dc field) it decreases sharply with the cw field intensity. It is shown that the asymptotic populations of the reactant and product states are non-Boltzmann; their ratio can be varied by 18 orders of magnitude. Contrary to the case with no ac field, the equilibrium constant exhibits a nonmonotonic dependence on the reaction heat, ε. In some regions of ε it is insensitive to the reaction heat. The analytical solution is verified via numerical solution of the original master equation. Excellent agreement is obtained.

Dynamic solvent effect on the kinetics of the inner-sphere reduction of cobalt(III) anchored to a mercury electrode in mixtures of water with an organic solvent

Unimolecular rate constants have been measured for the electroreduction of two similar pentaamminecobalt(III) complexes in mixtures of water with acetone and acetonitrile. The reactants were attached to mercury via the bridging ligands 3-(2-thienyl)acrylocarboxylate and 3-(2-thienyl)propionocarboxylate. Only the reactant with a bridge featuring uninterrupted conjugation is known to undergo the adiabatic electron transfer in aqueous solutions. For this reactant, it has been found that the rate constant evidently decreases with the addition of organic solvent to the mixture. This observation is discussed in terms of a change in the longitudinal relaxation times for the mixtures.

Time resolved studies of the photoinduced electron transfer in aqueous solutions of chloro- and cyanocuprate(I) complexes

Laser flash and continuous photolysis studies of equilibrated solutions of CuCl − 2CuCl −3 2, Cu(CN) − 2Cu(CN) 3 2− and Cu(CN) − 2Cu(CN) 2Cl 2− systems were carried out in the UV region. Neutral aqueous solutions of the complexes produce hydrated electrons when they absorb pulses of laser light at 266 nm. The electrons are scavenged by the ground-state copper(I) species itself. The second-order rate constants of these reactions are determined at various ionic strengths and concentrations. Illumination of chlorocuprate complexes in strong acidic media leads to formation of hydrogen (H 2) via electron scavenging by H +. In solutions of cyanocuprates, methylviologen (MV 2+) was added to scavenge the electron in a reaction resulting in a reducing intermediate MV .+. The formation of an oxidizing intermediate is demonstrated using NO 3 − and N 2O as electron scavengers. Continuous photolysis of the complexes reveals the dependence of quantum yield on concentration of Cl − ligand in CuCl 2 −CuCl 3 2− and Cu(CN) 2 −Cu(CN) 2Cl 2− systems, and the action spectra of Cu(CN) 2 − and Cu(CN) 2Cl 2− indicate an efficient excitation band at 265 nm for both complexes (the quantum yields are 0.37 and 0.21 respectively).

Phosphorylation coupled to electron transfer in aqueous solutions.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhosphorylation coupled to electron transfer in aqueous solutionsShu-I Tu and Jui Hsin WangCite this: Biochemistry 1970, 9, 23, 4505–4509Publication Date (Print):November 1, 1970Publication History Published online1 May 2002Published inissue 1 November 1970https://doi.org/10.1021/bi00825a007RIGHTS & PERMISSIONSArticle Views59Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (561 KB) Get e-Alerts Get e-Alerts

A theory of non-adiabatic electron-transfer in aqueous solution

A theory of non-adiabatic electron-transfer in aqueous solution