Simple Electron Transfer Processes Research Articles

Fluoroalkyl Iodides in Fluoroalkylative Difunctionalization of C−C Multiple Bonds

AbstractFluorinated alkyl iodides serve as a convenient and inexpensive source of fluoroalkyl radicals that can readily undergo addition to the C−C unsaturated bonds of alkynes and alkenes which is the foundation for a variety of useful synthetic protocols. Since 2010 this field has witnessed huge progress in several respects. First the portfolio of fluorinated alkyl iodides was extended beyond only simple perfluoroalkyl iodides (CnF2n+1I). In particular, employment of iododifluoro−methyl‐ carbonyls and phosphonates has enabled facile installation of the medicinally relevant difluoromethylene motif. Secondly, from a conceptual point of view, novel strategies for activation of fluoroalkyl iodides towards radical formation have been introduced, relying on the formation of electron donor‐acceptor (EDA) complexes, photoredox catalysis, frustrated Lewis pairs and transition metal catalysis, complementing prior approaches based on heat and UV induced C−I homolysis, radical initiators, and simple electron transfer processes. Based on these strategies a range of novel fluoroalkylative transformations of unsaturated systems have been added to classical iodoperfluoroalkylation. Broadly applicable protocols for simple fluoroalkylation and hydrofluoroalkylation, as well as for more sophisticated, complexity‐building methods of fluoroalkylation‐annulation and tandem multicomponent fluoroalkylations with concomitant installation of another functionality have been recently disclosed. This review summarizes the progress achieved since 2010 in the reactivity of fluoroalkyl iodides towards alkenes and alkynes with emphasis placed on the above‐mentioned advances.

Understanding molecular and electrochemical charge transfer: theory and computations.

Electron, proton, and proton-coupled electron transfer (PCET) are crucial elementary processes in chemistry, electrochemistry, and biology. We provide here a gentle overview of retrospective and currently developing theoretical formalisms of chemical, electrochemical and biological molecular charge transfer processes, with examples of how to bridge electron, proton, and PCET theory with experimental data. We offer first a theoretical minimum of molecular electron, proton, and PCET processes in homogeneous solution and at electrochemical interfaces. We illustrate next the use of the theory both for simple electron transfer processes, and for processes that involve molecular reorganization beyond the simplest harmonic approximation, with dissociative electron transfer and inclusion of all charge transfer parameters. A core example is the electrochemical reduction of the S2O82- anion. This is followed by discussion of core elements of proton and PCET processes and the electrochemical dihydrogen evolution reaction on different metal, semiconductor, and semimetal (say graphene) electrode surfaces. Other further focus is on stochastic chemical rate theory, and how this concept can rationalize highly non-traditional behaviour of charge transfer processes in mixed solvents. As a second major area we address ("long-range") chemical and electrochemical electron transfer through molecular frameworks using notions of superexchange and hopping. Single-molecule and single-entity electrochemistry are based on electrochemical scanning probe microscopies. (In operando) scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are particularly emphasized, with theoretical notions and new molecular electrochemical phenomena in the confined tunnelling gap. Single-molecule surface structure and electron transfer dynamics are illustrated by self-assembled thiol molecular monolayers and by more complex redox target molecules. This discussion also extends single-molecule electrochemistry to bioelectrochemistry of complex redox metalloproteins and metalloenzymes. Our third major area involves computational overviews of molecular and electronic structure of the electrochemical interface, with new computational challenges. These relate to solvent dynamics in bulk and confined space (say carbon nanostructures), electrocatalysis, metallic and semiconductor nanoparticles, d-band metals, carbon nanostructures, spin catalysis and "spintronics", and "hot" electrons. Further perspectives relate to metal-organic frameworks, chiral surfaces, and spintronics.

Elucidating Zn, Mg, and Ca Electrodeposition Mechanisms in Nonaqueous Electrolytes for Next-Generation Metal Batteries

Due to their significantly higher volumetric capacity, natural abundance, and decreased cost compared to Li metal, Zn, Mg, and Ca rechargeable metal batteries in nonaqueous electrolyte solutions have been the subject of extensive research and development. In this study, we employ the electrochemistry and computational simulation to examine the reversible electrodeposition processes of multivalent metals in nonaqueous electrolytes. Cyclic voltammetry (CV) obtained from Zn electroplating/stripping at an ultramicroelectrode (UME) shows that current density and scan rate are independent as predicted from a simple electron transfer process. However, CVs obtained from Mg and Ca deposition/stripping processes at an UME displays an inverse dependence between current density and scan rate. COMSOL simulations suggest that Zn deposition is governed by a one−step two−electron (E) transfer process while Mg and Ca plating requires a chemical step prior to electron transfer step, suggesting a chemical−electrochemical (CE) mechanism.

Switchable NLO response induced by rotation of metallacarboranes [NiIII/IV(C2B9H11)2]−/0 and C-,B-functionalized derivatives

The rotary motion based on metallacarboranes around a molecular axis can be controlled by simple electron transfer processes, which provides a basis for the structure-property relationship for the nonlinear optical (NLO) switching. However, this phenomenon has not been previously reported in the development of NLO properties of metallacarboranes. In this work, the metallacarboranes [Ni(III/IV)(C2B9H11)2](-/0) and their C-,B-functionalized derivatives are studied by the density functional theory (DFT) method. By calculating relative energies, we obtained the stable states before and after rotation controlled by simple electron transfer. Then, the static and frequency-dependent second-order NLO properties were calculated by several DFT functionals. According to the TDDFT results, the large NLO responses of the studied compounds are mainly caused by substituent group-to-carborane cage charge transfer (L'LCT) and substituent group-to-metal charge transfer (L'MCT) processes. The order of first hyperpolarizabilities (β values) illustrates that the NLO response can be enhanced by introducing a strong electron-donating group. Significantly, the geometric interconversions resulting from the redox reaction of 1C/1T-6C/6T allow the NLO responses to be switched "ON" or "OFF". The B(9,9')-methoxyphenyl-functionalized derivative of nickelacarborane, having low energetic cost and large different NLO responses between two states (from 0 to 20 998 a.u.), can be an excellent switchable NLO material.

Reexamination of the Rehm–Weller Data Set Reveals Electron Transfer Quenching That Follows a Sandros–Boltzmann Dependence on Free Energy

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.

ELECTROCHEMICAL STUDIES AND EXTRACTION OF CHEMICAL AND ELECTROCHEMICAL PARAMETERS OF ZN(II) ION AT MERCURY ELECTRODE VIA CONVOLUTIVE VOLTAMMETRY AND DIGITAL SIMULATION METHODS

aBstract The electrochemical behaviour of Zn(II) ion as example of C rev E irr system was studied at hanging mercury drop electrode in aqueous solution in the presence of 1M KCl using cyclic voltammetry and convolutive voltammetry methods. Digital simulation technique was used to test the chemical and electrochemical parameters of Zn(II) ion estimated experimentally as well as verifying the proposed electrode mechanism. Keywords: Electrochemical behaviour, C rev E irr system, cyclic voltammetry, convolutive Voltammetry, digital simulation. introduction The electroreduction of Zn(II) has been extensively studied in aqueous and non-aqueous solutions and found to be controlled by both diffusion and charge transfer. 1-6 At mercury electrode different mechanisms have been proposed for electrode reduction of metal cations, e.g., a simple electron transfer process 7,8 , stepwise electron transfer 9 , a CE or CEE mechanism. 10,11 Several literature reports a possible validity of a mechanism involving two one-electron transfer has been discussed.

Charge distribution effects in polyatomic reactants involved in simple electron transfer reactions

The role of charge distribution in a polyatomic reactant in determining its effective charge in the double layer is discussed. It is shown that the use of a corrected Tafel plot (cTp) requires knowledge of the effective charge on both the reactant and product of a simple electron transfer process. This leads to the use of so called apparent corrected Tafel plots. Data obtained in this laboratory for six different simple electron transfer reactions involving transition metal ions are reviewed, and discussed in terms of their kinetic parameters, namely, the effective charge on the reactant and the apparent Tafel coefficient.

Cyclopropyl Conjugation and Ketyl Anions: When Do Things Begin to Fall Apart?

Results pertaining to the electrochemical reduction of 1,2-diacetylcyclopropane (5), 1-acetyl-2-phenylcyclopropane (6), 1-acetyl-2-benzoylcyclopropane (7), and 1,2-dibenzoylcyclopropane (8) are reported. While 6*- exists as a discrete species, the barrier to ring opening is very small (<1 kcal/mol) and the rate constant for ring opening is >10(7) s(-1). For 7 and 8, the additional resonance stabilization afforded by the benzoyl moieties results in significantly lower rate constants for ring opening, on the order of 10(5)-10(6) s(-1). Electron transfer to 8 serves to initiate an unexpected vinylcyclopropane --> cyclopentene type rearrangement, which occurs via a radical ion chain mechanism. The results for reduction of 5 are less clear-cut: The experimental results suggest that the reduction is unexceptional, with a symmetry coefficient alpha </= 0.5, and reorganization energy consistent with a simple electron-transfer process (one electron reduction, followed by ring opening). In contrast, molecular orbital calculations suggest that 5*- has no apparent lifetime and that reduction of 5 may occur by a concerted dissociative electron transfer (DET) mechanism (i.e., electron transfer and ring opening occur simultaneously). These seemingly contradictory results can be reconciled if the increase in the internal reorganization energy associated with the onset of concerted DET is offset by a lowering of the solvent reorganization energy associated with electron transfer to a more highly delocalized LUMO.

Measurement of rapid electron transfer across a liquid/liquid interface from 7,7,8,8-tetracyanoquinodimethane radical anion in 1,2-dichloroethane to aqueous tris(2,2-bipyridyl)-ruthenium (III)

Rate constants for the reduction of tris(2,2-bipyridyl)-dichlororuthenium (III) (Ru(bipy)(3)(3+)) in water by 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ(.-)) in 1,2-dichloroethane (DCE) have been measured as a function of the Galvani potential across an ideally non-polarizable interface using scanning electrochemical microscopy (SECM). The Galvani potential was varied by changing the concentration of tetrabutylammonium chloride in the aqueous phase, with a fixed concentration of the organic electrolyte, tetrabutylammonium tetrakis(penta-fluoro)phenylborate, employed in the DCE phase. By selecting appropriate concentration ratios of the reactants in the two phases, fast electron transfer (ET) rate constants were measured which were found to increase with increasing driving force. The experimentally obtained ET rate constants have been compared to the corresponding theoretical values calculated from the sharp boundary and thick layer models of Marcus. The results are consistent with the sharp boundary model, providing further evidence for the applicability of this model for the description of simple ET processes across liquid/liquid interfaces. (c) 2007 Elsevier B.V. All rights reserved.

The electrochemical reaction mechanism of selenocystine on selenium-gold film modified glassy carbon electrode

A simple and selective voltammetric method based on selenium-gold film modified glassy carbon electrode has been developed for investigating electrochemical reaction mechanism of selenocystine. With N 2 saturated, redox reactions between selenocystine (SeC) and selenocysteine (SeCys) were judged to be two simple electron-transfer processes. With air saturated, the reduction reaction was diagnosed to be EC catalytic reaction (the chemical oxidation reaction of the SeCys by O 2 (C) following the electron-transfer reaction (E)) and oxidation reaction is a simple electron-transfer process. With pure O 2 saturated, only reduction peak was observed and the reaction was judged to be EC catalytic reaction. The electron-transfer numbers of redox reaction were calculated to be 2 by chronocoulometry and rotating disk electrode.

Electroanalytical applications of boron-doped diamond microelectrode arrays

The electrochemical characteristics of a novel all diamond fabricated boron-doped diamond microelectrode array (BDD-MEA) are critically appraised. The voltammetric response of simple electron transfer processes has been investigated and found to generate sigmoidal voltammetric curves. Furthermore, the device has been utilized for various analytical applications including, the direct detection of 4-nitrophenol over the concentration range 1.8–9.2 μM, manganese over the range 0.1–4.8 μM and the indirect determination of sulfide producing a limit of detection of 23 μM.

Electron Transfer Kinetics on Boron-Doped Diamond Part I: Influence of Anodic Treatment

Boron-doped diamond electrodes, both as-grown and polarized anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the role played by the surface-state density. Many different treatments were employed: as-grown (BDDag), mildly polarized (BDDmild), strongly polarized in perchloric acid (BDDsevererpar;, and strongly polarized in a sulphuric acid-acetic acid mixture (BDDAcOHrpar;. Charge transfer processes at the electrode surface were studied by cyclic voltammetry. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (FeIII/II;(CN)6rpar; and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H2Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The active surface area and the reaction rate constants decreased significantly upon anodic polarization. Important drops in the charge carrier concentration on the surface and in true surface area led to hindrance of electron transfer at the electrode.

Electrical or photocontrol of the rotary motion of a metallacarborane.

Rotary motion around a molecular axis has been controlled by simple electron transfer processes and by photoexcitation. The basis of the motion is intramolecular rotation of a carborane cage ligand (7,8-dicarbollide) around a nickel axle. The Ni(III) metallacarborane structure is a transoid sandwich with two pairs of carbon vertices reflected through a center of symmetry, but that of the Ni(IV) species is cisoid. The interconversion of the two provides the basis for controlled, rotational, oscillatory motion. The energies of the Ni(III) and Ni(IV) species are calculated as a function of the rotation angle.

Electron transfer from carbanionic salts to linear sequences of conjugated double bonds, 1. Mechanism of n doping of polyacetylene

AbstractIt is shown that the mechanism of polyacetylene (PA) reduction (or n “doping”) by alkaline carbanionic salts and butyllithium is a simple electron transfer without addition. In this reduction reaction the dicarbanionic dimer of 1,1‐diphenylethylene (DD2– Na2+) is quantitatively converted to 1,1‐diphenylethylene. The reduction by 3‐methyl‐1,1‐diphenylpentyllithium produces free radicals which deactivate in solution by coupling and disproportionation. But the strong dependence of the ratio of disproportionation to coupling of these radicals on solvent polarity is unusual: no coupling is observed in diethyl ether or pentane, while it is the most important reaction in THF. sec‐Butyllithium in pentane also reduces PA by a simple electron transfer process. The “doping” levels can be deduced accurately from the quantitive analysis of the by‐products by gas chromatography.

Variation of the electrochemical transfer coefficient with potential

The electrochemical electron-transfer rate constant has been determined as a function of the electrode potential for a series of simple electron-transfer processes to organic molecules in media containing acetonitrile or dimethylformamide and a quaternary ammonium salt as supporting electrolyte and using mercury as the electrode material. The reactions and the experimental conditions were selected so as to deal with outer-sphere processes and to minimize the magnitude of double-layer corrections. Convolution potential-sweep voltammetry and the impedance method were used for obtaining the kinetic data. Under these conditions, the electrochemical transfer coefficient was observed, in all cases, to vary, beyond experimental error, with the electrode potential. The magnitude of the variation is of the same order of magnitude as that predicted by the Marcus theory of outer-sphere electron transfer. A more complex reaction, the reduction of benzaldehyde in ethanol, involving dimerization steps following the initial electron transfer was also investigated. A definite variation of the transfer coefficient was again observed. This behaviour, observed for various solvents and functional groups, appears as a general phenomenon in the reduction of organic molecules in the case where charge transfer is fast and mainly governed by solvent reorganization.

The effect of electrode material on redox reactions of quinones in acetonitrile

Standard heterogeneous electron transfer rates for the one-electron reductions of 1,4-benzoquinone, 1,4-naphthoquinone and 9,10-anthraquinone in acetonitrile solution were measured at Pt, Au and graphite electrodes by rapid scan cyclic voltammetric methods. Observed values of the order, k s ∼ 0.5 cm s −1 are compared with conflicting earlier results. The quinone reactions conformed with the criteria necessary for simple electron transfer processes and are adequately interpreted in terms of the Marcus theory.

Oxidation of Ascorbate Anion by Electron Transfer to Phenoxyl Radicals

Most phenoxyl radicals rapidly oxidize ascorbate anion (rate constants from 4 to 20 x 10/sup 8/ M/sup -1/ sec/sup -1/) by a simple electron transfer process. The product radical anion is relatively unreactive and has a well-characterized absorption at 360 nm where it has an extinction coefficient of 3300 M/sup -1/ cm/sup -1/. In the case of a phenoxyl radical produced by OH attack on phenol, oxidation appears to be quantitative. Ascorbate is oxidized only slowly or not at all by less reactive radicals, such as the alcohol radicals, para-semiquinones, or the phenyl radical. Ascorbate can, therefore, be used to selectively remove phenoxyl radicals from many mixed radical systems. Because ascorbate radical anion absorbs only weakly above 390 nm, where phenoxyl and para-semiquinone radicals absorb more strongly, ascorbate can be used to examine the oxidation of substrates in cases where phenoxyl and semiquinone radicals are produced simultaneously. This application is illustrated by a study of the attack of OH at the fluorine position of para-fluorophenol. A second illustrative example is provided by a study of the tertiary oxidation of ascorbate following reduction of the bromophenols by e/sub aq//sup -/. It is shown, in agreement with previous optical and ESR studies,more » that phenoxyl radicals are produced by rapid protonation of the hydroxyphenyl radical anion in the case of the ortho- and para-isomers but not in the case of the meta-isomer.« less

On the theory of electron-transfer processes in aqueous solutions

It has been realized for some time that simple electron-transfer processes play an important part in the mechanism of many oxidation-reduction reactions in solution. An attempt has been made to give a quantum-mechanical interpretation of these processes on the basis of the earlier theories of electron transfer in the gaseous state (Landau 1932; Bates &amp; Massey 1943). The present treatment for solutions takes into account the role of the solvent, with particular reference to the operation of the Franck—Condon principle and it also leads to some definite picture of the transition state for the electron transfer process. A number of examples are discussed, including electron transfer between like ions of different valency and also reactions involving complex ions, e.g. metal porphyrins, the reactions of which are of importance in certain biochemical processes. It appears that the application of certain theoretical principles leads to a satisfactory understanding of electron-transfer processes in solution from a qualitative and, in some cases, also from a semi-quantitative point of view.

Simple electron transfer processes in systems of conjugated double bonds

Simple electron transfer processes in systems of conjugated double bonds