Fully adiabatic dissociative electrochemical electron transfer reactions induced by scanning tunneling microscopy

A theory of fully adiabatic dissociative electrochemical processes of the electron transfer that are induced by scanning tunneling microscopy is constructed. Adiabatic free energy surfaces are calculated and properties of their symmetry are examined under various conditions. Diagrams of kinetic regimes, which characterize possible kinetic processes, which may proceed in the system under consideration, are constructed in the space of model parameters. Dependence of activation free energy on the bias voltage, overvoltage, physical properties of a molecule, and intensity of interaction of a molecule with an electrode and the tip of the scanning tunneling microscope is explored.     

Effect of electron-electron correlations on the activation free energy of adiabatic electrochemical reactions of dissociative electron transfer

Adiabatic free energy surfaces for adiabatic electrochemical reactions of dissociative electron transfer are calculated with exact allowance for the effects of electron-electron correlations in a model of an electrode with an infinitely broad conduction band. The role of correlation effects in these reactions is discussed. It is shown that, as in common adiabatic electrochemical reactions of electron transfer, correlation effects play a substantial role and lead to a considerable decrease in the activation free energies.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 412–418.Original Russian Text Copyright © 2005 by Kuznetsov, Medvedev, Sokolov.     

The electron transfer rate of large TPA based compounds: a joint theoretical and electrochemical approach

A series of triphenylamine (TPA) based compounds is investigated by means of density functional theory and cyclic voltammetry. Using the Nicholson's formalism, the measured deltaE(p) are correlated with B3LYP/6-31G* calculated reorganisation energies (lambda), elucidating the trend followed by the electron transfer rate of these compounds. Besides the direct dependency upon the dimension of the cationic fragment contributing to the hole stabilisation, the lambdas are tuned by the symmetry local to the TPA units, as evidenced by the structural relaxation of the cations. MDTAB shows the interesting combination of low ionisation potential (IP) and low lambda. This can make this compound interesting for practical applications in organic light emitting diode (OLEDs) devices, due to the direct correlation of the IP and lambda with the hole transfer efficiency to the anode, along with the hole mobility.     

Intermolecular biological electron transfer: an electrochemical approach.

We investigated the electron transfer (ET) rates between a well-defined gold electrode and cytochrome c immobilized at the carboxylic acid terminus of alkanethiol self-assembled monolayers (SAMs) by using the potential modulated electroreflectance technique. A logarithmic plot of ET rates against the chain length of the alkanethiol is linear with long chain alkanethiols. The ET rates become independent of the chain length with short alkanethiols. It is proposed that the rate-limiting ET step through short alkyl chains results from a configurational rearrangement process preceding the ET event. This "gating" process arises from a rearrangement of the cytochrome c from a thermodynamically stable binding form on the carboxylic acid terminus to a configuration, which facilitates the most efficient ET pathways (surface diffusion process). We propose that the lysine-13 of mammalian cytochrome c facilitates the most efficient ET pathway to the carboxylate terminus and this proposal is supported by the ET reaction rate of a rat cytochrome c mutant (RC9-K13A) [Elektrokhimiya (2001) in press], in which lysine-13 is replaced by alanine. The ET rate of K13A is more than six orders of magnitude smaller than that of the native protein.     

Cross-coupling reactions of organosilicon compounds: new concepts and recent advances

This review highlights the rapid evolution of the newly-developed class of palladium-catalyzed cross-coupling reactions of organosilicon compounds. A myriad of heteroatom-containing silicon moieties (silyl hydrides, siletanes, silanols, silyl ethers, orthosiliconates, di- and polysiloxanes and pyridylsilanes) undergo mild and stereospecific cross-coupling. The diversity of methods for introduction of silicon groups into organic molecules and the range of organic electrophiles that can be used are emphasized.     

Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis-1,2-diphenylcyclobutane (1), cis-transoid-cis-cyclobutal[1,2-a:4,3-a′] diindene (2) and r-1,c-2-dimethyl-t-3,t-4-di(4-methoxyphenyl)cyclobutane (3) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10⁹ s⁻¹ without ionic dissociation. In the case of 3, however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10⁷ s⁻¹. The rapid ring cleavage of 1⁺ and 2⁺ is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3⁺ is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A⁻⁻−1⁺ and A⁻⁻−2⁺ pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A⁻⁻−D⁺ surface to the A–D surface. In the case of 3, this mechanism cannot be expected to operate in the geminate recombination.     

Digital simulation of electrochemical processes involving very fast chemical reactions : Part 1. A simple general approach

A simple approach based on the steady-state assumption and linear concentration profiles in the reaction layer is proposed to simulate electrochemical systems involving very fast homogeneous chemical reactions. The flux equations are derived in detail and the method is shown to be suitable for catalytic e.c.e and dimerization reaction mechanism. The accuracy of the proposed method is checked by comparing the results obtained with those already available in the literature.     

Cross-coupling reactions of organosilicon compounds in the stereocontrolled synthesis of retinoids

This paper presents a full account of the use of Hiyama cross-coupling reactions in a highly convergent approach to retinoids in which the key step is construction of the central C10-C11 bond. Representatives of two families of oxygen-activated dienyl silanes (ethoxysilanes and silanols) and of all reported families of "safety-catch" silanols (siletanes, silyl hydrides, allyl-, benzyl-, aryl-, 2-pyridyl- and 2-thienylsilanes) were regio- and stereoselectively prepared and stereospecifically coupled to an appropriate electrophile by treatment with a palladium catalyst and a nucleophilic activator. Both all-trans and 11-cis-retinoids, and their chain-demethylated analogues, were obtained in good yields regardless of the geometry (E/Z) and of the steric congestion in each fragment. This comprehensive study conclusively establishes the Hiyama cross-coupling reaction, with its mild reaction conditions and stable, easily prepared, ecologically advantageous silicon-based coupling partners, as the most effective route to retinoids reported to date.     

On the possibility of an STM-induced dissociative electron transfer

The process of dissociative adsorption of a molecule on an electrode in a system of the type in situ scanning tunneling microscopy (STM) is investigated theoretically. It is shown that, in the case of fully nonadiabatic or partly adiabatic electron transfer, the presence of the tip of STM may either accelerate (or even induce) or decelerate the process of dissociation of the molecule, depending on the sign of the bias voltage. The maximum effect takes place in the case of strong interaction of the molecule with both electrodes (fully adiabatic electron transfer). In this limit, diagrams of kinetic modes, which mark off the boundaries between processes of different types possible in a given system, are constructed.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 273–284.Original Russian Text Copyright © 2005 by Kuznetsov, Medvedev.     

Analysis of electrochemical electron transfer mechanisms in molten salts by the frontier orbital method

It has been demonstrated that the application of the frontier orbital method to the quantum-chemical analysis of possible variants of electron transfer in electrochemical systems makes it possible to sharply narrow the search for the transition state and to reduce computational burden by several orders of magnitude. The high informativity of the method allows us to recommend it as a tool for verifying any hypotheses about the mechanism of charge transfer in electrochemical systems.     

Role of electron transfer in reactions involving substitution of halogen in 2-halo-5-nitrofurans

The replacement of the halogen in 2-halo-5-nitrofurans (Hal=C1, Br, I) in dimethylformamide (DMF) under the influence of tetra-n-butylammonium hydroxide to give 2-hydroxy-5-nitrofuran salts was studied. The intermediate anion radicals of the halonitrofurans were recorded by EPR spectroscopy; bromo- and iodonitrofurans can give 5,5-dinitro-2,2-difuryl anion radicals under the influence of Bu₄NOH. The polarographic reduction of halonitrofurans in the presence of hydrogen peroxide also leads to 2-hydroxy-5-nitrofuran. A mechanism for the transformation is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1604–1607, December, 1979Original article submitted March 17, 1977; revision submitted October 30, 1978.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates

The electrochemical reduction of benzyl thiocyanate and p-nitrobenzyl thiocyanate was investigated in acetonitrile at an inert electrode. These two compounds reveal a change in the reductive cleavage mechanism, and more interestingly, they show a clear-cut example of a regioselective bond dissociation. Both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. While the effect of the standard oxidation potential of the leaving group seems to be predominant in understanding the change in the ET mechanism by changing the driving force, the regioselective cleavage is dictated by changes in the intrinsic barrier related to the nature of the substituent on the aryl moiety.     

Comparison of photoinduced electron transfer reactions of C60 and aromatic carbonyl compounds

The one-electron reduction potential of the triplet excited state of C₆₀ is similar to those of some aromatic carbonyl compounds. Thus, photoinduced electron transfer is expected to occur from the common electron donors to both C₆₀ and aromatic carbonyl compounds. In this paper comparison is made between photoinduced electron transfer from organosilanes and organostannanes used as the electron donors to the triplet excited states of C₆₀ and aromatic carbonyl compounds, providing valuable insight into their common mechanistic features for the C-C bond formation via photoinduced electron transfer as well as the new functionalization method of C₆₀.