Electron-transfer reactions of extremely small AgI colloids

Small colloidal AgI particles (particle diameter (20–50 Å) have been prepared in water and acetonitrile, and optical effects due to size quantization have been observed. Electron transfer reactions involving electron donors and electron acceptors with AgI have been studied by pulse radiolysis techniques. Both reduction and oxidation of the colloids led to transient bleaching of semiconductor absorption. The recovery of the bleaching has been attributed to corrosion processes. Electrons injected into AgI colloids produce metallic silver and hydrogen. Hydrogen evolution is catalyzed by metallic silver formation.     

Photocatalytic reactions at the graphite/ice interface

We have studied the photodissociation of water molecules at the ice/graphite interface in the presence of submonolayer amounts of potassium. The ice films were grown at cryogenic temperatures and ultrahigh vacuum conditions. They are transparent for 240-900 nm photons, but the strong light absorption in the uppermost layers of the graphite substrate generates energetic charge carriers that may drive photoreactions at the interface. Similar schemes have been demonstrated and investigated before in the monolayer regime, without an ice layer (D. Chakarov, M. Gleeson and B. Kasemo, J. Chem. Phys., 2001, 115, 9477). Here, using ice films of tens of monolayer thickness and with different morphologies, we have investigated the confinement effects due to the ice layer, and the ice permeability for reaction products.     

Electron-transfer reactions of amines with α-ferrocenylcarbonium ions

The production observed in the reactions of α-ferrocenylcarbonium ions with tertiary amines do not originate from ferrocenylcarbene intermediates. Evidence is presented in support of an electron-transfer mechanism leading to α-ferrocenylcarbinyl radicals as reaction intermediates.     

Electron-transfer reactions and conformational changes associated with the reduction of substituted bianthrones

The electrochemical reduction of several substituted bianthrones is similar to that of the parent compound. 3,3′-Dimethylbianthrone (II), 3,3′-di-n-heptylbianthrone (III). 3,3′-dimethoxybianthrone (IV) and 1,1′-dimethylbianthrone -(V) were studied in dimethylformamide using cyclic voltammetry and transmission mode spectroelectrochemistry. For each compound the low temperature A form is reduced in a two-electron irreversible reaction to a twisted dianion, B^2?. Upon oxidation, B^2? forms first B, then B, whose spectral properties are identical to those of the high-temperature thermochromic form of the bianthrones. Rate constants for the B-A reaction were determined for each compound. The reduction of 2,3,2′,3′-dibenzo-7,7′-dimethylbianthrone (VI) showed somewhat different features which were tentatively interpreted in terms of redox catalysis.     

Electron-transfer reactions with significant changes in structure. Unsymmetrical crowded ethylenes

The electrochemical reduction mechanisms of xanthylideneanthrone, 6, thioxanthylideneanthrone, 7, 10-(diphenylmethylene)anthrone, 8, and 9-(diphenylmethylene)-9H-fluorene, 9, have been studied in dimethylformamide. The reduction of the first two compounds proceeds from folded forms of the neutral to twisted forms of the anion radical according to a square scheme. The data for reduction of 8 can be well accounted for by the same square scheme. However, one-step reduction with concerted electron transfer and structural change cannot be ruled out. Compound 9, whose fluorene ring system cannot fold, exists only in twisted forms in the neutral, anion radical, and dianion. Consequently, there are no major changes in structure upon reduction, and the compound is reduced in two reversible steps with the second complicated by rapid loss of the dianion that is probably due to protonation by components of the medium.     

Surface reactions of carbon dioxide at the adsorbed water-iron oxide interface

Despite the fact that carbon dioxide is an abundant atmospheric gas with profound environmental implications, there is little information on the reaction of carbon dioxide at the adsorbed water-oxide interface. In this study, the chemistry of carbon dioxide at the adsorbed water-iron oxide interface is investigated with FTIR spectroscopy. As shown here, the thin water layer on the iron oxide surface plays an important role in the surface chemistry of carbon dioxide. In particular, adsorbed water enhances CO(2) uptake, undergoes isotope exchange with CO(2) in O(18)-labeled experiments, and influences the chemical nature of the predominant adsorbed product on the surface from bicarbonate to carbonate. The resultant thin water film is acidic in nature from the reaction of CO(2). The IR spectrum recorded of adsorbed carbonate at the adsorbed water-iron oxide interface is remarkably similar to that at the bulk liquid water-iron oxide interface. Since reactions in thin water films estimated to be approximately 2 layers will play a role in a number of environmental processes, it is essential to understand the chemistry of these "wet" interfaces with atmospheric gases.     

Electron-transfer mechanism in radical-scavenging reactions by a vitamin E model in a protic medium

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) or galvinoxyl radical (GO.) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO4)2 in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.     

Accelerated reactions of amines with carbon dioxide driven by superacid at the microdroplet interface

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk. Carbon dioxide undergoes C–N bond formation reactions with amines at the interface of droplets to form carbamic acids. Electrospray ionization mass spectrometry displays the reaction products in the form of the protonated and deprotonated carbamic acid. Electrosonic spray ionization (ESSI) utilizing carbon dioxide as nebulization gas, confines reaction to the gas–liquid interface where it proceeds much faster than in the bulk. Intriguingly, trace amounts of water accelerate the reaction, presumably by formation of superacid or superbase at the water interface. The suggested mechanism of protonation of CO₂ followed by nucleophilic attack by the amine is analogous to that previously advanced for imidazole formation from carboxylic acids and diamines.

Microdroplets display distinctive interfacial chemistry, manifested as accelerated reactions relative to those observed for the same reagents in bulk.     

Ionic components dependence of the charge transfer reactions at the silicon/HF solution interface

The actual requirements for circuit miniaturization and production economy require obtaining smooth silicon surfaces using diluted chemicals, especially HF treatment. This fundamental research deals with the electrochemical corrosion of n- and p-type silicon substrates in 0.25 M dilute HF solutions, and examines the influence of fluoride ions or protons additives. All experiments were conducted both in the dark and under constant light flux, with solutions thoroughly degassed by high purity argon bubbling. Polarization resistance measurements near the open circuit potential lead to the value of the corrosion current. The kinetics of charge transfer reactions, studied by linear voltammetry, were interpreted as a function of the carrier density in the energy levels of the semiconductor and the concentration of acceptor species in the solution. The influence of these parameters on the surface roughening of the silicon samples was also studied by ex situ atomic force microscopy profile measurements. Received: 13 December 1998 / Accepted: 29 March 1999     

Electron-transfer reactions and functionalization of cytochrome P450cam monooxygenase system in reverse micelles

Enzyme-based electron-transfer reactions involved in the cytochrome P450 monooxygenase system were investigated in nanostructural reverse micelles. A bacterial flavoprotein, putidaredoxin reductase (PdR), was activated and shown to be capable of catalyzing the electron transport from NADH to electron-carrier proteins such as cytochrome b5 (tCyt-b5) and putidaredoxin (Pdx) in reverse micelles. Ferric tCyt-b5 in reverse micelles was effectively converted to its ferrous form by the exogenous addition of separately prepared reverse micellar solution harboring PdR and NADH. The fact that direct interactions of macromolecular proteins should be possible in the reverse micellar system encouraged us to functionalize a multicomponent monooxygenase system composed of the bacterial cytochrome P450cam (P450cam), putidaredoxin (Pdx), and PdR in reverse micelles. The successful camphor hydroxylation reaction catalyzed by P450cam was significantly dependent on the coexistence of Pdx, PdR, and NADH but not H2O2, suggesting that the oxygen-transfer reactions proceeded via a "monooxygenation" mechanism. This is the first report of a multicomponent cytochrome P450 system exhibiting enzymatic activity in organic media.     

Electron correlation effects in the adiabatic charge transfer reactions at the metal/polar liquid interface

New simple expressions for average number of electrons in the valence orbital of a reacting ion and the charge susceptibility are obtained that allow one to calculate adiabatic free energy surfaces (AFES) and corresponding kinetic regime diagrams (KRD) for adiabatic processes of electron transfer from the ion, located in a polar liquid, to a metal within the framework of the exactly solvable (in the limit T-->0) model of the metal with the infinitely wide conduction band. This model represents one of limiting cases of the Anderson model that may be applied to s-p metals. Unlike previous studies of the adiabatic reactions in the model of the metal with the infinitely wide conduction band, the present work takes into account the electron-electron correlation effects in an exact manner. General results are illustrated with KRD which determine the regions of the physical parameters of the system corresponding to various types of electron transfer processes. AFES are calculated for some typical parameters sets. The exact AFES are compared with those calculated within the Hartree-Fock approximation. It is shown that the correlation effects are of importance and results not only in a considerable decrease of the activation free energy but also to qualitatively different shapes of AFES in some regions of the system parameters.     

529—Chemical reactions and oscillating potential difference variations at an oil-water interface

Oscillatory potential and difference interfacial tension variation can be observed at an oil-water interface containing charge species when the conditions are such that hydrodynamic instabilities can occur. We propose a mechanism based on an experimental study accountable for the relaxation-type oscillations observed. It involves the coupling of a chemical reaction occurring in the bulk in the vicinity of the interface with an interfacial transfer by diffusion and adsorption-desorption processes.