Electron transfer reactions high pressure

For my first volume as Editor, I have invited Professor Colin D. Hubbard (University of Erlangen-Niirnberg, Erlangen, Germany and University of New Hampshire, Durham, NH, USA) as co-editor. Professor Hubbard studied chemistry at the University of Sheffield, and obtained his PhD with Ralph G. Wilkins. Following post-doctoral work at MIT, Cornell University and University of California in Berkeley, he joined the academic staff of the University of New Hampshire, Durham, where he became Professor of Chemistry in 1979. His interests cover the areas of high-pressure chemistry, electron transfer reactions, proton tunnelling and enzyme catalysis. 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

A number of electron-transfer reactions of biological interest have been studied using high-pressure techniques (4, 5). These include the oxidation of L-ascorbic acid by [Fe(CN)6]3- (148), [Fe(CN)5N02]3 - (149), and Fe(phen)2(CN)2] (150). The first two reactions are characterized by volumes of activation of -16 and 10 cm3 mol-1, respectively, which indicate that solvent rearrangement as a result of an increase in electrostriction must account for the volume collapse on going to... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

Reviews " of pentacyanoferrate substitution kinetics have included a detailed consideration of high-pressure studies of thermal and photochemical substitution and electron transfer reactions of pentacyanoferrates-(II) and -(III). Photochemical activation can result in the loss of L or of CN . The best way to study the latter is through photochemical chelate ring closure in a pentacyanoferrate complex of a potentially bidentate ligand LL [Fe(CN)5(TL)]" rFe(CI 4(LL)] " +... 

The focus here, in so far as the value of employing high-pressure kinetics measurements in electron transfer reactions is concerned, will be on OSET reactions,... 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

Electrochemical methods have also been adopted for application of high pressure [41-43] (see Chapter 5). Correlations emerging from these investigations have valuable application in the interpretation of partial molar volume changes associated with electron transfer reactions (see Sect. 1.3.4). A potential future interest is in reactions carried out at elevated pressures in a suprercritical fluid medium in view of this a special optical cell has been developed for studying organometallic reactions initiated by flash photolysis in supercritical fluids [20] (see Chapters 12 to 14). 

Electron Transfer Reactions Under High Pressure Application of Spectroscopic and Electrochemical Techniques... 

A challenging question concerns the feasibiUty of the apphcation of high-pressure kinetic and thermodynamic techniques in the study of long distance electron transfer reactions. Do such processes exhibit a characteristic pressure... 

Application of electrochemical methods can be suitable for certain oxidation-reduction reactions. Both homogeneous and heterogeneous electron transfer reactions have been investigated electrochemically. The technique and apparatus can range from reasonably standard cyclic voltammetry and rapid scan voltammetry to quite specialized arrangements.In the latter case, the method often needs to be adapted to the context of a particular reaction. An example of high-pressure electrochemistry apparatus is shown in Figure 6. 

Three cases of Type la activations illustrate a class of reactions expected to give positive results. The first one is provided by SrnI or ETC processes. Figure 1 shows the chain mechanism of the reaction of lithium nitronate with 4-nitro-benzyl bromide established by Komblum and Russell. This reaction was expected to display sonochemical switching, which was indeed foimd. The mechanism suggests that the sonochemical activation should find its origin either in creating species 1 or 2 (no direct entry to 3 seems plausible). The creation of 1 within a cavitation bubble could result either from high-pressure-promoted electron transfer (activation volumes for some electron transfer reactions may be found in Ref. 9) or local conditions at the interface between the cavitation bubbles and the bxilk solution (Qi. 1). The creation of radical 2 could result from a direct sonolysis of the benzylic C-Br bond (p. 86) but... 

For more than a century, a number of different aluminum alloys have been commonly used in the aircraft industry These substrates mainly contain several alloying elements, such as copper, chromium, iron, nickel, cobalt, magnesium, manganese, silicon, titanium and zinc. It is known that these metals and alloys can be dissolved as oxides or other compounds in an aqueous medium due to the chemical or electrochemical reactions between their metal surfaces and the environment (solution). The rate of the dissolution from anode to cathode phases at the metal surfaces can be influenced by the electrical conductivity of electrolytic solutions. Thus, anodic and cathodic electron transfer reactions readily exist with bulk electrolytes in water and, hence, produce corrosive products and ions. It is known that pure water has poor electrical conductivity, which in turn lowers the corrosion rate of materials however, natural environmental solutions (e g. sea water, acid rains, emissions or pollutants, chemical products and industrial waste) are highly corrosive and the environment s temperature, humidity, UV light and pressure continuously vary depending on time and the type of process involved. ... 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

---