Pressure effects electron transfer

The solubility of iodine in many organic vapors behaves similarly at moderate pressures, but at pressures below atmospheric the concentration of iodine is much less than in the pure saturated vapor. Jepson and Rowlinson38 suggested that this is due to strong absorption of the vapors on the surface of the iodine. The effect is most marked with vapors of substances such as pyridine which are known to form strong electron-transfer complexes with iodine. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

These equations may be used directly to predict the effect of pressure on the chemical reactions preceding or following the electron transfer step and, by use of standard thermodynamic formulae, they may be modified to allow a consideration of the electron transfer step itself. For example, the electrode reaction... 

It is apparent that, as in chemical systems, the magnitude of these effects will become useful and interesting from a practical viewpoint only when the pressure is increased above one kilobar. Thus for a typical electron transfer reaction with JF"=—20 cm mole , AE will be 211 mV when the pressme is ten kilobars. This shift could be important in the not uncommon situation where, at atmospheric pressure, the oxidation of a neutral substrate occurs at around the same potential as the anion of the base electrolyte. An increase in the pressure to ten kilobars will result in a separation of the processes... 

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. 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

Freeman et al (Ref 78) reported on the effect of X-rays and gamma rays on the chemical reactivity of AP with respect to thermal de-compn. The effect was shown by DTA and TGA tests under ambient and reduced pressures. The low temp stages of reaction are affected by pre-irradiation. It was suggested that the increased reactivity was due to the presence of positive holes which would indicate an electron transfer... 

These measurements cannot be used to quantify the electron transfer from the semiconductor to the metal deposit, but an estimate has been drawn from studies of oxygen photoadsorption on Pt/Ti(>2 samples in a pressure range such that nearly all of the free electrons are captured to form adsorbed 05 ion-radicals. Increasing Pt contents corresponded to decreasing amounts of photoadsorbed oxygen, which corroborates the effect of deposited Pt on the Ti(>2 free electron density. For Ti(>2 samples evacuated at 423 K and... 

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 optical studies performed on most samples of table 1 were aimed at different aspects of the f-electron properties. A considerable amount of the work was concerned with the energy level shifts under pressure. From these shifts, variations of free-ion parameters, crystal-field parameters or crystal-field strengths with pressure have been deduced. Other studies concentrated on changes in lifetimes or intensities, the efficiency of energy transfer between rare earths or rare earths and other impurities or on electron-phonon coupling effects under pressure. The various aspects investigated under high pressure will be presented within the next sections. 

In the preceding section the pressure effects on the energy transfer from electronic states of constituents of the host lattice to the f element have been discussed. Different to this case, another source of energy transfer can be the electronic state of some point defect in the host lattice which will be considered now. As simple examples, the point defect can be another f element (either of the same or a different kind) or a transition metal element. 

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