Electron reactions, pressure effect

Farhataziz et al. (1974a, b) studied the effect of pressure on eam and found that as the pressure is increased from 9 bar to 6.7 Kbar at 23° (1) the primary yield of e decreases from 3.2 to 2.0 (2) hv increases from 0.67 to 0.91 eV (3) the half-width of the absorption spectrum on the high-energy side increases by 35% and (4) the extinction coefficient decreases by 19%, which is similar to eh. The pressure effects are consistent with the large volume of ean (98 ml/M), whereas the reduction in the observed primary yield at 0.1 ps is attributable to the reaction eam + NH4+. Some of the properties of eam have been discussed by several authors in Solvated Electron (Hart, 1965). 

The hexacyanoferrate(II)/(III) electron exchange reaction is strongly catalyzed by cations such as K" ". However if the K" " is complexed by, e.g., 18-crown-6 or the cryptand [2.2.2] then the rate constant for the uncatalyzed reaction can be determined. Carbon-13 NMR spectroscopy has established that is 240 s (at 298 K), with AVyyJ = —11.3 cm mol Pressure effects on... 

These observations are all consistent with the proposed tandem-catalysis mechanism shown in Fig. 8. Central to this proposal is that the reaction proceeds via several parallel reaction steps as might be expected for formal eight-electron coupling of two methane molecules to acetic acid. The observation that the reaction is effectively stopped by high concentrations of added CO but proceeds at low levels of CO can be explained if the rate-limiting step is oxidation of Pd(0) by sulfuric acid. From independent experiments we have found that under high CO pressure... 

Scott JR, Fairris JL, McLean M, et al. Intramolecular electron-transfer reactions of cytochrome 5 covalently bonded to ruthenium(II) polypyridine complexes reorgani-zational energy and pressure effects. Inorg Chim Acta 1996 243 193-200. 

Reviews have appeared on the luminescence of inorganic solids, photophysics of metal complexes, pressure effects on photochemical reactions, photoredox reactions of mixed-valence compounds, photochemical electron treuisfer reactions, ... 

Much of our earlier work on pressure effects on the kinetics of electron transfer reactions focused upon the rate constants kex and corresponding volume of activation for self-exchange reactions... 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

A negligible pressure effect on the diastereoselectivity was also observed for the cycloaddition of the enamine carbaldehyde (81a) carrying an electron-withdrawing group at position 3 and 61a to yield the dihydropyrans 82 and 83 (Scheme 8.21). This reaction was again studied by direct quantitative infrared spectroscopy up to 300 MPa between 45 and 95 °C in different solvents. The activation volume was found to be —(25.1 1.7) cm mol in dichloromethane and —(25.0 + 1.8) cm mol in isodurene. Thus, in this reaction solvent polarity had no influence on the pressure dependence of the rate coefficient in addition, the ratio of the two diastereo-meric products is not changed under high pressure thus the AAV value is very small (AAV < 1 cm mol ). 

The effect of pressure on a number of inner-sphere electron transfer reactions has also been investigated. By way of example, the reaction of Co(NH3)5X2-" with Fe(H20)6 -" exhibits Avalues of +10.7 (X = F), +8.7 (X = Cl), +6.4 (X = Br), and +13.0 (X = N3 ), which are mainly ascribed to the release of a solvent molecule during the formation of the bridged inner-sphere species, [(NH3)5Co-X-Fe(H20)5] (24). Other examples of pressure effects on inner-sphere electron transfer reactions, also including some intramolecular reactions induced by pulse radiolysis, have been reported in the htera-ture (i, 25, 26). 

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