Pulse radiolysis pressure effects

In one case pulse-radiolysis techniques were employed to study the effect of pressure on such reactions. The oxidation of [Cu lphenhl by dioxygen proceeds via a Cu1—02 transient in which a copper—oxygen bond is formed, followed by the rapid formation of [Cun(phen)2] and 02 (110). This process is characterized by a AV of 22 cm3 mol1, which is close to the reaction volume expected for the binding of dioxygen. 

In some cases pulse-radiolysis techniques were employed to study the effect of pressure on inorganic reactions. For instance the oxidation of [CuI(phen)2] by dioxygen via the formation of a C -C transient species was studied using this technique (see Section III,A). Other examples include the formation and cleavage of metal-carbon (7-bonds, which formally involve a change in the oxidation state of the metal. A typical example of a volume profile for the formation and cleavage of a Co-CH3 bond is reported in Fig. 21 for the reaction (162)... 

Pulse Radiolysis A technique related to flash photolysis pulse radiolysis uses very short (nanosecond) intense pulses of ionizing radiation to generate transient high concentrations of reactive species. See Salmon, G. A. and Sykes, A.G., Pulse radiolysis, Methods Enzymol. 227, 522-534, 1993 Maleknia, S.D., Kieselar, J.G., and Downard, K.M., Hydroxyl radical probe of the surface of lysozyme by synchrotron radiolysis and mass spectrometry. Rapid Commun. Mass Spectrom. 16, 53-61, 2002 Nakuna, B.N., Sun, G., and Anderson, V.E., Hydroxyl radical oxidation of cytochrome c by aerobic radiolysis, Free Radic. Biol. Med. 37, 1203-1213, 2004 BataiUe, C., Baldacchino, G., Cosson, R.P. et al., Effect of pressure on pulse radiolysis reduction of proteins, Biochim. Biophys. Acta 1724, 432-439, 2005. 

BataiUea C, Baldacchinoa G, Cosson RP, Coppoc M, Trehenc C, Vignerona G, Renaulta JP, Pina S. (2005) Effect of pressure on pulse radiolysis reduction of proteins. Biochim Biophys Acta 1724 432 39. 

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

To measure the back reaction, it is necessary to generate H in alkaline solution, and this was achieved by Matheson and Rabani [71] who developed a pulse radiolysis cell capable of operating at a pressure of 100 bar. In this way they were able to produce H effectively during the pulse (0.4 ps) by the following reaction (Eq. 60) ... 

The kinetics of the disappearance of the BrO transient, formed during the pulse radiolysis of 02 + Br or N20 + Br2 mixtures, have been investigated.149 Second-order kinetics were confirmed although additional effects, due to gas pressure and dosage, were detected. Bromine(m) oxide, Br203, has been prepared by the thermal decomposition of Br204.150 The vibrational spectrum of Br2Os shows the presence of a Br—O—Br bond, but it has not been... 

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). 

His research interests include electron transfer (particularly over long distances using oligopeptides and proteins as structural motifs), radiation chemistry, inorganic reaction mechanisms, the reactivity of energetic inorganic species produced by radiation, the effects of high pressure on chemical reactivity, reactions in supercritical solvents, and the development of new accelerators and new detection techniques for pulse radiolysis. 

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