Chromium pulse radiolysis

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. 

The pressure dependences of formation, homolytic and heterolytic fission of chromium-carbon bonds in aqueous solutions were investigated for the [(H20)5CrR]2+ complexes, where R = different alkyl groups [32-34], The reactions (Eqs. 5 and 6) were followed by spectrophotometry [35], the alkyl radicals being generated in situ by pulse radiolysis [36]. 

Other Studies.— The kinetics and mechanisms of reactions of tptz (5) complexes of cobalt(ii), copper(ii), and nickel(u) with water and with hydroxide have been established and compared. Covalent hydrates are believed to be important intermediates in aquation of the [M(tptz)(OH2)3] + complexes. Kinetics of aquation of the anions [M(acac)a] (M = Co, Cr, or Ru) have been studied in pulse radiolysis experiments. All three steps were monitored for the cobalt(n) complex, but the first step for the chromium(ii) complex was too fast to follow and, predictably, all steps for the ruthenium(ir) complex were too slow to follow by this technique. The mechanism of acac loss is thought to involve equilibrium... 

Chromium(ii) is rapidly oxidized by molecular oxygen to yield the [Cr(OH)2Cr] + dimer as product. Pulse-radiolysis studies have been made of the reaction, the initial step,... 

The reduction of ferricytochrome c by hydrated electrons and by several free radicals has been studied by pulse radiolysis. The reduction of oxidized cytochrome c by [Fe(edta)] - follows first-order kinetics for both protein and reductant, with a rate constant of 2.57 x 10 1 mol" s" at pH 7 and activation enthalpy and entropy of 6.0 kcal mol" and —18 cal K" mol", respectively. These values are comparable to those for outer-sphere cytochrome c reductions and redox reactions involving simple iron complexes, and are compatible with outer-sphere attack of [Fe(edta)] " at the exposed haem edge, although the possibility of adjacent attack through the haem pocket is not ruled out. The rate data at pH 9 are consistent with [Fe(edta)] " reduction of two slowly interconverting forms of the protein, native kt = 2.05 X10 1 mol" S" ) and high-pH kt = 2.67 x 10 1 mol" s" ) isomers. A possible route for the transfer of the electron from Cr + to ferricytochrome c has been suggested as a result of the chemical analysis of the chromium(m) product. The reduction by Cr + of the native protein and of ferricytochrome c carboxy-methylated at the haem-linked methionine (residue 80) has been studied kinetically. At pH 6.5 the former process is simple and corresponds to a second-order rate constant of 1.21 x 10 1 mol" s". The latter, however, is complex - two chromium-... 

The [(H20)5CrH] ion can be prepared by uv flash photolysis of aqueous chromium(II) perchlorate. Previously it was made by pulse radiolysis. The ion is characterized by a uv peak at 385 nm, and is shortlived owing to its rapid reaction with HsO ion [equation (1)]. At an... 

The formation of chromium-carbon a-bonds in aqueous media has been investigated, the specific rates of reaction of chromium(ii) with aliphatic radicals being measured. Pulse radiolysis of solutions of Cr and organic molecules generates OH and H radicals which react with the organic species... 

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