Pulse Radiolysis of Potassium Bromide Solutions

Halide Photoredox Chemistry

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X-X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-to-ligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights.

Sequential radiation chemical reactions in aqueous bromide solutions: pulse radiolysis experiment and spur model simulation

Pulse radiolysis experiments were carried out to observe transient absorptions of reaction intermediates produced in N₂O- and Ar-saturated aqueous solutions containing 0.9–900 mM NaBr.

The radiolysis of aqueous solutions of oxybromine compounds; the spectra and reactions of BrO and BrO 2

Pulse radiolysis supplemented by steady state radiolysis of aqueous solutions containing some of the following solutes: N 2 O, Br - , BrO - , BrO - 2 , BrO - 3 , CO 2- 3 , and OH - has been used to identify the absorption spectra of BrO ( λ max = 350 nm) and BrO 2 ( λ max = 475 nm) and to evaluate the following rate constants (units, M -1 s -1 ): e - aq. + BrO - → Br - + O - (2.3 ± 0.5 x 10 10 ); e - aq. + BrO - 2 → BrO + O 2- (1.8 ± 0.2 x 10 10 ); e - aq. + BrO - 3 → BrO 2 + O 2- (4.1 ± 0.2 x 10 9 ); OH + BrO - → OH - + BrO (4.5 x 10 9 ); O - + BrO - → BrO + O 2- (4.6 x 10 9 ); OH + BrO - 2 → OH - + BrO 2 (1.9 x 10 9 ); Br - 2 + BrO - → BrO + 2Br - (8.0 ± 0.7 x 10 7 ); Br - 2 + BrO - 2 → BrO + Br - + BrO - (8.0 ± 0.8 x 10 7 ); BrO + BrO - 2 → BrO - + BrO 2 (3.4 ± 0.7 x 10 8 ); 2BrO 2 ⇌ Br 2 O 4 ( k = 1.4 x 10 9 and K = 19 M -1 ); Br 2 O 4 + OH - → H + + BrO - 2 + BrO - 3 (7 x 10 8 ); 2BrO + H 2 O → BrO - + BrO - 2 + 2H + (4.9 ± 1.0 x10 9 ); CO - 3 + BrO - → CO 2- 3 + BrO (4.3 ± 0.4 x 10 7 ); CO - 3 + BrO - 2 → CO 2- 3 + BrO 2 (1.1 ± 0.1 x 10 8 ). In contrast to their chlorine analogues, little is known of the chemistry of the bromine oxides in aqueous solution. In this paper we describe the methods of formation and identification of the radicals BrO and BrO 2 , and their reactions with various oxybromine species, as elucidated by the techniques of pulse radiolysis and kinetic spectroscopy applied to aqueous solutions containing oxybromine anions.

The oxidation of ferrocytochrome c by Br2-, (SCN)2-, N3 and OH radicals studied by pulsed-electron and gamma-ray radiolysis

The reactions of ferrocytochrome c with Br2-, (SCN)2-, N3 and OH radicals were followed by measuring the change in the optical spectra of cytochrome c on gamma-irradiation as well as the rate of change of absorbance upon pulse irradiation. Ferrocytochrome c is oxidized to ferricytochrome c by Br2-, (SCN)2- or N3 radical with an efficiency of about 100% through a second-order process in which no intermediates were observed. The rate constants in neutral solutions at I = 0.073 are 9.7 . 10(8) M-1 . s-1, 7.9 . 10(8) M-1, 1.3 . 10(9) M-1 . s-1 for the oxidation by Br2-, (SCN)2- and N3 radicals, respectively. The rate constants do not vary appreciably in alkaline solutions (pH 8.9). The ionic strength dependence was observed for the rate constants of the oxidation by br2- and (SCN)2-. Those rate constants estimated on the assumption that the radicals react only with the amino acid residues with the characteristic steric correction factors were less than one-tenth of the observed ones. These results suggest that the partially exposed region of the heme is the probable site of electron transfer from ferrocytochrome c to the radical. Hydroxyl radicals also oxidize ferrocytochrome c with a high rate constant (k greater than 1 . 10(10) M-1 . s-1), but with a very small efficiency (5%).

Pulse radiolysis of lactate dehydrogenase

Pulse radiolysis has been used to investigate the rates and transient spectra for the reactions of free radicals with beef heart lactate dehydrogenase at pH 7. Analysis of the results leads to second-order rate-constants for eaq-, .OH, .I, .Br2-, .I2- and .(CNS)2- which are, respectively, 24, 21, 10, 0.55, 0.43 and 0.15 in units of 10(10) M-1 s-1 with uncertainties of +/- 20 per cent. Those for .I and .I2- are similar to the corresponding rate-constants for the related enzyme alcohol dehydrogenase. The spectra of the transient species produced by .OH, .Br2- and .(CNS)2- all showed evidence for reactions with tyrosine and tryptophan residues, and in general terms the magnitudes of the rate-constants appeared to increase with the oxidizing abilities of the radicals. The implication of the results for understanding the mechanism of deactivation by free radicals is discussed.