| A pulse radiolysis facility for the study of fast chemical kinetics in aqueous solution, based on a Febetron 706 electron beam accelerator, has been established at the University of Florida Radiation Chemistry Laboratory.; In previous work on the radiolysis of tetraphenylborate (TPB) solutions carried out in this laboratory, it was found that several organic products, including benzene, phenol, and biphenyl, are produced with substantial yield. However, the reaction mechanism has not been determined. Since the tetraphenylborate anion, TPB{dollar}sp-{dollar}, is a reducing species, it should be more readily subject to attack by OH{dollar}cdot{dollar} than by e{dollar}sb{lcub}rm aq{rcub}sp{lcub}-{rcub}{dollar} or by H{dollar}cdot{dollar}. The lack of reactivity between TPB and e{dollar}sb{lcub}rm aq{rcub}sp{lcub}-{rcub}{dollar} has been confirmed by directly monitoring the transient signal due to e{dollar}sb{lcub}rm aq{rcub}sp{lcub}-{rcub}{dollar}. Concerning the reaction with OH{dollar}cdot{dollar}, two schemes were investigated: (1) a rapid electron transfer from B(C{dollar}sb6{dollar}H{dollar}sb5)sb4sp-{dollar} to OH{dollar}cdot{dollar}; (2) OH{dollar}cdot{dollar} addition to B(C{dollar}sb6{dollar}H{dollar}sb5)sb4sp-{dollar}. Comparison of transient absorption spectra resulting from the two different schemes above suggests that the OH{dollar}cdot{dollar} addition is the dominant reaction under conditions of N{dollar}sb2{dollar}O saturation, with an experimentally determined second-order rate constant of 6.2 {dollar}times{dollar} 10{dollar}sp9{dollar} M{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}. A mechanism based on an initial first-order self-decomposition of the OH{dollar}cdot{dollar} adduct, (C{dollar}sb6{dollar}H{dollar}sb5)sb3{dollar}BC{dollar}sb6{dollar}H{dollar}sb5{dollar}OH{dollar}spdiv{dollar}, is proposed.; Reactions initiated by OH{dollar}cdot{dollar} radicals or e{dollar}sb{lcub}rm aq{rcub}sp{lcub}-{rcub}{dollar} in aqueous IrCl{dollar}sb{lcub}6{rcub}sp{lcub}3-{rcub}{dollar} solutions were studied. The rate constant for the respective reactions were found to be 4.9 {dollar}times{dollar} 10{dollar}sp9{dollar} M{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} and 6.1 {dollar}times{dollar} 10{dollar}sp9{dollar} M{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}. The oxidation product, IrCl{dollar}sb{lcub}6{rcub}sp{lcub}2-{rcub}{dollar} disappears rapidly in N{dollar}sb2{dollar}O-saturated basic solution or in either neutral N{dollar}sb2{dollar}-saturated or aerated solution, but is nearly inert in neutral solution with N{dollar}sb2{dollar}O present. The complex IrCl{dollar}sb{lcub}6{rcub}sp{lcub}2-{rcub}{dollar} reacts rapidly with hydrogen peroxide in basic media, as confirmed on the benchtop and by stopped-flow kinetics. It is therefore inferred that reaction with HO{dollar}sb{lcub}2{rcub}sp{lcub}-{rcub}{dollar} may account for the loss of IrCl{dollar}sb{lcub}6{rcub}sp{lcub}-2{rcub}{dollar} under basic conditions. Loss of Ir(IV) in neutral N{dollar}sb2{dollar}-saturated solution without added N{dollar}sb2{dollar}O may involve electron transfer from Ir(II), and loss of Ir(IV) in aerated solution is attributed to reduction by superoxide ion, O{dollar}sb{lcub}2{rcub}sp{lcub}-{rcub}{dollar}.; Kinetic modeling on the respective mechanistic schemes of the above systems gives good agreement with our experimental results. |
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