Stopped-flow kinetic analysis of the oxidation of semicarbazide by hexachloroiridate(IV)
Stopped-flow kinetic analysis of the oxidation of semicarbazide by hexachloroiridate(IV)
复制标题
六氯铱(IV)氧化氨基脲的停流动力学分析
DOI：
10.1007/s11243-016-0100-1
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发表时间：
2017-02-01
影响因子：
1.7
通讯作者：
Shi，Tiesheng
中科院分区：
文献类型：
--
作者：
Nan，Chunxia;Dong，Jingran;Shi，Tiesheng
A kinetic analysis of the oxidation of semicarbazide (SEM) by the single-electron oxidant [IrCl6]2−has been carried out by stopped-flow spectrometric techniques. The reaction proved to be first order each in [IrCl62−] and [SEM]tot, leading to overall second-order kinetics. The variation in the observed second-order rate constantk′ with pH was explored over the pH range of 0–7.11. Spectrophotometric titration revealed a stoichiometry of Δ[IrCl62−]/Δ[SEM]tot= 4:1 for the redox reaction. On the basis of the rate law, the redox stoichiometry, and the rapid scan spectra, a reaction mechanism is proposed which involves parallel attacks of [IrCl6]2−on both H2NCONHNH3+and H2NCONHNH2as rate-determining steps, followed by several rapid reactions. The rate expression, derived from the reaction mechanism, was utilized to simulate thek′–pH profile yielding a virtually perfect fit and indicating that the reaction path involving H2NCONHNH3+does not make a significant contribution to the overall rate. The reaction between [IrCl6]2−and H2NCONHNH2was further studied as a function of both temperature and ionic strength. From the temperature dependence, activation parameters were obtained as: ∆H2‡= 34.9 ± 1.5 kJ mol−1and ∆S2‡= −78 ± 5 J K−1mol−1. The observed ionic strength dependence suggests that the rate-determining step is between [IrCl6]2−and a neutral species of SEM. Hence, both the temperature and ionic strength dependency studies are in good agreement with the proposed reaction mechanism, in which the rate-determining step involves an outer sphere electron transfer.
