Kinetic studies of the electron transfer reaction in iron(II) and iron(III) systems. VIII. The effect of hexamethylphosphoric triamide on the electron transfer reaction in aqueous solution.
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概要
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The kinetics and mechanism of the electron transfer reaction between Fe(II) and Fe(III) in mixed solvents of water and hexamethylphosphoric triamide (HMPA) were studied. The apparent rate constant <I>k</I> varies depending upon [HMPA] and [H<SUP>+</SUP>]; <I>k</I> exhibits a minimum value at [HMPA]=1.5 M at constant [H<SUP>+</SUP>] and increases linearly with respect to [H<SUP>+</SUP>]<SUP>−1</SUP> at constant [HMPA]. Judging from the basicity constant of HMPA, the complex formation constant of Fe(hmpa)<SUP>3+</SUP>, and the absorption spectra of aqueous solutions containing Fe(II), Fe(III), HMPA, and HClO<SUB>4</SUB>, the existing species in the reaction systems are mostly Fe<SUP>2+</SUP><SUB>aq</SUB>, Fe<SUP>3+</SUP><SUB>aq</SUB>, Fe(OH)<SUP>2+</SUP>, Fe(hmpa)<SUP>3+</SUP>, and Fe(hmpa)(OH)<SUP>2+</SUP> at low concentrations of HMPA. Therefore, the probable reaction paths are Fe<SUP>2+</SUP>–Fe<SUP>3+</SUP>, (<I>k</I><SUB>0</SUB>); Fe<SUP>2+</SUP>–Fe(OH)<SUP>2+</SUP>, (<I>k</I><SUB>H</SUB>); Fe<SUP>2+</SUP>–Fe(hmpa)<SUP>3+</SUP>, (<I>k</I><SUB>L</SUB>); and Fe<SUP>2+</SUP>–Fe(hmpa)(OH)<SUP>2+</SUP>, (<I>k</I><SUB>HL</SUB>) at the concentration range of [HMPA]=0–1 M and [HClO<SUB>4</SUB>]=0.03–0.2 M. The occurrence of a minimum value of <I>k</I> <I>vs.</I> [HMPA] is reasonably understood by taking the complex formation between Fe(III) and HMPA into account in reaction rate equation. If the hydrolysis constants of both Fe(OH)<SUP>2+</SUP> and Fe(hmpa)(OH)<SUP>2+</SUP> are considered to be nearly of the same order, the value of <I>k</I><SUB>H</SUB> are approximately ten times larger than that of <I>k</I><SUB>HL</SUB>. At higher concentrations of HMPA, the higher Fe(III)-complexes with HMPA might take part in the electron transfer reaction.
- 公益社団法人 日本化学会の論文
著者
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Abe Yuriko
Department Of Chemistry Faculty Of Science Nara Women's University
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Wada Goro
Department of Chemistry, Faculty of Science, Nara Women s University
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Nakago Sayuri
Department of Chemistry, Nara Women's University
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