Kinetic Studies of the Electron Transfer Reaction in Iron(II) and Iron(III) Systems. IV, The Reaction in Mixed Solvents of Dimethyl Sulfoxide and Water

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The rate constants, kapp, of the electron transfer reaction between iron(II) and iron(III) in mixed solvents of DMSO and water were measured at various acid and DMSO concentrations at μ=0.50M and 25°C. The kapp decreased linearly with the increase in [DMSO] at a constant [H+] and increased linearly with the increase in 1/[H+] at a constant [DMSO]. The reaction is considered to proceed through two paths: Fe2++*Fe3+→Fe3+*Fe2+ and Fe2++*FeOH2+→FeOH2++*Fe2+, with the rate constants of kO and kH respectively. Both kO and kH were suppressed by the addition of DMSO to water because of the occurrence of molecular association between DMSO and water. The hydration number of DMSO in a very dilute aqueous solution was approximately six, as revealed by the cryoscopic measurements. The suppression of kO and kH was proportional to [DMSO], with a common proportional constant. Consequently, DMSO was deduced to destroy the chains of water molecules connected by hydrogen bondings and to interfere with the ease of hydrogen atom transfer between the reducing and oxidizing iron species. DMSO itself did not serve as a ligand to catalyze the reaction as N-methylacetamide, N,N-dimethylacetamide, and various other anions do.
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