title: Mechanistic study on the electrochemical reduction of 9,10-anthraquinone in the presence of hydrogen-bond and proton donating additives.
abstract: The electrochemical reduction of 9,10-anthraquinone (AQ) was investigated in CH(3)CN in both the absence and presence of the hydrogen-bond and proton donating additives, CH(3)OH, CH(CF(3))(2)OH, phenol, 4-methoxyphenol, 4-cyanophenol, 2,4,6-trichlorophenol, and benzoic acid (BA).  Three clearly different types of electrochemical behavior were observed with increasing concentrations of the additives, and were simulated to analyze the reaction mechanisms.  Type I was observed for weakly interacting additives, such as CH(3)OH, characterized by positive shifts of the two well-separated reduction waves, corresponding to the formation of AQ(•-) and AQ(2-), with no loss of reversibility.  The second wave shifted more strongly, and finally merged with the first.  These behaviors are explained by the association of AQ(2-) with the additives via strong hydrogen-bonding.  Type II is attributed to a reduction mechanism involving quantitative formation of strong hydrogen-bonded complexes of AQ(2-) with additives, such as CH(CF(3))(2)OH, phenol and 4-methoxyphenol, showing a reversible or quasireversible two-electron reduction wave with increasing concentrations of the additives.  The behavior of Type III, observed in the presence of strongly interacting additives, such as 2,4,6-trichlorophenol and BA, is characterized by a voltammogram composed of the 2-electorn cathodic and the broad anodic waves without keeping reversibility, facilitated by proton transfer in the hydrogen-bonded complexes, AQ(•-)-BA and AQ(2-)-BA.  The effects of hydrogen-bonding and protonation on the electrochemistry of AQ have been systematically demonstrated in terms of the potentials and reaction pathways of the various species, which appear in quinone-hydroquinone systems.
authors.affiliation: Graduate School of Drug Discovery and Medical Information Sciences, Gifu University, Japan.
authors.forename: Jiro
authors.initials: J
authors.lastname: Katsumi
authors.forename: Tatsushi
authors.initials: T
authors.lastname: Nakayama
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Esaka
authors.forename: Bunji
authors.initials: B
authors.lastname: Uno
mesh_terms: D000880:Anthraquinones / Q000737:chemistry*; D000970:Antineoplastic Agents / Q000737:chemistry; D004563:Electrochemistry; D004579:Electron Transport; D006860:Hydrogen Bonding; D011522:Protons
chemical_list: D000880:Anthraquinones; D000970:Antineoplastic Agents; D011522:Protons; C042834:9,10-anthraquinone