Analysis of the Substituent Effect on the Reactivity Modulation during Self-Protonation Processes in 2-Nitrophenols

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• 作者：Jimmy Alexander Morales-Morales ; Carlos Frontana ; Martha Aguilar-Martí ; nez ; Jos&eacute ; Antonio Bautista-Martí ; nez ; Felipe J. Gonzá ; lez ; Ignacio Gonzá ; lez
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：September 20, 2007
• 年：2007
• 卷：111
• 期：37
• 页码：8993 - 9002
• 全文大小：300K
• 年卷期：v.111,no.37(September 20, 2007)
• ISSN：1520-5215

文摘

A voltammetric and spectroelectrochemical ESR study of the reduction processes of five substituted 4-R-2-nitrophenols (R = -H, -OCH₃, -CH₃, -CN, -CF₃) in acetonitrile was performed. In the potential rangeconsidered here (-0.2 to -2.5 V vs Fc⁺/Fc), two reduction signals (I_c and II_c) were detected; the first onewas associated with the formation of the corresponding hydroxylamine via a self-protonation pathway. Thevoltammetric analysis at the first reduction signal showed that there are differences in the reduction pathwayfor each substituted 4-R-2-nitrophenol, being the E_1/2 values determined by the inductive effect of the substituentin the meta position with respect to the nitro group, while the electron-transfer kinetics was determined bythe protonation rate () of the anion radical electrogenerated. However, at potential values near the firstreduction peak, no ESR signal was recorded from stable radical species, indicating the instability of theradical species in solution. Nevertheless, an intense ESR spectrum generated at the second reduction peakwas detected for all compounds, indicating the monoelectronic reduction of the corresponding deprotonated4-R-2-nitrophenols. The spin-coupling hyperfine structures revealed differences in the chemical nature of theelectrogenerated radical; meanwhile, the -CF₃ and -CN substituents induced the formation of a dianionradical structure, and the -H, -CH₃, and -OCH₃ substituents provoked the formation of an anion radicalstructure due to protonation by acetonitrile molecules of the initially electrogenerated dianion radical. Thisbehavior was confirmed by analyzing the ESR spectra in deuterated acetonitrile and by performing quantumchemical calculations of the spin densities at each site of the electrogenerated anionic radicals.