Godsk Larsen A, Hjarbaek Holm A, Roberson M, Daasbjerg K
Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark.
J Am Chem Soc. 2001 Feb 28;123(8):1723-9. doi: 10.1021/ja003811b.
Oxidation (E(1/2)(ox)) and reduction potentials (E(1/2)(red)) of a series of para-substituted phenylthiyl radicals XC(6)H(4)S* generated from the pertinent disulfides or thiophenols have been measured by means of photomodulated voltammetry in acetonitrile. The values of E(1/2)(ox) are of particular interest as they give access to the hitherto unknown thermochemistry of short-lived phenylsulfenium cations in solution. Both E(1/2)(OX) and E(1/2)(red) decrease as the electron-donating power of the substituent raises, resulting in linear correlations with the Hammett substituent coefficient sigma(+) with slopes rho(+) of 4.7 and 6.4, respectively. The finding of a larger substituent effect on than is a consequence of a corresponding development in the electron affinities and ionization potentials of XC(6)H(4)S* as revealed by quantum-chemical calculations. Solvation energies extracted for XC(6)H(4)S(+) and XC(6)H(4)S(-) from thermochemical cycles show the expected substituent dependency; i.e., the absolute values of the solvation energies decrease as the charge becomes more delocalized in the ions. Acetonitrile is better in solvating XC(6)H(4)S(+) than XC(6)H(4)S(-) for most substituents, even if there is a substantial delocalization of the charge in the series of phenylsulfenium cations. The substituent effect on is smaller in aqueous solution than acetonitrile, which is attributed to the ability of water to stabilize in particular localized anions through hydrogen bonding.
通过在乙腈中进行光调制伏安法，测量了由相关二硫化物或硫酚生成的一系列对取代苯硫基自由基XC₆H₄S•的氧化电位（E(1/2)(ox)）和还原电位（E(1/2)(red)）。E(1/2)(ox)的值特别令人感兴趣，因为它们能让我们了解溶液中迄今未知的短寿命苯锍阳离子的热化学性质。随着取代基给电子能力的增强，E(1/2)(ox)和E(1/2)(red)均降低，分别与哈米特取代基系数σ⁺呈现斜率为4.7和6.4的线性相关性。正如量子化学计算所揭示的，发现取代基效应比对XC₆H₄S•的电子亲和能和电离势的相应变化更大。从热化学循环中提取的XC₆H₄S⁺和XC₆H₄S⁻的溶剂化能显示出预期的取代基依赖性；即，随着电荷在离子中更离域，溶剂化能的绝对值降低。对于大多数取代基，乙腈对XC₆H₄S⁺的溶剂化作用比对XC₆H₄S⁻更好，即使在苯锍阳离子系列中电荷有大量离域。取代基效应在水溶液中比在乙腈中小，这归因于水通过氢键特别是稳定局域阴离子的能力。
