Bird Matthew J, Pearson Matthew A, Asaoka Sadayuki, Miller John R
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11793-5000, United States.
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
J Phys Chem A. 2020 Jul 2;124(26):5487-5495. doi: 10.1021/acs.jpca.0c02948. Epub 2020 Jun 18.
A novel method to determine redox potentials without electrolyte is presented. The method is based on a new ability to determine the dissociation constant, °, for ion pairs formed between any radical anion and any inert electrolyte counterion. These dissociation constants can be used to determine relative shifts of redox potential as a function of electrolyte concentration, connecting referenced potentials determined with electrochemistry (with 0.1 M electrolyte) to electrolyte-free values. Pulse radiolysis created radical anions enabling determination of equilibrium constants for electron transfer between anions of donor and acceptor molecules as a function of electrolyte concentration in THF. The measurements determined "composite equilibrium constants", , which contain information about the dissociation constant for the electrolyte cations, X, with the radical anions of both the donor, °(D,X) and the acceptor, °(A,X). Dissociation constants were obtained for a selection of radical anions with tetrabutylammonium (TBA). The electrolyte was found to shift the reduction potentials of small molecules 1-methylpyrene and -stilbene by close to +130 mV whereas oligo-fluorenes and polyfluorenes experienced shifts of only (+25 ± 6) mV due to charge delocalization weakening the ion pair. These shifts for reduction of aromatic hydrocarbon molecules are smaller than shifts of +232 and +451 mV seen previously for benzophenone radical anion with TBA and Na respectively where the charge on the radical anion is localized largely on one C═O bond, thus forming a more tightly bound ion pair.
提出了一种无需电解质即可测定氧化还原电位的新方法。该方法基于一种新的能力，即能够确定任何自由基阴离子与任何惰性电解质抗衡离子之间形成的离子对的解离常数°。这些解离常数可用于确定氧化还原电位随电解质浓度的相对变化，将通过电化学方法（使用0.1M电解质）测定的参考电位与无电解质值联系起来。脉冲辐解产生自由基阴离子，从而能够测定供体和受体分子阴离子之间电子转移的平衡常数，该平衡常数是四氢呋喃中电解质浓度的函数。测量确定了“复合平衡常数”，其中包含有关电解质阳离子X与供体°(D,X)和受体°(A,X)的自由基阴离子的解离常数的信息。获得了一系列带有四丁基铵（TBA）的自由基阴离子的解离常数。发现电解质使小分子1-甲基芘和-芪的还原电位偏移接近+130mV，而寡聚芴和聚芴由于电荷离域减弱了离子对，仅经历了(+25±6)mV的偏移。对于芳烃分子还原的这些偏移小于之前分别观察到的二苯甲酮自由基阴离子与TBA和Na的+232和+451mV的偏移，其中自由基阴离子上的电荷主要集中在一个C═O键上，从而形成了更紧密结合的离子对。
