On the Kinetic and Thermodynamic Properties of Aryl Radicals Using Electrochemical and Theoretical Approaches

Abstract

In this work, sampled‐current voltammetry performed on a series of aryldiazonium, diaryliodonium, and triarylsulfonium salts allows the determination of the reduction potential of aryl radicals in acetonitrile. Specifically, this is accomplished by measuring the number of electrons consumed in the reduction process as a function of the applied potential. For the phenyl, 4‐bromophenyl, and 4‐nitrophenyl radicals, the reduction potential is found to be −0.91±0.06, −0.90±0.10, and −0.98±0.06 V vs. SCE, respectively. Furthermore, from measurements on an extended series of substituted compounds, it is concluded that the substituent effect on the reduction potential is small, which can be explained by the σ nature of the aryl radical as evidenced from theoretical calculations. At the same time this yields a mean value for the reduction potential of the aryl radical of −0.87 V±0.03 V vs. SCE. Determination of the intrinsic barrier and the standard potential from the data obtained are more uncertain since it is unknown to which extent the competing reference reaction, the electrochemical grafting reaction, is affected by the applied potential. From calculations using density functional theory, the intrinsic barrier for the reduction of the phenyl radical is determined to be 0.32 eV.

Steric and Electronic Effects of Electrochemically Generated Aryl Radicals on Grafting of the Graphite Surface

“…Typically, negative overpotentials of -0.3 V --0.5 V from current peak maximums (Epc) of the reduction waves of the aryldiazonium salts were applied to maximize the reduction of the aryldiazonium salts and avoid the reduction of the aryl radicals. 46…”

Morphological Changes in Electrografted Aryl‐Based Thin Films Induced by using Diazonium Salts or Aryl Iodides

“…One crucial point to understand the difference between the morphology of the NBD and INB generated films is related with the value of the potential at which aryl radical is reduced. Following Scheme , aryl radical is reduced to aryl anion and subsequently to benzene at around −0.98 V vs SCE . When an aryl diazonium salt is used the grafting process occurs at a positive potential enough to impede the reduction of the radicals to the aryl anion.…”

“…Following Scheme 2, aryl radical is reduced to aryl anion and subsequently to benzene at around À0.98 V vs SCE. [18] When an aryl diazonium salt is used the grafting process occurs at a positive potential enough to impede the reduction of the radicals to the aryl anion. On the opposite side, for INB, the reduction potential is negative enough to produce a certain reduction of aryl radicals and promoting a decrease in the amount of radicals that can then undergo the electrografting reaction.…”

“…Although the chemical structure of these molecules is very different, the proposed mechanism for all of them includes the generation of an aryl radical that is grafted on the surface. Nevertheless, the conditions where the grafting occurs for each molecule are extremely dependent on the facility of the production of the aryl radical …”

“…Nevertheless, the conditions where the grafting occurs for each molecule are extremely dependent on the facility of the production of the aryl radical. [18] One of the most recent proposal of organic molecules that could act as precursors of radical species is the use of aryl iodides. [19] Although alkyl iodides are known as grafting agents to anchor alkyl chains on conductive surfaces, [20,21] aryl iodides have not been yet deeply explored.…”