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

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.