Potential inversion in the reduction of trans-2,3-dinitro-2-butene

The title compound (1) is reduced in a stepwise two-electron process to its dianion. It has been confirmed that potential inversion occurs both in acetonitrile and acetonitrile+water (83%/17%, v/v). By this it is meant that insertion of the first electron is actually more difficult than insertion of the second, i.e. E°1−E°2 is negative (where E°1 and E°2 are the standard potentials for the two reduction steps). The value of E°1−E°2 in the acetonitrile+water mixed solvent is −0.08±0.01 V which corresponds to about 8 kJ mol−1. Other aspects of the electrochemical reduction of 1 are discussed, including the fact that the dianion is unstable on the voltammetric timescale in pure acetonitrile but is stabilized by the presence of water. The electron-transfer kinetics are more sluggish on solid electrodes (platinum or glassy carbon) than on mercury, which was used for the determination of E°1−E°2. Theoretical calculations (DFT and AM1) have been carried out to indicate the structural changes that accompany the reduction of 1 to its radical anion and dianion, and the role of these structural changes in the development of potential inversion is discussed.