Hydrodynamic cryoelectrochemical ESR: The reduction of ortho-bromonitrobenzene in acetonitrile

Novel methodology permitting cryoelectrochemical ESR measurements under hydrodynamic conditions allows new insights into the mechanism of multistep electron transfer processes in general. Specifically the electroreduction of ortho- and para-bromonitrobenzene in acetonitrile is investigated at low temperatures, employing a tubular electrode under conditions of hydrodynamic flow. At room temperature, a full two electrons per molecule are transferred in the ortho-case due to an ECE reaction in which the nitrobenzene radical anion is generated, but at 233 K only a single electron transfer takes place (as is the case for the para-derivative at room temperature). The analytical theory for ECE reactions at a tubular electrode is developed under the assumption of a thin diffusion layer and applied to the above system in order to extract rate constants from the limiting current data. The variation of these kinetic parameters with temperature is used to then determine an activation energy for the homogeneous chemical reaction. A newly designed coaxial-tube electrochemical ESR cell is utilised for the detection of paramagnetic species electrogenerated in situ at variable temperatures. At room temperature, the para-bromonitrobenzene reduction yields a spectrum indicative of the corresponding radical anion which is found to be stable on the experimental timescale, but in the ortho-case the nitrobenzene radical anion spectrum is observed as a result of the ECE process. Upon sufficient cooling, however, we observe a spectral change to that of the ortho-bromonitrobenzene radical anion, as the kinetics of the chemical reaction are slowed.