ELECTROCHEMISTRY IN NEAR-CRITICAL AND SUPERCRITICAL FLUIDS .4. NITROGEN-HETEROCYCLES, NITROBENZENE, AND SOLVATED ELECTRONS IN AMMONIA AT TEMPERATURES TO 150-DEGREES-C

The electrochemistry of pyrazine, quinoxaline, phenazine, and solvated electrons in near-critical and supercritical ammonia was investigated by cyclic voltammetry and chronoamperometry and compared to that in liquid ammonia at -40°C. The reductions of pyrazine, quinoxaline, and phenazine at room temperature, and in the supercritical fluid (SCF), occur reversibly or quasi-reversibly and result in stable products (anion radicals or dianions) on the voltammetric time scale. The electrochemistry of nitrobenzene was investigated in the presence of water, and reactions of the dianion at higher temperature, similar to those previously reported at -40°C, were observed. The diffusion coefficient of the four aromatics increased by an order of magnitude between -40°C and +150°C, in agreement with the Stokes-Einstein relationship over the temperature range studied. Solvated electrons were electrochemically generated in the supercritical fluid and found to be stable on the voltammetric time scale. The thermodynamics of electrode reactions in the SCF is discussed, and the apparatus for performing electrochemistry in the SCF is described. © 1987 American Chemical Society.