Pulse Radiolysis of Supercritical Water. 2. Reaction of Nitrobenzene with Hydrated Electrons and Hydroxyl Radicals

The rate constants for the reactions of nitrobenzene with the hydroxyl radical (OH • ) and hydrated electron ((e - ) aq ) in water have been measured from room temperature to 400 °C using electron pulse radiolysis and transient absorption spectroscopy. The diffusion-limited reaction of nitrobenzene with (e - ) aq exhibits temperature-insensitive activation energy up to 300 °C, indicating that the activation energy for electron diffusion remains high over this range. The (e - ) aq reactivity is explained as a long-range electron transfer, and the results are interpreted in terms of extended Marcus theory and Smoluchowski relationships. At 380 °C, the rate constant has a density dependence similar to that previously reported for other (e - ) aq scavenging reactions. The reaction rate of nitrobenzene with OH • is very insensitive to temperature from room temperature up to 300 °C, in agreement with previous studies. Above 300 °C, the rate constant increases as the critical temperature is approached and exceeded. Time-resolved electronic absorption spectra of the nitrobenzene radiolysis transients reveal complex kinetics involving multiple absorbing species.