Temperature dependence of ketyl radical in aqueous benzophenone solutions up to 400 °C: A pulse radiolysis study

Abstract

Pulse radiolysis studies with aqueous benzophenone solutions were performed over the range 25–400 °C, including conditions beyond the critical point of water. The addition of e_aq⁻ to benzophenone forms either ketyl anion in alkaline solutions or neutral ketyl radical in acidic/neutral solutions. Although both have an intense and sharp absorption band in the UV region and a weak but broad band in the visible region, their spectra are clearly different. The peak of the visible band is at 545 nm for the neutral ketyl radical and 610 nm for the anion at room temperature. With increasing temperature, the absorption peak of the ketyl radical shifts to a shorter wavelength while that of the anion shifts to a substantially longer wavelength. The decay of the ketyl radical is not monotonically temperature-dependent: its rate constant (2k/ε) increases with temperature up to 100 °C and then decreases at higher temperatures. The rate constant for the addition of e_aq⁻ to benzophenone can be considered to be diffusion controlled and the Arrhenius plot can be fitted with a curved line. The temperature dependence of the rate constant for the reaction of e_aq⁻ with proton was determined up to 400 °C by the competition method using benzophenone as the reference solute. It was also found that the addition of OH˙ radical to benzophenone forms a transient species with a strong absorption band centered at 380 nm and the peak seems to be slightly blue-shifted with temperature. The decay of this species increases rapidly with temperature, indicating the enhanced conversion to stable products at high temperatures.