Deciphering the reaction between a hydrated electron and a hydronium ion at elevated temperatures.

The formation of a H˙ atom in liquid water from the reaction of a hydrated electron with a hydronium cation is a very challenging subject in chemical processes. Here picosecond pulse radiolysis measurements are performed at elevated temperatures, up to 350 °C, of acidic H2O and D2O solutions (up to 0.1 mol L(-1) HClO4) with the aim of investigating several issues related to this reaction. First, the red shift of the solvated electron absorption band in D2O with increasing temperature is found to be affected by the presence of D3O(+) in solution. The modified absorption spectra demonstrate the formation of a transient pair between D3O(+) and solvated electrons at elevated temperature (200-250 °C) when the concentration of D3O(+) is higher than 0.05 mol L(-1). For higher temperatures at 300 and 350 °C when the rate constant is almost diffusion controlled, the pair is no longer observable. Second, the presolvated electron in D2O is not scavenged up to 250 °C in solution containing 0.1 mol L(-1) D3O(+). Third, the decays in the picosecond range obtained under different conditions show that the rate constants are strongly affected by the temperature and by ionic strength. The model with an equation of log(k/k0) = 2A(I)(0.5)/1 + b(I)(0.5) gives a good fit of the experimental results by taking into account the variation of ionic strength, I, and, in particular, by considering the temperature dependence of the constant A.