Non-linear arrhenius behaviour of the rate constants of some activation-controlled reactions of the hydrated electron in the temperature range 20–200 °C

Rate constants for the reaction of e–aq with N2O, NO–3, NO–2, phenol, SeO2–4, S2O2–3 and Mn2+ have been measured in the temperature range 20–200 °C by pulse radiolysis. In each case non-linear Arrhenius behaviour of the rate constant has been observed for the activation-controlled part of the overall process of diffusion and reaction. The activation energies (E)a decrease with increasing temperature and, except for Mn2+, become negative. The data are interpreted in terms of a model for reversible electron transfer which implies that activation-controlled reactions of e–aq may be reversible. In the case of Mn2+, Ea first decreases and then increases with increasing temperature. It is suggested that a second channel becomes operative at elevated temperatures for the reaction of e–aq with Mn2+.