Probing the ultrashort-lived and powerful oxidizing radical cation H2O•+ – H2O°+-Rad

Chemical reactions triggered by the radiation change profoundly the composition of the medium through the destruction of existing bonds and the build-up of new chemical bonds. Knowledge of the mechanisms of these reactions is of principal importance. Among the reactions induced by ionizing radiation, oxidation reactions are very present and initiate corrosion of materials, or induce oxidative stress. Although the hydroxyl radical is often seen as a major oxidative species, recent experimental results suggest that the cation radical H2O•+, with a life-time less than 100 fs, is also itself a strong oxidant. Characterization of the positive hole induced in water by radiation is an important issue. Irradiated highly concentrated media (like nuclear waste treatment in highly concentrated nitric acid) or solid /water interface (as storage medium), water in the first solvation shell of DNA or water/ organic molecule mixtures could be hangouts for the reactions of the radical cation H2O•+. Up to now, the knowledge of the properties of this radical cation produced by UV, X, gamma radiation and also by energetic charged particles in water is very poor. The chemical moieties induced by water radical could be different than that by OH• radical which is considered as the precursor of other oxidizing radicals. It is therefore very important to characterize this very short-lived radical and to highlight its responsiveness. In order to reach this goal, following studies will be undertaken:

1 – Direct observation of H2O+• in neat water: The first task concerns the exact nature and lifetime of H2O+• in neat water. The lifetime of the radical cation in pure water defines the time window on which the oxidizing reaction can take place. Ultrashort UV-Vis fs pump-probe measurements, with a time resolution of 25 fs will be performed in neat water (and in heavy water). The objective is to follow its absorption spectrum and to observe the kinetics of OH• (and OD•) formation.
2 - The second task is to achieve atomistic numerical simulations combining quantum and molecular mechanics approaches, with the aim to provide complementary insights into experimental data as well as intimate details on the reaction mechanisms. Our first objective will be to characterize the degree of delocalization of the hole centred on the ionized water molecule. Our second, main objective will be to investigate the reactivity of H2O•+, and more specifically the competition between proton transfer reactions and redox reactions. Reactions in heavy water will also be investigated.
3 – The third task is the study of the electron transfer involving the radical water hole by observing the reactions in heavy water. The electron transfer reaction will be studied in highly concentrated sulfuric acid (using D2SO4 in heavy water) using the picosecond pulse radiolysis technique.
4 - The fourth task is the demonstration of the reactivity of the radical cation at interfaces that is crucial to account for radiation-induced corrosion. To distinguish the oxidation effect of H2O•+ from another oxidative species on polycrystalline material, silver samples will be irradiated by the alpha (4He2+) beam from the cyclotron of CEMHTI (at Orléans, CNRS) at a few tens of MeV.
In conclusion, the present project aims at characterizing H2O+• by determining its lifetime, its absorption spectrum, the related charge delocalization and its redox potential in neat water and to demonstrate the important role of this radical in oxidation processes in concentrated solutions, and at interfaces which was never mentioned before. Knowing this role could provide further answers on the mechanisms and guidelines to prevent oxidation phenomena at the interface.