Reactivity of the Strongest Oxidizing Species in Aqueous Solutions: The Short-Lived Radical Cation H2O(•.)

The radical cation H2O(•+) formed under irradiation of liquid water undergoes an ultrafast proton transfer reaction and consequently exhibits an extremely short lifetime. The proton transfer yields an oxidizing OH(•) radical whose reactivity has been extensively studied. By contrast, H2O(•+) reactivity with molecules other than water has not been established experimentally and was subject to controversy. The direct oxidation by H2O(•+) can take place in various situations. In highly concentrated solutions, the radical cation H2O(•+) may also be involved in ultrafast electron transfer reactions. We have applied picosecond pulse radiolysis conducted at the electron accelerator ELYSE on solutions with various H2SO4 concentrations to determine the scavenging yield of H2O(•+). The yield of H2O(•+) at a few tens of femtoseconds is estimated to be around 5.3 × 10(-7) mol J(-1), and its reactivity is quantitatively determined. Moreover, a simple estimation of the reduction potential of this short-lived radical cation shows that it is the most powerful oxidizing species.