Kinetics of the Reaction of Solvated Electrons with Water in Liquid Ammonia

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AbstractSolvated electrons (< 10−6 M) produced by flash photolysis in ammonia react with water according to the rate law d[e−]/dt = k · [e−] · [H2O] with the rate constant k = 0.16 ± 0.03 M−1 sec−1 at – 40°C. The rate law is compatible with the reaction e− + H2O → H + OH−, but the activation energy Ea (e−) = 6 ± 1 kJ/mol is too small for this endothermic process with ΔH0 = 52 kJ/mol. Therefore we conclude that in the rate determining step an unknown intermediate is formed which in turn recombines to molecular hydrogen. Evidence for this intermediate was given already 15 years ago in the pure aqueous system in which solvated electrons a few msec after their reaction could be regenerated by a photo flash too soft to photo‐generate them directly. – The reaction of very low concentrated metal ammonia solutions with water follows the same kinetics. But at higher concentrations (0.1 M) the relative reaction rate is very low. With sodium the reaction is slower than with cesium, in deutero‐ammonia it is slower than in normal ammonia. It is almost of zero order with respect to the metal and of one ha1f order with respect to water and has an activation energy Ea (Na) = 32 ± 3 kJ/mol (normal sodium‐ammonia solution). The observed kinetics suggest that the solvated electrons are still the only reactive species in these solutions; but their concentration is lower than the metal concentration by many orders of magnitude due to the well known spin pairing equilibrium, which must include a metal cation and a solvent molecule. The stability of the spin paired species increases with decreasing cation size and with increasing tendency of the solvent to form hydrogen bridges. The high experimental activation energy is partly due to the temperature dependence of the spin pairing equilibrium.