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

Abstract

We are very grateful to Professor E. U. Franck (UniversitLt Karlsruhe) for his interest in the research program of our laboratory and for his kindness in providing us with the cooperation of the skilful personnel of the workshop at the Institut fur Physikalische Chemie und Elektrochemie, who built the cell employed in this work (method I[) following a design which was developed at the Institut.The authors are also grateful to SUBCYT (Argentina) for partial economic support. [I1 121 131 References R. Crovetto, R. Fernandez-Prini, and M. L. Japas, J. Chem. Chemical Kinetics / Isotope Effects / Photochemistry Solvated electrons (< M) produced by flash photolysis in ammonia react with water according to the rate law d[e-]/dt = k . [e-] . [H20]with the rate constant k = 0.16 k 0.03 M-' sec-' at -40°C. The rate law is compatible with the reaction e-+ H 2 0 + H + OH-, but the activation energy E, (e-) = 6 ?C 1 kJ/mol is too small for this endothermic process with AH' = 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 half order with respect to water and has an activation energy E,(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.exothermic and very fast. The corresponding reactions in ammonia (-40°C)

Controlled Radical Polymerization Initiated by Solvated Electrons

“…[17,18] Considering the electron transfer nature of alkali metals, they X. Li, Z. Pan, Y. Xia, J. Rui, M. Zhu, H. Ren, J. Huang College of Materials Science and Engineering Nanjing Tech University No. 30 Puzhu Road (S), Nanjing, Jiangsu Province 211816, P.R. China E-mail: jhuang@njtech.edu.cn…”

“…In the reduction reactions of alkali metals/NH 3 , Telse and Colombara indicated the higher electron transfer capacities of low concentrated metal solutions compared with high concentrations. [ 29,30 ] It was reasonably inferred that, at low metal concentrations, non‐paired e sol − inclined to apply outer‐sphere electron transfer mechanism and exhibited higher reactivity, otherwise, paired e sol − adopted inner‐sphere mechanism and presented lower reactivity. [ 29 ] Indeed, limited by pairing energies, non‐paired e sol − should be highly reactive relative to paired one.…”

Collisions of Sodium Atoms with Liquid Glycerol: Insights into Solvation and Ionization

“…2,3,8−10 They may also be generated at low energies by alkali atom 11 or electron attachment to solvent clusters. 2 There exists a large body of kinetic and mechanistic data for reactions of e s − in bulk water, 5,12 ammonia, 13 and alcohols 14 as well as in water clusters, 15,16 which has been extended recently to studies of electron solvation in the interfacial region of liquid water. 9,17−21 A potentially gentle method for producing solvated electrons at or near the surface of a protic liquid is the gas-phase deposition of sodium atoms, which should rapidly ionize into Na + and e s − .…”

“…The solvated electron, e s – , is a fascinating species both structurally and dynamically, − and its high reduction potential makes it a powerful reagent in solution. − Solvated electrons may be generated from high-energy sources − utilizing megavolt electrons, X-rays, and γ rays and from lower-energy techniques that rely on photodetachment from anions and neutral molecules. ,,− They may also be generated at low energies by alkali atom or electron attachment to solvent clusters . There exists a large body of kinetic and mechanistic data for reactions of e s – in bulk water, , ammonia, and alcohols as well as in water clusters, , which has been extended recently to studies of electron solvation in the interfacial region of liquid water. ,− …”