Isotopic enrichment by electron exchange

Abstract

The importance of isotope separation has increased due to the wide use of isotopes in both energy- and life-science-oriented applications¹. This, coupled with our observation² that the solution electron affinities³ of perdeuterated hydrocarbons are less than those of corresponding undeuterated species, led us to investigate the possibility of isotopic enrichment through electron exchange equilibria. Here we report the observation that nitrobenzene containing ¹⁵N has a higher affinity for solvated electrons in liquid ammonia than does nitrobenzene containing the most common isotope of nitrogen (¹⁴N). When an equal-molar mixture of ¹⁴N-nitrobenzene (PhNO₂-¹⁴N) and ¹⁵N-substituted nitrobenzene (PhNO₂-¹⁵N) is added to a solution of solvated electrons in liquid ammonia, the anion radical of PhNO₂−¹⁵N is formed predominantly over that of PhNO₂-¹⁴N. Furthermore, as the chemical and physical properties of the resulting anion radicals (PhNO₂-¹⁵N^−. and PhNO₂-¹⁴N^−.) are very different from those of the neutral molecules, the electron exchange equilibrium between these two isotopic species can be utilized to enrich samples selectively with respect to the two isotopic species. Analogous results have been observed for the case of benzophenone-¹²C and benzophenone-¹³C (carbonyl ¹³C).