Gerald R. Stevenson

Contrasting free energies of electron transfer from [6]- and [8]annulenes to their perdeuteriated and Per-13C analogues

A very large equilibrium isotope effect (confirmed via physical separation of the isotopes involved) was observed via the EPR analysis of a mixture of benzene and perdeuteriated-per- 13 C-benzene competing for a deficient number of electrons in tetrahydrofuran (THF) in the presence of 18-crown-6 (18C6). TheK eq for the reaction C 6 H 6 .- ,K + (18C6)+ 13 C 6 D 6 =C 6 H 6 + 13 C 6 D 6 ,K + (18C6) is 0.096±0.008 at -100 o C

Diminished solution electron affinities of carbon-13 and deuterium-substituted anion radical precursors allow isotopic enrichment.

Mixtures of carefully measured quantities of naphthalene (NP) and perdeuterated naphthalene (NP-d/sub 8/) were reduced with very deficient amounts of potassium metal in tetrahydrofuran (THF). The relative intensities of the two simultaneously observed ESR spectra show that the equilibrium constant at -120/sup 0/C is 0.26 +/- 0.04 and ..delta.. G/sup 0/, which is identical with the enthalpy change, is 0.41 +/- 0.15 kcal/mol. Analogous results are obtained for /sup 13/C-labelled benzene in THF reduced with K. 12 references, 1 figure.

Isotopic enrichment by electron exchange

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).

Contrasting free energies of electron transfer from [6] and [8]annulenes to their erdeuteriated-per-13C analogues

EPR analysis shows that the equilibrium isotope effect upon the equilibrium constant for electron transfer between isotopic isomers is much larger in [6]annulene (i.e. C6H6−. + 13C6D6 = C6H6 + 13C6D6−.) than in the [8]annulene systems (i.e. C8H8−. + 13C8D8 = C8H8 + 13C8D8−.). This is due to the effects of aromaticity.