Equilibrium Reactions of Excess Electrons with Aromatics in Non‐polar Solvents

Abstract

The excess electron in non‐polar solvents is shown to undergo the reversible reaction: e− + A ⇄ A−, with the solutes (A): triphenylene, phenanthrene, naphthalene, styrene and α‐methylstyrene. This was demonstrated by a pulse conductivity method which permitted measurement of both the attachment and detachment rates. The attachment rate constants are very large, near 1013 mol−1 · l · s−1 in (CH3)4Si and 2,2,4‐trimethylpentane solvents and a little less than 1012 mol−1 · l · s−1 in n‐hexane. The detachment rates are very solvent and temperature dependent, characterized by large pre‐exponential factors (1018 ‐ 1023 s−1) which facilitate the very endothermic detachment reactions. The equilibrium data show that these reactions occur with large decreases in enthalpy (‐67 to −109 kJ/mol) and large decreases in entropy (‐84 to −204 J mol K). The equilibrium constants at 298° increase with electron affinity of the solute and also change with solvent. The results show that the ground state energy of the electron changes from liquid to liquid: for tetramethylsilane ΔHsoln) (e−) = −0.64 eV, for 2,2,4‐trimethylpentane ΔHsoln(e−) = −0.40 eV and for n‐hexane ΔHsoln(e−) = −0.18 eV.

Electrons in condensed media

“…Electron solvation in hydrocarbon liquids is found to be accompanied by negative enthalpy and positive entropy changes, thus both factors contributing significantly to the solvation process. These conclusions are qualitatively similar to those found earlier on the basis of the usual two-state model (Holroyd, 1977), but quantitative differences exist. In another context the states of the electron in dielectric liquids turn out to be important, viz.…”

“…The entropy change on trapping from the quasi-free state is negative, indicating that the trapped state is more ordered and that the trapping process is driven entirely by enthalpy. Following Holroyd (1977), the thermodynamic parameters of electron solvation from vacuum are obtained from considerations of a few typical reversible attachment-detachment reactions. Electron solvation in hydrocarbon liquids is found to be accompanied by negative enthalpy and positive entropy changes, thus both factors contributing significantly to the solvation process.…”

States of the electron in hydrocarbon liquids

“…Finally, the nature of the trapped state is poorly understood, even though its entropy relative to the quasi-free or vacuum state can be determined from experiments (Holroyd, 1977;Mozumder, 1996). The purpose of the present paper is to critically examine these questions and provide some tentative answers.…”

“…In this model, the electron exists in equilibrium in two states; a state of high mobility, called the quasi-free state and a state of negligible mobility, called the trapped state. Within the context of this model, thermodynamic parameters of trapping (from the quasi-free state) and solvation (from vacuum) may be obtained using the experimental data for overall attachment-detachment reactions with a few suitable solutes, subject to certain reasonable assumptions (Holroyd, 1977). A slightly modified version of this procedure has been suggested by Mozumder (1996) in view of the so-called quasi-ballistic model that allows finite momentum relaxation time in the quasi-free state in competition with trapping.…”

Experimental study of a diffusion-controlled reaction involving a chain molecule with terminal reactive sites

“…The electron transfer to a bromoalkane molecule is rather effective because the diphenylsilane molecule has a weakly negative gas-phase electron affinity, while the stability of the radical anion is provided by solvent polarization energy [19].…”