Thermodynamics of disproportionation of radical anions. The effect of counterions and solvents

Abstract

Publication costs assisted by the National Science FoundationThe previously described technique has been used in thermodynamic studies of disproportionation of radical anions of perylene and tetracene. The pertinent heats and entropies of disproportionation were determined for the Li' , Na+, K+, and Cs+ salts in tetrahydrofuran (THF) and in dimethoxyethane (DME). The peculiar behavior of the sodium salts in THF was confirmed and attributed to the desolvation of Na+ ions which retain their solvation in the radical-anion-cation pairs, but lose it on aggregation with dianions. In DME both the sodium and the potassium salts behave like the sodium salt in THF. The higher solvating power of the bidentate DME allows even the K+ ion to retain its solvation shell in the pair but not in the dianion aggregate. The above technique was applied in thermodynamic studies of electron transfer process, sodium trans-stilbenide + anthracene F! trans-stilbene + sodium anthracenide. Heat and entropy of this reaction were determined.A simple technique described' in a previous publication from this laboratory allows us to study thermodynamics of disproportionation of radical anions and the effect of cations on the respective AH"'s and AS"'s. The effect of cation's nature on AS" is particularly illuminating and thought provoking.This work was expanded now to two systems, perylene and tetracene in two solvents tetrahydrofuran and dimethoxyethane. The results and their interpretation are reported in this communication. They refer to reactions described by the equation Experimental SectionThe details of the technique used in this investigation are given in the earlier publication.' In essence, it is a potentiometric technique. One measures the potential difference between two electrodes, one immersed in a 50:50 solution of the parent hydrocarbon and its radical anion and the other in a 50:50 solution of the radical anion and its dianion. Bulbs containing these solutions are linked through a liquid junction and the whole unit is placed in a thermostat that allows one to vary up and down the temperature from -55 to -25 "C. The potential difference between the electrodes is measured with voltmeter of infinite resistance, its scale allowing us to read its value to 0.1 mV. The resistance of the apparatus is in the range of 50 Mohm. The measured potential is reproducible; the same reading is recorded when after changing the temperature we return to its original value. Moreover, the potential remains constant a t any fixed temperature a t least for 1 h.The purification of tetrahydrofuran (THF) and dimethoxyethane (DME) was performed on a high-vacuum line using the procedure2 well known by now. Perylene, tetracene, anthracene, and trans-stilbene acquired from Aldrich Co. were crystallized and vacuum sublimed before being used. Alkali metals (mirrors for Na, K, and Cs) were used as reducing agents.The solutions of radical-anion pairs or dianion aggregates were kept a t concentrations of lo-* to M. The fraction of free radical anions in those solutio...

Solvent-Shared and Solvent-Separated Ion Multiples of Perylene Radical Anions and Dianions: An Exemplary Case of Alkali Metal Cation Solvation

“…The knowledge gathered helps to navigate the reactivity of such compounds in solution by effective solvation of their countercations to the crystallization of salts containing “naked anions”, in which radical anions or dianions can be studied without the disturbing influence of a given countercation. In addition, changes of the countercation or the solvent influence the disproportionation equilibrium of radical anions to the dianion and the neutral molecule and, therefore, help to selectively prepare and crystallize radical anion or dianion salts. The network of equilibria involved is rather complex, and continuing investigations of cation solvation phenomena 2,3 are needed to improve the understanding of them.…”

Polymerization of Styrene-Butadiene Block Copolymers Using a Dicarbanion Initiator Made by the Reaction of Lithium with .alpha.-Methylstyrene

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