Solvent effect on reversible self‐termination reactions of aromatic free radicals

Rates and thermodynamic data have been obtained for the reversible self‐termination reaction:\documentclass{article}\pagestyle{empty}\begin{document}$${\rm R}^ \cdot + {\rm R}^ \cdot \mathop{\buildrel\longleftarrow\over\longrightarrow}^{2k1}_{2k_{-1}}D $$\end{document}Involving aromatic 2‐(4′dimethylaminophenyl)indandione‐1,3‐yl (I), 2‐(4′diphenylaminophenyl)indandione‐1,3‐yl (II), and 2,6 di‐tert‐butyl‐4‐(β‐phthalylvinyl)‐phenoxyl (III) radicals in different solvents. The type of solvent does not tangibly affect the 2 k 1 of Radical(I), obviously due to a compensation effect. The log(2 k 1 ) versus solvent parameter E T (30) curves for the recombination of radicals (II) and (III) have been found to be V shaped, the minimum corresponding to chloroform. The intensive solvation of Radical (II) by chloroform converts the initially diffusion‐controlled recombination of the radical into an activated reaction. The log (2 k −1 ) of the dimer of Radical (I) has been found to be a linear function of the Kirkwood parameter (ε ‐ 1)/(2ε + 1), the dissociation rate increasing with the dielectic constant of the solvent. The investigation revealed an isokinetic relationship for the decay of the dimer of Radical (I), an isokinetic temperature β = 408 K and isoequilibrium relationship for the reversible recombination of Radical (I) with β° = 651 K. For Radical (I) dimer decay In(2 k −1 ) = const + 0.8 In K , where K is the equilibrium constant of this reversible reaction. The transition state of Radical (I) dimer dissociation reaction looks more like a pair of radicals than the initial dimer. The role of specific solvation in radical self‐termination reactions is discussed.