The influence of crown ethers on cation migration processes. Part 5. The acenaphthene radical anion

Abstract

The radical anion of acenaphthene has been prepared in tetrahydrofuran and in tetrahydrofuran in the presence of dibenzo-18-crown-6, 18-crown-6, or [2.2.2]cryptand as cation complexing agent, with either Li⁺, Na⁺, or K⁺ as counterion. Linewidth alternation has been observed in the K⁺/(acenaphthene)^–˙/tetrahydrofuran system as a result of cation migration from one side of the molecular plane to the other. The activation energy for this process has been determined (6.6 kJ mol^–1) by a procedure which does not require a knowledge of the methylene proton splitting constants in the ‘slow’ exchange limit. Linewidth alternation is not observed in this system upon addition of any one of the foregoing cation complexing agents. However, equilibria between contact (or ‘tight’) and solvent-separated (or ‘loose’) ion pairs have been observed in the Na⁺/(acenaphthene)^–˙/tetrahydrofuran and K⁺/(acenaphthene)^–˙/18-crown-6/tetrahydrofuran systems (ΔH° 10.2 and 14.1 kJ mol^–1, respectively). Values of da/dT, determined for each system, appear to be significantly influenced by the counterion. In particular da(4-H)/dT is positive in pure tetrahydrofuran but negative in the presence of the cation complexing agents.