Formation and dimerization of vinylic radical anions and ion pairs. A pulse radiolysis study

Abstract

Absorption spectra of the free-radical anions and their ion pairs with Na⁺ and Li⁺ of 1,1-diphenylethylene (D), α-methylstyrene (MS) and styrene (S) were recorded by pulse radiolysis of solutions of the hydrocarbon in either neat tetrahydrofuran or tetrahydrofuran containing sodium or lithium tetrahydridoaluminate salts. The spectra indicate that radical anions of 1,1-diphenylethylene and α-methylstyrene associate with Na⁺ and Li⁺ to give solvent-separated ion pairs; but styrene radical anions form contact ion pairs with Na⁺. Rates of reaction of e^–_s, (Na⁺C, e^–_s) and (Na⁺, e^–_s) with these hydrocarbons and with pyrene and biphenyl were determined and were found to follow the order e^–_s > (Na⁺C, e^–_s) > (Na⁺, e^–_s) for each solute. Except for 1,1-diphenylethylene, the reactivity of (Na⁺, e^–_s) with these and some related hydrocarbons was found to increase slightly with the electron affinity of the hydrocarbon. The rates of dimerization of the radical anions of D, MS and S have been determined and the influence of ion pairing with Na⁺ and Li⁺ has been studied. The dimerization rates of (Na⁺, MS^˙–), (Li⁺, MS^˙–) and (Na⁺C, MS^˙–) are 6.6 × 10⁸, 1.23 × 10⁹ and 6.42 × 10⁹ dm³ mol^–1 s^–1, respectively, and the latter value was found to be independent of ionic strength. Absorption spectra of the dimer dianions of D and MS were also recorded.