Pulse radiolysis of solutions of trans-stilbene. Radical-anions and ion-pairs in tetrahydrofuran

Abstract

On pulse radiolysis of solutions of trans-stilbene (t-St) in THF the radical-anion of t-St is formed by reaction of e^–_s with t-St [reaction (5)] with k₅=(1.16 ± 0.03)× 10¹¹ dm³ mol^–1 s^–1. The transient absorption spectrum observed with λ_max at 500 and 720 nm is attributed to the unassociated St^˙–. The subsequent decay of the radical-anion is accounted for by reaction with the counter-cation of THF formed on radiolysis and with radiolytically generated radicals; rate constants for these processes are estimated. Addition of sodium tetrahydridoaluminate (NAH) results in the radical-anion being associated with Na⁺ as a contact ion-pair and a shift of λ_max to 490 nm. In the presence of the lithium salt the absorption spectrum of the radical-anion reverts to 500 nm and this is interpreted in terms of formation of solvent-separated ion-pairs. On pulse radiolysis of solutions containing NAH the main reaction forming St^˙– is that of (Na⁺, e^–_s) ion-pairs with t-St [reaction (12)] with k₁₂=(1.29 ± 0.04)× 10¹⁰ dm³ mol^–1 s^–1. In addition there is a delayed formation of St^˙– over a period of microseconds and a possible mechanism for this is considered. The presence of tetrahydridoaluminate salts also greatly enhances the stability of St^˙– and at high doses per pulse little decay was observed over 700 µs. Reasons for the enhanced stability of St^˙– are considered. The variation of G(St^˙–) with [NAH] was studied and was found to attain a plateau value of 2.0 at the higher concentrations. Pulse radiolysis of NAH/THF solutions containing crown ether yielded an absorption spectrum closely resembling that of e^–_s, but kinetic studies on the solutions containing trans-stilbene indicated that the species is an ion-pair between e^–_s and Na⁺ in which the sodium is complexed with the crown ether, i.e.(Na⁺C, e^–_s). This species reacts with trans-stilbene with a rate constant of (3.86 ± 0.07)× 10¹⁰ dm³ mol^–1 s^–1.