Reactions of radical anions. Part IX. The radical anion of 1-phenyl-2-trimethylsilylacetylene

Abstract

The e.s.r. spectra of the radical anion of 1-phenyl-2-trimethylsilylacetylene with sodium, potassium, and caesium as gegenions in tetrahydrofuran have been obtained and analysed. Simple Hückel molecular orbital calculations were made, and coulomb and resonance integrals found which are consistent with the correct spin densities. For the carbon–silicon bond of the neutral molecule the calculated π-bond order is ca. 0·3. The radical anions are stable at –80 °C but when the temperature is raised they dimerize and the rate of dimerization has been followed in tetrahydrofuran over the temperature range –70 to –40 °C. The following values were obtained for the reaction at –60 °C: Na⁺ as gegenion, k= 41·1 l mol^–1 s^–1, ΔG‡= 10·8 kcal mol^–1, ΔH‡= 9·3 kcal mol^–1, ΔS‡=–6·9 cal mol^–1 K^–1; K⁺ as gegenion, k= 0·59 l mol^–1 s^–1, ΔG‡= 13·5 kcal mol^–1, ΔH‡= 6·3 kcal mol^–1, ΔS‡=–33·8 cal mol^–1 K^–1; Cs⁺ as gegenion, k= 0·26 l mol^–1 s^–1ΔG‡= 13·9 kcal mol^–1, ΔH‡= 4·5 kcal mol^–1, ΔS‡=– 44 cal mol^–1 K^–1.