Activation energies for pyramidal inversion of sulfonium selenonium and telluronium ylides based on ab initio MO calculations

Abstract

Ab initio MO calculations of sulfonium, selenonium and telluronium ylides R¹₂X⁺–C^–R²₂(X = S, Se, Te; R¹= H, Me; R²= H, CN) have been performed to estimate their thermal stabilities toward the pyramidal inversion (vertex inversion) reaction. Geometrical parameters at both the global minima and the saddlepoints are independent of the levels of theory employed. However, the activation energies for the pyramidal inversion reaction depend on the levels of theory employed; a method superior to the Hartree–Fock level of theory was necessary for the present purposes. The activation energies for the pyramidal inversion were also found to depend on the substituents. The present computations clearly indicate that the telluronium ylides require higher activation energies for the pyramidal inversion reaction than those of the sulfonium and selenonium ylides.