Free Cyclooctatetraene Dianion: Planarity, Aromaticity, and Theoretical Challenges.

Abstract

The planarity and 10 π-electron aromaticity of the free cyclooctatetraene dianion (C8H82–, COT2–) have been questioned recently on the basis of conflicting density functional and second-order Moller–Plesset perturbation computations. Rigorous coupled-cluster methods are employed here to establish the structure and properties of COT2–. Like many multiply charged anions, COT2– exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT2– rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT2– but mainly describe COT in a continuum of free electrons. All-electron coupled cluster theory extended through triple excitations [AE-CCSD(T)] yields a planar D8h symmetry COT2– structure. Final focal point analyses place the COT2– resonance state 61.6 kcal mol–1 above neutral COT. Nonetheless, COT2– exhibits structural, magnetic, and energet...