Abstract
The planarity and 10 Ï-electron aromaticity of the free cyclooctatetraene dianion (C8H82â, COT2â) have been questioned recently on the basis of conflicting density functional and second-order MøllerâPlesset perturbation computations. Rigorous coupled-cluster methods are employed here to establish the structure and properties of COT2â. Like many multiply charged anions, COT2â exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT2â rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT2â 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 COT2â structure. Final focal point analyses place the COT2â resonance state 61.6 kcal molâ1 above neutral COT. Nonetheless, COT2â exhibits structural, magnetic, and energetic properties characteristic of aromatic compounds. Comparison with all-trans octatetraene indicates that COT2â has a substantial aromatic stabilization energy (25 kcal molâ1) approaching that of benzene (33 kcal molâ1), but this favorable influence is swamped by Coulomb repulsion. Charge-compensating complexation of COT2â with two sodium cations results in a thermodynamically stable Na2COT compound (D8h symmetry), for which high-level structures are also presented.
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