The ethylenedione anion: Elucidation of the intricate potential energy hypersurface

Abstract

Ab initio molecular orbital theory has been used to study the controversial potential energy surface of the ethylenedione anion C2O−2. Seven different basis sets, the largest being triple zeta plus two polarization functions and one set of higher angular momentum functions (TZ2Pf) in quality, were utilized in conjunction with five correlated methods, the highest-level being coupled-cluster theory including single, double, and perturbative triple excitations [CCSD(T)]. Equilibrium geometries and harmonic vibrational frequencies of the predicted 2Au trans-bent ground state are presented. The Renner–Teller potential energy surface resulting from the splitting of the doubly degenerate linear 2Πu transition state into the nondegenerate bent 2Au and linear 2Bu surfaces is also characterized by means of energy predictions for these three states. Several recent peak assignments in the experimental spectrum, as well as the isotopic shifts associated with them, are supported by theory. A correct description of the potential energy hypersurface is obtained only by application of large basis sets in conjunction with methods including high-level treatment of electron correlation effects. The TZP+/CCSD(T) methodology predicts the OCC bond angle to be 146.5°.