Diatomic Halogen Anions and Related Three-Electron-Bonded Anion Radicals: Very Contrasted Performances of Møller−Plesset Methods in Symmetric vs Dissymmetric Cases

Abstract

The bonding distances, vibrational frequencies, and dissociation energies of a series of dihalogen radical anions, X∴Y- (X, Y = F, Cl, Br, I), held together by three-electron bonds, are calculated at the MP2 and MP4 levels and compared to the CCSD(T) level and to experimental values when available. In agreement with a qualitative model which is expressed in valence bond terms, it is found that Moller−Pesset calculations offer a good description of symmetrical homonuclear three-electron bonds by providing fairly accurate equilibrium bond lengths, stretching frequencies, and dissociation energies. By contrast, the Moller−Plesset method fails for some unsymmetrical systems, leading to largely erroneous equilibrium distances and stretching frequencies. The qualitative model predicts such errors to be related to an inaptitude of the UHF reference determinant to properly describe the sharing out of the charge between the two fragments. This inadequacy results in a set of optimized molecular orbitals that is poo...