Diatomic halogen anions and related three-electron-bonded anion radicals:: Very contrasted performances of Moller-Plesset methods in symmetric vs dissymmetric cases

The bonding distances, vibrational frequencies, and dissociation energies of a series of dihalogen radical anions, X therefore 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-Plcsset 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 poorly adapted to the subsequent perturbation calculation, and carries over to the MP2 and MP4 levels. In such a case, the three-electron bond is systematically found too short, while the corresponding stretching frequency is found too high. The MP2 error is shown to linearly correlate with a simple function of the calculated net charges, thus providing a simple way to check the validity of MPn calculations for dissymmetrical three-electron-bonded radical anions. The thumb rules that follow are further confirmed by applicatory test calculations on some three-electron-bonded anions of chemical interest: HO therefore SH-, HO therefore SCH3-, HO therefore CF3-, HS therefore SCH3-, and H3CS therefore CF3-.