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 o f 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 a vailable. In agreement with a qualitative model which is expressed in valen ce bond terms, it is found that Moller-Pesset calculations offer a good des cription of symmetrical homonuclear three-electron bonds by providing fairl y accurate equilibrium bond lengths, stretching frequencies. and dissociati on energies. By contrast, the Moller-Plcsset method fails for some unsymmet rical systems, leading to largely erroneous equilibrium distances and stret ching 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 resul ts in a set of optimized molecular orbitals that is poorly adapted to the s ubsequent perturbation calculation, and carries over to the MP2 and MP4 lev els. In such a case, the three-electron bond is systematically found too sh ort, while the corresponding stretching frequency is found too high. The MP 2 error is shown to linearly correlate with a simple function of the calcul ated net charges, thus providing a simple way to check the validity of MPn calculations for dissymmetrical three-electron-bonded radical anions. The t humb rules that follow are further confirmed by applicatory test calculatio ns on some three-electron-bonded anions of chemical interest: HO therefore SH-, HO therefore SCH3-, HO therefore CF3-, HS therefore SCH3-, and H3CS th erefore CF3-.