Natural populations and charge transfer in hydrogen and alkali bihalide ions

Equilibrium structural parameters and dissociation energies of the bihalide ions, (MXY) (M = H, Li or Na; X and Y = F or Cl) have recently been calculated [l] at ab initio HF and post-HF(MPn) levels using 6-31+G* and 6-31 l+G* basis sets. All the species were predicted to have a linear structure with a double-minima potential at R(X . . . Y) > &(X...Y). In the case of heterobihalide ions the structure (X-M . . . Y)), where X is a smaller atom than Y, corresponds to the global minimum in the potential energy surface. For the (HXY)species &(X . . . Y) is shorter than the sum of the van der Waals radii, Rx + RY. Owing to the presence of core electrons in Mf (M = Li or Na) and also its larger size, R, (X . . . Y) is longer than Rx + RY in alkali bihalide ions. These anions are binary complexes of a halide ion with hydrogen or alkali halides. Their nature of bonding was deduced by us [l] on the basis of atomic charges, charge transfer (CT) and bond indices obtained at the HF/6-31 l+G* level using the Mulliken population analysis (MPA) [2]. It was found that the CT or valence interaction plays an almost equal important role both in (HXY)and (MXY)-. This is an unacceptable result. In view of the well-known deficiency [3,4] of MPA, both atomic charges and the amount of CT are highly exaggerated especially for the alkali bihalide ions.