Bonding Analysis of the Shortest Bond between Two Atoms Heavier than Hydrogen and Helium: O₂²

Quantum chemical calculations using ab initio methods at the CCSD(T) level with large basis sets and DFT calculations using the BP86 functional have been carried out for O₂²⁺ and N₂. An energy decomposition analysis of the chemical bonds suggests that the shorter bond in O₂²⁺ compared with isoelectronic N₂ is due to the weaker Pauli repulsion in the dication, which overcompensates the weakening of attractive interactions that are operative in N₂. At the equilibrium distance of N₂, the orbital (covalent) bonding in O₂²⁺ is weaker than in N₂, and the attractive Coulomb interactions in the neutral diatomic system become repulsive in the dication, but the weakening of the Pauli repulsion caused by the shrinking of the orbitals in O₂²⁺ compensates for these forces and leads to a shortening of the bond. The results also show that the bond dissociation energy is not a reliable indicator for the strength of bond, which is more faithfully given by the (local) force constant.