Bonding Analysis of the Shortest Bond between Two Atoms Heavier than Hydrogen and Helium: O2 2

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 O2 2+ and N2. An energy decomposition analysis of the chemical bonds suggests that the shorter bond in O2 2+ 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 O2 2+ 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 O2 2+ 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.