Why is the F-2 bond so weak? A bond energy decomposition analysis

The bond energies of F-2, Cl-2, Br-2 and I-2 have been studied computationally using gradient-corrected, relativistic density functional theory with large all-electron basis sets. The bond energies of all four compounds have been broken down into electrostatic, Pauli (exchange) and orbital terms at the equilibrium internuclear separation and as a function of r(X-X) using the Ziegler-Rauk energy decomposition scheme. The comparative weakness of the F-2 bond is traced to the electrostatic energy, which is considerably smaller than would be expected on extrapolation of the trend in the I-2-->Cl-2 data. The small electrostatic term is itself a result of the F-F bond being c. 0.1 Angstrom longer than might be expected, due to the large gradient in the Pauli repulsion around the equilibrium internuclear separation. The weakness of the F-2 bond is therefore a combination of quantum mechanical (Pauli) and classical (electrostatic) factors. The present conclusions are placed throughout in the context of previous research.