The strength of a chemical bond

We discuss the physical criteria that have been proposed as a measure of the strength of a chemical bond. Critical examination of the strengths and weaknesses of the various quantities shows that the force constant is the most general measure for determining the strength of a chemical bond in molecules. The force constant of a bond A-B is a direct indicator of the interatomic forces between the fragments A and B at the equilibrium distance, which is independent from the energies and electronic structures of the free species A and B. This is a great advantage compared to the use of energy differences for determining the bond strength, which are subject to the choice of the reference system that is often not uniquely defined. This is particularly evident when considering the strength of a chemical bond in strongly bonded but metastable species such as [O₂]²⁺. The change in the electronic structure of the reference system obscures the intrinsic bond strength at equilibrium. Energy changes along the course of chemical reactions are elementary quantities in chemistry; they provide crucial information for the kinetics and thermodynamics of processors. However, they are not suitable as a measure of bond strength. The definition of local vibrational constants by Konkoli and Cremer eliminates the coupling of the stretching vibration with other vibrational modes, which makes it possible to use force constants as direct measure for bond strength also in larger molecules.