Identification of the strongest bonds in chemistry.

Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH](+)((1)Σ(+)), [HCO](+)((1)Σ(+)), [HNN](+)((1)Σ(+)), and [HNNH](2+)((1)Σg(+)). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 is found for [HNNH](2+) using scaled CCSD(T)/CBS frequencies determined for CCSD(T)/CBS geometries. Less strong is the NN bond in [FNNH](2+) and [FNNF](2+).