Sensitivity of noncovalent bonds to intermolecular separation: hydrogen, halogen, chalcogen, and pnicogen bonds

Abstract

It is well known that noncovalent bonds are weakened when stretched from their equilibrium intermolecular separation. Quantum chemical calculations are used to examine and compare the sensitivity to stretches of hydrogen, halogen, chalcogen, and pnicogen bonds. NH₃ was taken as the universal electron donor, paired with HOH and FH in H-bonds, as well as with FPH₂, FSH, and FCl. Even though the binding energies span a wide range, stretching the intermolecular separation by 1 Å cuts this quantity by the same proportion, roughly in half, for each system. Taking the sum of van der Waals radii as an arbitrary cutoff, the H-bond energy in FH⋯NH₃ remains at 5.5 kcal mol⁻¹ while the binding energy of the other three bond types is only slightly smaller at 4.5–4.7 kcal mol⁻¹.