Effect of Lone-Pair Hybridization on the Stability of Weak Hydrogen Bonds

In the past, various explanations have been given for the fact that hydrogen bonds involving a common proton donor become stronger as the acceptor lone‐pair orbital contains less $s$ character (e.g., amines form stronger H bonds than nitriles). Calculations are now presented which provide a more coherent rationalization for this phenomenon. The calculations are based on Murrell's second‐order perturbation theory of intermolecular forces—including exchange—and they have been carried out for a linear model system A–H···B which was described previously. The results show that there are three contributions to the H‐bond energy which favor the formation of H bonds with low lone pair $s$ character, viz., the penetration part of the Coulomb energy, the exchange energy, and the charge‐transfer energy. Differences in dipolar interaction are less important except for very polar A–H bonds. A comparison with some experimental data is given in the final section.