"Union is strength": how weak hydrogen bonds become stronger

Recently reported rotational spectroscopic studies on small dimers and oligomers bound by weak hydrogen bonds show that the driving forces, the spatial arrangement and the dynamical features displayed are very different from those involved in stronger and conventional hydrogen bonds. The very small binding energies (similar to those of van der Waals interactions) imply that the stabilization of the dimer is often obtained by networks of weak hydrogen bonds. Even in the presence of multiple bonds the partner molecules show a high degree of internal freedom within the complex. This paper analyses several examples of molecular adducts bound by weak hydrogen bonds formed in free jet expansions and recently characterized by rotational spectroscopy. They include weakly bound complexes of weak donors with strong acceptors (C-H center dot center dot center dot O, N, S-H center dot center dot center dot O, N), strong donors (O-H, N-H) with weak acceptors such as the halogen atoms and pi systems but also the elusive interactions between weak donors and weak acceptors (C-H center dot center dot center dot pi and C-H center dot center dot center dot halogen). Examples are also given where rotational spectroscopy highlights that weak hydrogen bonds are extremely important in chiral recognition phenomena and as driving forces of the conformational landscape of important biomolecules.