The challenge of H-bonding

Hydrogen bonds are ubiquitous in biomolecules and a key to understanding their functionality. This was first appreciated by Pauling and exploited by Watson and Crick to decode the structure of DNA. It is also the origin of the unique and amazing properties of water. There is a helpful review article Hydrogen bonding in the solid state by Steiner. Here are a few things I learnt.

Energies vary by 2 orders of magnitude, 0.2-40 kcal/mol [10 meV to 2 eV]. This spans the energy range from van der Waals to covalent and ionic bonds. The amount of electrostatic, covalent, and dispersion character of the bond varies within this range.

For O-H ... O bonds the shift in frequency of the O-H bond correlates with the distance between the oxygen atoms. The O-H distance is correlated with the O..H distance.

All hydrogen bonds can be considered as incipient hydrogen transfer reactions.

Hydrogen bonds exhibit some unexplained isotope effects. Simple zero-point motion arguments suggest that deuterium substitution should lead to weaker bonds, as is observed in some cases. However, some bonds exhibit a negligible effect and others a negative effect.