Hydrogen Bonding

Hydrogen bonding is one of the most important interactions in both chemistry and biology. It arises whenever a hydrogen atom is covalently bonded to a small, highly electronegative atom -- fluorine, oxygen or nitrogen -- which pulls electron density away from the hydrogen and leaves it strongly polarised (bearing a partial positive charge). This polarised hydrogen can then form a weak, additional electrostatic attraction to another electronegative atom nearby -- this attraction is the hydrogen bond, conventionally drawn as a dotted line ($\cdots$), with a typical strength of 20-50 kJ mol⁻¹.

A hydrogen bond is weaker than a covalent bond (> 100 kJ mol⁻¹) but stronger than a van der Waals interaction (< 20 kJ mol⁻¹). Its presence has a profound effect on physical properties -- vapour pressure (contrast H₂O with H₂S, which cannot hydrogen-bond as effectively), boiling point, the miscibility of liquids (e.g. H₂O with C₂H₅OH), surface tension, density, viscosity, and the heats of vaporisation and fusion, among others.

Hydrogen bonds occur in two settings:

Intramolecular hydrogen bonding occurs entirely within a single molecule, when a suitably placed H-bond donor and acceptor both exist on the same molecule close enough to interact. Two textbook examples (Figure 4.6) are ortho-nitrophenol, where the phenolic -OH hydrogen bonds to an oxygen of the adjacent nitro group, and salicylaldehyde, where the phenolic -OH hydrogen bonds to the oxygen of the adjacent aldehyde group -- in both cases the ortho (adjacent) positioning of the two groups on the benzene ring is what allows the intramolecular bond to close into a small ring.

Intermolecular hydrogen bonding occurs between two separate molecules -- of the same substance or of two different substances -- wherever a hydrogen-bond donor on one molecule and an acceptor on another are positioned so as to interact. It can happen, for instance, between ammonia molecules themselves, between water molecules themselves, or between ammonia and water.

Water is an especially strongly hydrogen-bonded liquid: each water molecule links to four neighbouring water molecules through hydrogen bonds. In this arrangement, the shorter separations (about 100 pm) correspond to the ordinary covalent O-H bonds within each molecule, while the longer separations (about 180 pm) correspond to the hydrogen bonds reaching out to neighbouring molecules (Figure 4.7b). …