Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole–dipole intermolecular attraction that arises under a very specific condition: hydrogen must be covalently bonded directly to one of the three small, highly electronegative elements — fluorine, oxygen or nitrogen. This combination produces a bond dipole with an unusually large partial positive charge concentrated on the hydrogen atom (because hydrogen has no core electrons to shield its nucleus, and because F, O and N are so strongly electronegative), and this exposed, electron-poor hydrogen is then strongly attracted to a lone pair on an F, O or N atom of a neighbouring molecule, forming the hydrogen bond, conventionally written with a dotted or dashed line: $\text{X}-\text{H} \cdots \text{Y}$ (where X and Y are F, O or N).

Hydrogen bonds can be intermolecular (between two separate molecules, as in liquid water or hydrogen fluoride) or intramolecular (within a single molecule, when geometry allows a suitably placed F, O or N to reach back to an N–H or O–H group elsewhere in the same molecule). Although a single hydrogen bond ($\approx 10$–$40\ \text{kJ mol}^{-1}$) is still far weaker than a genuine covalent or ionic bond, it is markedly stronger than an ordinary dipole–dipole attraction, and — critically — many hydrogen bonds acting together across a whole liquid or solid can have a dramatic cumulative effect on physical properties: elevated boiling and melting points, increased viscosity, and unusual solid-state structures. …