Hydrogen bond

Hydrogen bonding — a special, strong case of dipole-dipole attraction

Hydrogen bonding, introduced already in Unit 4, is best thought of as a particularly powerful variety of dipole-dipole interaction (Section 5.1). It shows up in molecules where hydrogen is bonded to a strongly electronegative atom, producing a highly polar bond — chiefly N–H, O–H, or H–F. While hydrogen bonding is normally associated only with N, O and F, other reasonably electronegative atoms such as Cl can take part too.

How the bond forms:

The H of one H–F molecule (carrying $\delta^{+}$) is drawn towards the lone pair on the F of a neighbouring molecule (carrying $\delta^{-}$) — shown as a dotted line ($\cdots$) — while each H–F pair within a molecule stays a normal, much stronger covalent bond.

Strength and importance

The energy of a hydrogen bond typically lies in the range 10 to 100 kJ mol$^{-1}$ — a genuinely significant amount, which is why hydrogen bonds strongly influence the structure and properties of substances such as proteins and nucleic acids. The strength of any particular hydrogen bond comes from the coulombic (electrostatic) interaction between the lone-pair electrons on the electronegative atom of one molecule and the hydrogen atom of the other.

Molecules also repel each other …