As already mentioned in section (5.1); this is special case of dipole-dipole interaction. This is found in the molecules in which highly polar N–H, O–H or H–F bonds are present.
Although hydrogen bonding is regarded as being limited to N, O and F; but species such as Cl may also participate in hydrogen bonding. Energy of hydrogen bond varies between 10 to 100 kJ mol–1.
This is quite a significant amount of energy; therefore, hydrogen bonds are powerful force in determining the structure and properties of many compounds, for example proteins and nucleic acids.
Strength of the hydrogen bond is determined by the coulombic interaction between the lone-pair electrons of the electronegative atom of one molecule and the hydrogen atom of other molecule. Following diagram shows the formation of hydrogen bond.
δ + δ − δ + δ −
H − F H − F
Intermolecular forces discussed so far are all attractive. Molecules also exert repulsive forces on one another.
When two molecules are brought into close contact with each other, the repulsion between the electron clouds and that between the nuclei of two molecules comes into play.
Magnitude of the repulsion rises very rapidly as the distance separating the molecules decreases.
This is the reason that liquids and solids are hard to compress. In these states molecules are already in close contact; therefore they resist further compression; as that would result in the increase of repulsive interactions.