When an electronegative atom B (such as F,N,O,S,...) approaches (to short distances of about 2) a hydrogen atom which is bound to another electronegative atom A, then the A - H distance increases to some extent allowing for the A - H...B interaction. This is the well known hydrogen bond which is present in several molecular systems and has a role to play in various processes from the strengths of fibers, genetic coding and even in human memory.
The changes in boiling points among closely related systems such as Ne, Ar, Kr or dipolar molecules with similar dipole moments can be generally explained in terms of changes in polarizability or changes in dipole moments. Ethanol and ethylene oxide have dipole moments 1.7D and 1.9 D but ethanol boils at 352K while ethylene oxide boils at 284K. Ethylene oxide should have had a higher boiling point if the dipole moments were solely responsible for changes in boiling points. Extensive hydrogen bonding in ethanol is responsible for the higher energy (through higher temperature) required to separate ethanol molecules at the surface of the liquid from the bulk.
The strength of hydrogen bond is greater than dispersive interactions but lower than covalent bond dissociation energies. The magnitudes are in the range of 3 to 10 kcal/mol and hence the name "bond". The hydrogen bond length (B...H) in B...H-A is in the range of 1.5 to 2.5. Examples of hydrogen bonding in HF, water, acetic acid dimmer, salicylic acid (intramolecular hydrogen bond), polyamide fiber and DNA are shown in figure 10.4.
Figure 10.4 (a) and (b): Some examples of hydrogen bonds.
) a hydrogen atom which is bound to another electronegative atom A, then the A - H distance increases to some extent allowing for the A - H...B interaction. This is the well known hydrogen bond which is present in several molecular systems and has a role to play in various processes from the strengths of fibers, genetic coding and even in human memory. 
