The hydrogen bond

A hydrogen atom bound to an electronegative atom like N, O or F might be interact in a noncovalent way with other electronegative atom. The resultant hydrogen bond has an energy within the range 10-60 kJ mol^-1, weak by standards of covalent bonds but strong as compared with other intermolecular forces. The strongest hydrogen bonds are created while a fluoride ion is involved, for instance in the symmetrical [F-H-F]^- ion. Symmetrical bonds are sometime formed with oxygen but in several cases the hydrogen is not symmetrically disposed, a general instance being in liquid water in which the normal O-H bond has a length of 96 pm and the hydrogen bond a length approximately 250 pm. Hydrogen bonding increases from a combination of electrostatic (ion-dipole and dipole-dipole) forces and orbital overlap; the latter influence may be treated through a three-center molecular orbital approach.

Hydrogen bonding is essential for the secondary structure of biological molecules like nucleic acids and proteins, and for the operation of the genetic code. Its effect can be seen in the boiling points of simple hydrides. The exceptional values for H₂O,  NH₃, and HF result from strong hydrogen bonding within the liquid.