Hydrogen bond

Hydrogen bondis a type of electrostatic interaction between molecules occurring in molecules that have hydrogen atoms bound to electronegative atoms (F, N, O). It can be regarded as a strong dipole-dipole attraction caused by the electron-withdrawing properties of the electronegative atom. Thus, in the water molecule the oxygen atom attracts the electrons in the O-H bonds. The hydrogen atom has no inner shells of electrons to shield the nucleus, and there is an electrostatic interaction between the hydrogen proton and a lone pair of electrons on an oxygen atom in a neighboring molecule. Each oxygen atom has two lone pairs and can make hydrogen bonds to two different hydrogen atoms. The strengths of hydrogen bonds are about one tenth of the strengths of normal covalent bonds. Hydrogen bonding does, however, have significant effects on physical properties. Thus it accounts for the unusual properties of water and for the relatively high boiling points of H₂O, HF and NH₃ (compared with H₂S, HC1 and PH₃). It is also of great importance in living organisms. Hydrogen bonding occurs between bases in the chains of DNA. It also occurs between the C=O and N-H groups in proteins and is responsible for maintaining the secondary structure. Hydrogen bonds are not purely electrostatic and can be shown to have some covalent character. Bond energy is an amount of energy associated with a bond in a chemical compound. It is obtained from the heat of atomization. For instance, in methane the bond energy of the C-H bond is one quarter of the enthalpy of the process:

CH₄(g) ® C(g) + 4H(g).

Bond energies (or bond enthalpies) can be calculated from the standard enthalpy of formation of the compound and from the enthalpies of atomization of the elements. Energies calculated in this way are called average bond energies or bond-energy terms. They depend to some extent on the molecule chosen. The C-H bond energy in methane will differ slightly from that in ethane. The bond dissociation energy is a different measurement, being the energy required to break a particular bond: e.g. the energy for the process.

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