The hydrogen bond is an interaction between a covalently bonded hydrogen atom on a donor group (i.e., -OH or ) and a pair of nonbonded electrons on an acceptor group (i.e., or ) as shown in Figure 2.7. The atom to which hydrogen is covalently bonded is called the hydrogen bond donor and the atom with the nonbonded electron pair is called the hydrogen bond acceptor. Hydrogen bond donors tend to be highly electronegative atoms, such as N and O, because they can withdraw negative charge from the hydrogen atom, thus making it partially positive and, thus, more strongly attracted to the electron pair of the hydrogen bond acceptor.

Hydrogen bonds have characteristics of covalent and noncovalent bonds. For example, the attraction between the partially positive hydrogen and the negative charge of the electron pair is like a charge-charge interaction (Figure 2.2a). At the same time, there is electron sharing (as in a covalent bond) between the hydrogen atom and the hydrogen bond acceptor.

Other features that suggest hydrogen bonds are partially covalent in character include the following:

- Hydrogen bond lengths are fixed at about 0.33 nm (See Table 2.3)

- Hydrogen bonds are highly directional - the donor H bond tends to point directly at the acceptor electron pair.

- The energy of hydrogen bonds is greater than most other noncovalent interactions.

Hydrogen bonds provide forces that help stabilize the structures of macromolecules, such as DNA and proteins, give a molecule like water its unusual chemical characteristics for its size (see Table 2.4, Table 2.5), and the hydrogen bonds of water also assist in solubilizing polar compounds.