<P> The hydrogen bond can be compared with the closely related dihydrogen bond, which is also an intermolecular bonding interaction involving hydrogen atoms . These structures have been known for some time, and well characterized by crystallography; however, an understanding of their relationship to the conventional hydrogen bond, ionic bond, and covalent bond remains unclear . Generally, the hydrogen bond is characterized by a proton acceptor that is a lone pair of electrons in nonmetallic atoms (most notably in the nitrogen, and chalcogen groups). In some cases, these proton acceptors may be pi - bonds or metal complexes . In the dihydrogen bond, however, a metal hydride serves as a proton acceptor, thus forming a hydrogen - hydrogen interaction . Neutron diffraction has shown that the molecular geometry of these complexes is similar to hydrogen bonds, in that the bond length is very adaptable to the metal complex / hydrogen donor system . </P> <P> The dynamics of hydrogen bond structures in water can be probed by the IR spectrum of OH stretching vibration . In the hydrogen bonding network in protic organic ionic plastic crystals (POIPCs), which are a type of phase change material exhibiting solid - solid phase transitions prior to melting, variable - temperature infrared spectroscopy can reveal the temperature dependence of hydrogen bonds and the dynamics of both the anions and the cations . The sudden weakening of hydrogen bonds during the solid - solid phase transition seems to be coupled with the onset of orientational or rotational disorder of the ions . </P> <P> Hydrogen bonding is a key to the design of drugs . According to Lipinski's rule of five the majority of orally active drug tend to have between five and ten hydrogen bonds . These interactions exist between nitrogen--hydrogen and oxygen--hydrogen centers . As with many other rules of thumb, many exceptions exist . </P> <Ul> <Li> Occurrence of proton tunneling during DNA replication is believed to be responsible for cell mutations . </Li> <Li> High water solubility of many compounds such as ammonia is explained by hydrogen bonding with water molecules . </Li> <Li> Deliquescence of NaOH is caused in part by reaction of OH with moisture to form hydrogen - bonded H O species . An analogous process happens between NaNH and NH, and between NaF and HF . </Li> <Li> The presence of hydrogen bonds can cause an anomaly in the normal succession of states of matter for certain mixtures of chemical compounds as temperature increases or decreases . These compounds can be liquid until a certain temperature, then solid even as the temperature increases, and finally liquid again as the temperature rises over the "anomaly interval" </Li> <Li> Smart rubber utilizes hydrogen bonding as its sole means of bonding, so that it can "heal" when torn, because hydrogen bonding can occur on the fly between two surfaces of the same polymer . </Li> </Ul>

Why is hydrogen bond not a chemical bond