Hydrogen is the first element
in the periodic table and is unique. Hydrogen and Helium are the two elements
which are present in the first period. Hydrogen is quite reactive but helium is
inert. There is no difficulty in relating the structure and properties of helium
to those of the other noble gases in group 8th but the properties of hydrogen
cannot be correlated with any of the main groups in the periodic table, And
hydrogen is considered best on its own.
On the basis of unique properties, hydrogen may be placed in group I-A, IV-A and
VII-a and may be not in any group.
An example of intermolecular hydrogen bonding in a self-assembled dimer complex
reported by Meijer and coworkers..
A hydrogen attached to carbon can also participate in hydrogen bonding when the
carbon atom is bound to electronegative atoms, as is the case in chloroform,
CHCl3. The electronegative atom attracts the electron cloud from around the
hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a
positive partial charge. Because of the small size of hydrogen relative to other
atoms and molecules, the resulting charge, though only partial, represents a
large charge density. A hydrogen bond results when this strong positive charge
density attracts a lone pair of electrons on another heteroatom, which becomes
the hydrogen-bond Acceptor.
The hydrogen bond is often described as an electrostatic dipole-dipole
interaction. However, it also has some features of covalent bonding: it is
directional and strong, produces interatomic distances shorter than sum of van
der Waals radii, and usually involves a limited number of interaction partners,
which can be interpreted as a type of valence. These covalent features are more
substantial when acceptors bind hydrogens from more electronegative donors.
The partially covalent nature of a hydrogen bond raises the following questions:
"To which molecule or atom does the hydrogen nucleus belong?" and "Which should
be labeled 'donor' and which 'acceptor'?" Usually, this is simple to determine
on the basis of interatomic distances in the X−H…Y system: X−H distance is
typically ≈110 pm, whereas H…Y distance is ≈160 to 200 pm. Liquids that display
hydrogen bonding are called associated liquids.