Chemistry HYDROGEN BONDING AND ITS APPLICATIONS

Hydrogen Bonding :

Ahydrogen bond is a type of attractive intermolecular force that exists between two partial electric charge of opposite polarity. Although stronger than most other intermolecular forces, the hydrogen bond is much weaker than both the `text(ionic bond)` and the `text(covalent bond)`. Within macromolecules such as proteins and nucleic acids, it can exist between two parts of the same molecule, and figures as an important constraint on such molecules' overall shape.

As the name "hydrogen bond" implies, one part of the bond involves a `text(hydrogen atom)`. The hydrogen must be attached to a strongly electronegative heteroatom, such as `text(oxygen, nitrogen)` or `text(fluorine)` which is called the hydrogen-bond donor. This electronegative element 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, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a `text(lone pair)` of electrons on another heteroatom, which becomes the hydrogen-bond acceptor.

The hydrogen bond is not like a simple attraction between point charges, however. It possesses some degree of orientational preference, and can be shown to have some of the characterist ics of a covalent bond. This covalency tends to be more extreme when acceptors bind hydrogens from more electronegative donors.

In the species `X-H--X` as the electronegativity of `X` increases the strength of hydrogen bond (`H- - - X`) also increases. Thus the order of increases of the and (`F- H- -- - F`) is shown below

`N - H-- --N` < `O - H-- --O` < `F - H- --- F`

[The electronegativity of `N = 3.0`, `O = 3.5`, `F = 4.0` in Pauling scale]

Conditions for Hydrogen Bonding :

- Hydrogen should be linked to a highly electronegative element.

- The size of the electronegative element must be small.

These two criterias are fulfilled by `F`, `O`, and `N` in the periodic table. Greater the electronegativity and smaller the size, the stronger is the hydrogen bond which is evident from the relative order of energies of hydrogen bonds.

Although the electronegativities of nitrogen and chlorine are same (`3.0`), nitrogen can form hydrogen bond but `Cl` does not form effective hydrogen bond due to its larger size.

Types of Hydrogen Bonding :

(i) `text(Intermolecular Hydrogen Bonding)` : This type of bonding takes place between two molecules of the same or different types. For example, See fig.1.

Inter-molecular hydrogen bonding leads to molecular association in liquids like water etc. Thus in water only a few percent of the water molecules appear not to be hydrogen bonded even at `90^oC`. Breaking of those hydrogen bonds throughout the entire liquid requires appreciable heat energy. This is indicated in the relatively higher boiling points of hydrogen bonded liquids.

Crystalline hydrogen fluoride consists of the polymer `(HF)_n`. This has a zig-zag chain structure involving `H`-bond. See fig.2.

(ii) `text(Intramolecular Hydrogen Bonding)` : This type of bonding occurs between atoms of the same molecule present on different sites. Intramolecular hydrogen bonding gives rise to a closed ring structure for which the term chelation is sometimes used. Examples are, `o`-nitrophenol, salicylaldehyde. See fig.3.

Importance of Hydrogen Bonding in Biological Systems :

Hydrogen bonding plays a vital role in physiological systems. Proteins contain chains of amino acids. The amino acid units are arranged in a spiral form somewhat like a stretched coil spring (forming a helix). The `N-H` group of each amino acid unit and the fourth `C= O` group following it along the chain, establishes the `N- H---O` hydrogen bonds. These bonds are partly responsible for the stability of the spiral structure.