Hydrogen bonding is of universal importance. It is central to the understanding of the microscope structures and functions in many complex systems, for example hydrogen bonded water or alcohol networks, organic compounds in solution crystal engineering, proteins and DNA building blocks of life.
Hydrogen bonding can be either an important or intramolecular phenomenon. An intermolecular H-bond is one for which the donor and acceptor units are found in two different molecules, for an intramolecular H-bond they are in the same molecule.
Strength of Hydrogen BondBack to Top
The hydrogen bond strength depends on its length and angle and hence, it has directionality. Nevertheless small derivations from linearity in the bond angle have marginal effect on H-bond strength. The dependency of the same on H-bond length is very important and has been shown to decay exponentially with distance. Strengthening hydrogen bonding has particularly important effects on viscocity and diffusion as indicated by the large changes occurring in super cooled normal water.
For example, the $\alpha$-helix conformation of a polypeptide is stabilized by intra-chain H-bonding , while the $\beta$-sheet conformation is stabilized by inter-chain H-bonding. The strength of H-bond is strongly dependent on the solvent polarity; the addition of a polar solvent can decrease the H-bond strength significantly, over several orders of magnitude, because the solvent molecule can also take part in H-bonding interactions. As a result, nonpolar solvents are mostly used for the preparation of H-bonded supramolecular materials.
Hydrogen Bonding in WaterBack to Top
Water is a major chemical constituent of the planets surface and as such it has been indispensable for the genesis of the life on earth. Hydrogen bonding is responsible for the unusual properties of water. Water is a very polar molecule and an excellent solvent. Water is capable of forming hydrogen bonds. Water can donate and simultaneously accept two of these H-bonds. Hydrogen bonds are examples of non covalent interactions.
Although hydrogen bonds are relatively weak compared to covalent bonds within the water molecule itself., the ability of H-bonds to provide cooperative interaction between many water molecule is responsible for a number of waters physical properties. One such property is its relatively high melting point and boiling points. The chemically similar compound hydrogen sulfide (H2S), which has much weaker hydrogen bonding , is a gas at room temperature even though it has twice the molecular weight of water. The extra bonding between water molecules also gives liquid water a large specific heat capacity.
Types of Hydrogen BondingBack to Top
Two types of hydrogen bonding have been recognized; intramolecular and intermolecular hydrogen bonding. Due to hydrogen bonding there is an increase in inter molecular aggregation forces which is reflected in the boiling point and solubility of the organic compound. There is an increase in the boiling point since energy is required to separate the hydrogen bonded molecules in their translation to the gaseous state. Hydrogen bonds exist in the liquid and solid phases and in solution. Compounds which form strong hydrogen bonds may be associated even in the gas phase. Thus acetic acid exists as a dimer in the gas phase. An important way to detect hydrogen bonding is IR and NMR spectroscopy.
Intermolecular hydrogen bond formation lowers the strength of an acid. For a substance to be soluble in water, it should be able to make hydrogen bond with water, for example, ethanol and methanol are miscible with water in all proportions because of coordination with water, while ether higher alcohols and hydrocarbons are not soluble in water because they can not associate with water.
Intramolecular hydrogen bonding occurs within the same molecule. A large number of molecule are capable of forming intramolecular hydrogen bond between two groups present in the same molecule. This results in the formation of a five or six membered rings. This type of association also alters the properties of molecule. Intramolecular hydrogen bonding increases the strength of an acid.