2.2.2      Hydrogen bonds

Four conditions need to be fulfilled for hydrogen bonding

1.  Hydrogen is a must for hydrogen bonding

2.  The hydrogen must be sandwich between
     two highly electronegative atom

3.  The distance between the electronegative
     atom and hydrogen must be 2.8 A°

4.  The bond must be in a plane.

Hydrogen bonds are relatively weak interactions, which are crucial for biological macromolecules such as DNA and proteins. The solubility of any molecule in water is also dependent upon hydrogen bonding. Hydrogen bonds are fundamentally electrostatic interactions. Normally hydrogen bonding is formed by fluorine, oxygen and nitrogen. The electronegative atom (like F, O, N) to which the hydrogen atom is covalently bonded pulls shared electron density away from the hydrogen atom because of this the hydrogen develops a partial positive charge (\delta+) and the electronegative atom develops partial negative charge (\delta). Thus, the covalently bonded hydrogen of one molecule can interact with another atom having a partial negative charge (\delta) through an electrostatic interaction.

The large difference in electronegativities between hydrogen and any of fluorine, nitrogen and oxygen, coupled with their lone pairs of electrons cause strong electrostatic forces between molecules. Hydrogen bonds are responsible for the high boiling points of water and ammonia with respect to their heavier analogues.

2.2.3.     Van der Waals interactions.

The basis of Van der Waals interaction is that the distribution of electronic charge around an atom changes with time. The distribution changes with time are because of the movement of electrons in the orbit. The electron is not static, it revolves around the orbit.

Due to the movement of an electron at any instant, the charge distribution is not perfectly symmetric. This transient asymmetry in the electronic charge around an atom acts through electrostatic interactions to induce a complementary asymmetry in the electron distribution around its neighbouring atoms.

The resulting attraction between two atoms increases as they come closer to each other. The minimum distance between the molecule is known as Vander Walls contact distance. At a shorter distance, very strong repulsive forces become dominant because the outer electron clouds overlap to each other. Thus atoms repel to each other. That's the reason Vander Walls forces is the sum of attractive and repulsive forces.

Three types of Van der Walls forces

1.            Dipole-Dipole

2.            Dipole - induced Dipole

3.            London - forces

London dispersion force :

A type of Van der Walls force is known as the London dispersion force. The London dispersion force arises due to instantaneous dipoles in neighbouring atoms. As the negative charge of the electron is not uniform around the whole atom, there is always a charge imbalance. This small charge will induce a corresponding dipole in a nearby molecule; causing an attraction between the two. The electron then moves to another part of the electron cloud and the attraction is broken. London dispersion force is the weakest intermolecular force. London dispersive force is a temporary attractive force that results when electrons in two-atom occupy positions that make the atoms form temporary dipoles. This force is also known as induced-dipole-induced dipole attraction.

2.2.4.     Hydrophobic interaction

Hydrophobic force is not a force, is an interaction of non-polar molecules like lipids which come close to each other only in presence of water. The hydrophobic force is a tendency of water molecules to form a bigger cage around the lipid molecules. The hydrophobic force does not exist without water. It is a tendency of water molecule only. Hydrophobic are generally non-polar molecules and have a long chain of carbon molecule which does not interact with water molecules. 

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