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Intermolecular Forces in Nonpolar Molecules

In this intermolecular forces in nonpolar molecules post we have briefly explained about covalent bond, ionic bonds, hydrogen bonds, hydrophobic interactions, van der waals forces between molecules. Read on to learn more about intermolecular forces between molecules!!

Forces Between Molecules

Intermolecular forces between molecules are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions ). These forces forces between molecules are weak compared to the intramolecular forces, such as the covalent or ionic bonds between atoms in a molecule. For example, the covalent bond present within a hydrogen chloride (HCl) molecule is much stronger than any bonds it may form with neighbouring molecules.

1. Covalent Bond

Elements with extremely high ionisation energies are unable to transport electrons, whereas those with extremely low electron affinity are unable to absorb electrons. Such elements’ atoms tend to share electrons with atoms of other elements or with atoms of the same element in such a way that both atoms attain an octet configuration in their respective valence shells and therefore achieve stability. The term “covalent bond” refers to a link formed by the sharing of electron pairs among distinct or similar kinds of molecules.

Intermolecular Forces in Nonpolar Molecules

Covalent bond 

2. Ionic Bonds

Ionic bonds result from the electrostatic attraction between two ionized groups of opposite charges. They are formed by transfer of one or more electrons from the outermost orbit of an electropositive atom to the outermost orbit of an electronegative atom. This transfer results in the formation of a ‘cation’ and an ‘anion’, which get consequently bound by an ionic bond. Common examples of such compounds include NaCl, KBr and NaF.

Intermolecular Forces in Nonpolar Molecules

Ionic bond

With regard to protein chemistry, positive charges are produced by epsilon amino group of lysine, guanidium group of arginine and imidazolium group of histidine. Negative charges are provided by beta and gamma carboxyl groups of aspartic acid and glutamic acid.

Ionic bonds used in protein interactions

3. Hydrogen Bonds

These are formed by sharing of hydrogen between two electron donors. Hydrogen bonds result from electrostatic attraction between an electro-negative atom and a hydrogen atom that is bonded covalently to a second electronegative atom. Normally, a hydrogen atom forms a covalent bond with only one other atom. However, a hydrogen atom covalently bonded to a donor atom, may form an additional weak association, the hydrogen bond with an acceptor atom. In biological systems, both donors and acceptors are usually nitrogen or oxygen atoms, especially those atoms in amino (NH2) and hydroxyl (OH) groups.

Intermolecular Forces in Nonpolar Molecules: Forces Between Molecules

With regard to protein chemistry, hydrogen releasing groups are -NH (imidazole, indole, and peptide); -OH (serine, threonine) and -NH2 (arginine lysine). Hydrogen accepting groups are COO–, (aspartic, glutamic) C=O (peptide); and S–S (disulphide). The DNA structure is maintained by hydrogen bonding between the purine and pyrimidine residues.

4. Hydrophobic Interactions

Non-polar groups have a tendency to associate with each other in an aqueous environment; this is referred to as hydrophobic interaction. These are formed by interactions between nonpolar hydrophobic side chains by eliminating water molecules. The force that causes hydrophobic molecules or nonpolar portions of molecules to aggregate together rather than to dissolve in water is called the ‘hydrophobic bond’. This serves to hold lipophilic side chains of amino acids together. Thus, nonpolar molecules will have minimum exposure to water molecules.

Intermolecular Forces in Nonpolar Molecules

Hydrophobic interaction

5. Van Der Waals Forces

These are very weak forces between molecules of attraction between all atoms, due to oscillating dipoles, described by the Dutch physicist Johannes van der Waals (1837-1923). He was awarded Nobel prize in 1910. 

These are short range attractive forces forces between molecules chemical groups in contact. Van der Waals interactions occur in all types of molecules, both polar and nonpolar. The energy of the van der Waals interaction is about 1 kcal/mol and are unaffected by changes in pH. 

This forces between molecules will drastically reduce, when the distance between atoms is increased. Although very weak, van der Waals forces collectively contribute maximum towards the stability of protein structure, especially in preserving the nonpolar interior structure of proteins.

Further Readings