Weak Chemical Forces or Non-covalent Interactions

Non-covalent or supramolecular interaction refers to a variety of interactions that are not covalent in nature, but are strong enough to provide the attractive force between molecules or parts of molecules so as to hold them together, usually in a specific orientation or conformation. These non-covalent interactions include coordination bonds, dipole-dipole interactions, hydrogen bonds, pi−pi stacking or van der Waals interactions. A comparative overview of these different types of non-covalent interactions is as presented. 

These bonds are weak by nature and must therefore work together to have a significant effect. In addition, the combined bond strength is greater than the sum of the individual bonds. This is because the free energy of multiple bonds between two molecules is greater than the sum of the enthalpies of each bond due to entropy effects.    

Hydrogen bonding

Hydrogen bonding is one of the most significant interactions among the weak intermolecular attractive forces. The bond enthalpy of a conventional hydrogen bond is usually within the range of 3–7 kcal mol^-1. However, much higher values have been observed for halogen anions X−H∙∙∙X⁻ (X = F, Cl. Br and I), with a maximum of 38.6 kcal mol^‑1 for F−H∙∙∙F⁻ in LiHF₂. A few important hydrogen bonds along with their typical bond enthalpy values are listed in below.

Hydrogen bond results from a dipole-dipole force between an electronegative atom and a hydrogen atom bonded to nitrogen, oxygen or fluorine. The energy of a hydrogen bond is comparable to that of weak covalent bonds, and a typical covalent bond is only about 20 times stronger than an intermolecular hydrogen bond. These bonds can occur between molecules (intermolecular hydrogen bonds), or within different parts of a single molecule (intramolecular hydrogen bonds). Although, hydrogen bond is a very strong fixed dipole-dipole van der Waals-Keesom force, it is weaker than covalent, ionic or metallic bonds. The hydrogen bond is somewhere between a covalent bond and an electrostatic intermolecular attraction. Some important hydrogen-bonding interactions are shown in the figure below. A hydrogen atom attached to a electronegative atom is a potential hydrogen bond donor (D). This electronegative atom is usually fluorine, oxygen, or nitrogen. An electronegative atom such as fluorine, oxygen, or nitrogen is a hydrogen bond acceptor (A), regardless of whether it is bonded to a hydrogen atom or not. Ethanol, which has a hydrogen atom bonded to oxygen, is a suitable example of hydrogen bond donor and on the other hand, an example of a hydrogen bond acceptor which does not have a hydrogen atom bonded to it is the oxygen atom on diethyl ether.

Van der Waals Forces

Van der Waals forces are the types of weak attractive forces that are operating in between the molecules (intermolecular). There are two types of van der Waals forces viz., London dispersion forces and dipole-dipole forces. 

London Dispersion Forces

The interactions observed between ions, dipoles and induced dipoles are well understood and are purely electrostatic in nature. Many of the properties of molecules such as higher melting point, boiling point, stability etc. are responsible for these interactions. However, some non-polar molecules also show such behavior, indicating that there are some type of intermolecular interactions that cannot be attributed to simple electrostatic interactions. These interactions are known as dispersion forces. It is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms to form temporary dipoles in it. 

Dipole-Dipole Forces

Dipole-dipole forces are similar to that of London dispersion forces, but these interactions are observed in molecules that are permanently polar in. nature. In this type of intermolecular attraction,  a polar molecule such as water attracts (with the negative end of its dipole) the positive end of an another polar molecule. The attraction between these two molecules is the dipole-duple force.

To be continued...