Strength of the Hydrogen Bond

Hydrogen bond was a controversial topic in the early 20th century. However, during the mid-20th century, the concept of weak conventional hydrogen bonds was understood and accepted by masses as it evolved from the term hydrogen. Unlike the covalent bond, whose strength varies within the factor of ∼4 (30–120 kcal mol−1), the hydrogen bond is less constrained in its physical and geometrical properties. Hydrogen bonds vary in strength by 20-fold (2–40 kcal mol−1). 

What are Hydrogen Bonds? 

A hydrogen bond, also called H-bond, is primarily an electrostatic attraction force between the hydrogen atom denoted as H and a highly electronegative atom or the group. This intricate system is generally denoted as Dn–H···Ac. Here, the polar covalent bond is denoted by the solid line, whereas dashed or the dotted line denotes the hydrogen bond. Hydrogen bonds can easily take place between separate molecules, which is why these are also referred to as intermolecular. 

Hydrogen bonds depend on various factors. These include the environment, the geometry and acceptor atoms, and the specific donor’s nature. It generally varies between 1 and 40 kcal/mol. As a result, compared to the covalent or ionic bond, the hydrogen bond is weaker. However, it is stronger than the Van der Waals forces. Experts classified hydrogen bonds as a type of weak chemical bond.

Hydrogen Bonding 

Hydrogen bonding is described as the formulation of the hydrogen bond, which is a type of attractive intermolecular force caused by the dipole-dipole interaction among hydrogen atoms and highly electronegative atoms. For example, in the case of water molecules chemically represented as H2O, hydrogen is bonded covalently to the electronegative oxygen atom. Further, hydrogen bonding is divided into two major categories – intermolecular hydrogen bonding and intramolecular hydrogen bonding:

• Intermolecular Hydrogen Bonding is where hydrogen bonding occurs between different molecules that have either the same or different compounds. Common examples of intermolecular hydrogen bonding include hydrogen bonding in alcohol, water, ammonia, etc. 

• Intramolecular Hydrogen Bonding mainly occurs within the molecule. This type of hydrogen bonding takes place in compounds that have two groups. Out of the two groups, one group has a hydrogen atom, whereas the other group has a highly electronegative atom. 

Strength of the Hydrogen Bond 

The hydrogen bond may vary in strength on various parameters. It starts from weak (1–2 kJ mol−1) to strong (161.5 kJ mol−1 in the ion HF−2). It includes the following:

• F−H···: F (161.5 kJ/mol or the 38.6 kcal/mol), demonstrated uniquely by the HF−2, bifluoride
• O−H···: N (29 kJ/mol or the 6.9 kcal/mol), demonstrated by the water-ammonia
• O−H···:O (21 kJ/mol or the 5.0 kcal/mol), demonstrated by the water-water, alcohol-alcohol
• N−H···: N (13 kJ/mol or the 3.1 kcal/mol), demonstrated by the ammonia-ammonia
• N−H···:O (8 kJ/mol or the 1.9 kcal/mol), demonstrated by the water-amide
• OH+3···: OH2 (18 kJ/mol or 4.3 kcal/mol)

In all, a hydrogen bond is described as a weak bond as its strength lies between the weak Van der Waals forces and robust covalent bonds. 

Van der Waals Forces 

Van der Waals forces can be described as weak intermolecular forces which are mainly dependent on the total distance between molecules and atoms. These forces rise when the interaction between uncharged molecules or atoms occurs. For example, the Van der Waals forces rise from the polarisation fluctuation of two different particles, which are closely packed. 

There are primarily three types of Van der Waals forces, which are as follows:

• Keesom interactions
• Debye forces
• London dispersion forces 

Examples of Hydrogen Bonding 

The following are some examples of hydrogen bonding: 

• Water: The highly electronegative oxygen atom gets connected with the hydrogen atom in a water molecule. The electron’s shared pairs are closely attracted to the atoms of oxygen, which is why the molecule’s end becomes negative, whereas the hydrogen atoms appear to be positive. 
• Hydrogen fluoride: In hydrogen bonding, fluorine forms the strongest hydrogen bond featuring the highest electronegativity. 
• Hydrogen bonding in carboxylic acid and alcohols: Alcohol is referred to as an organic molecule with an -OH group. In this case, any molecule with a hydrogen atom is directly linked to nitrogen or oxygen. Later, hydrogen bonding becomes easier.
• Hydrogen bonding in ammonia: It has electronegative atom nitrogen connected to hydrogen atoms.
• Hydrogen bonding in the polymer: To determine 3D structures and properties acquired by natural and synthetic proteins, hydrogen bonding is a crucial factor. It also plays a significant role in defining the cellulose structure along with derived polymers, including flax or cotton.

Conclusion 

Hydrogen bonding can be described as the process of forming hydrogen bonds that are an attractive intermolecular force’s special class that arises because of the dipole-dipole interaction between the hydrogen atom and highly electronegative atom. It mainly occurs between an electronegative atom and a hydrogen atom. Unlike the covalent bond, whose strength varies within the factor of ∼4 (30–120 kcal mol−1), the hydrogen bond is less constrained in its physical and geometrical properties. Hydrogen bonds vary in strength by 20-fold (2–40 kcal mol−1).