HYDROGEN BRIDGE BONDING: MAIN FEATURES - CHEMISTRY - 2025

The link hydrogen bond is an electrostatic attraction between two polar groups that occurs when a hydrogen atom (H) bonded to a highly electronegative atom attraction exerted on the electrostatic field electronegatively charged atom another nearby.

In physics and chemistry there are forces that generate interaction between two or more molecules, including forces of attraction or repulsion, which can act between these and other nearby particles (such as atoms and ions). These forces are called the intermolecular forces.

Two molecules self-assemble into a dimer complex through four hydrogen bonds.

Intermolar forces are weaker in nature than those that bind the parts of a molecule from the inside out (the intramolecular forces).

Among the attractive intermolecular forces there are four types: ion-dipole forces, dipole-dipole forces, van der Waals forces, and hydrogen bonds.

Characteristics of hydrogen bonding

The hydrogen bonding is between a "donor" atom (the electronegative one that has hydrogen) and a "receptor" atom (the electronegative without hydrogen).

It usually generates an energy of between 1 to 40 Kcal / mol, making this attraction considerably stronger than that occurred in the van der Waals interaction, but weaker than covalent and ionic bonds.

It usually occurs between molecules with atoms such as nitrogen (N), oxygen (O) or fluorine (F), although it is also observed with carbon (C) atoms when they are attached to highly electronegative atoms, as in the case of chloroform (CHCl ₃).

Why does the union happen?

This bonding occurs because, being bonded to a highly electronegative atom, hydrogen (a small atom with a typically neutral charge) acquires a partially positive charge, causing it to begin to attract other electronegative atoms towards itself.

From here arises a bond that, although it cannot be classified as totally covalent, bonds hydrogen and its electronegative atom to this other atom.

The first evidences of the existence of these bonds were observed by a study that measured the boiling points. It was noted that not all of these increased by molecular weight, as expected, but there were certain compounds that required a higher temperature to boil than predicted.

From here, the existence of hydrogen bonds in electronegative molecules began to be observed.

Link length

The most important characteristic to measure in a hydrogen bond is its length (the longer it is, the less strong), which is measured in angstrom (Å).

In turn, this length depends on the bond strength, temperature and pressure. The following describes how these factors influence the strength of a hydrogen bond.

Bond strength

The bond strength itself depends on the pressure, temperature, the bond angle and the environment (which is characterized by a local dielectric constant).

For example, for linear geometry molecules the bond is weaker because hydrogen is further away from one atom than from another, but at tighter angles this force grows.

Temperature

It has been studied that hydrogen bonds are prone to form at lower temperatures, since the decrease in density and increase in molecular movement at higher temperatures causes difficulties in the formation of hydrogen bonds.

The bonds can be broken temporarily and / or permanently with increasing temperature, but it is important to note that the bonds also make the compounds have greater resistance to boiling, as is the case with water.

Pressure

The higher the pressure, the greater the strength of the hydrogen bond. This happens because at higher pressures, the atoms of the molecule (such as in ice) will compact more and this will help to reduce the distance between the components of the bond.

In fact, this value is almost linear when studying for ice on a graph where the bond length found with pressure is appreciated.

Hydrogen bridge bonding in water

Hydrogen-bonded water molecule.

The water molecule (H ₂ O) is considered a perfect case of hydrogen bonding: each molecule can form four potential hydrogen bonds with nearby water molecules.

There is the perfect amount of positively charged hydrogens and non-bonded electron pairs in each molecule, allowing all of them to become involved in hydrogen bonding.

This is why water has a higher boiling point than other molecules, such as ammonia (NH ₃) and hydrogen fluoride (HF).

In the case of the first, the nitrogen atom only has one free pair of electrons, and this means that in a group of ammonia molecules there are not enough free pairs to satisfy the needs of all the hydrogens.

It is said that for each molecule of ammonia a single hydrogen bond is formed and that the other H atoms are "wasted".

In the case of fluoride, there is rather a hydrogen deficit and electron pairs are "wasted". Again, there is the right amount of hydrogen and electron pairs in water, so this system bonds perfectly.

Hydrogen bonding in DNA and other molecules

In proteins and DNA, hydrogen bonding can also be observed: in the case of DNA, the double helix shape is due to the hydrogen bonds between its base pairs (the building blocks that make up the helix), which allow these molecules are replicated and life as we know it exists.

In the case of proteins, hydrogens form bonds between oxygens and amide hydrogens; Depending on the position where it occurs, different resulting protein structures will be formed.

Hydrogen bonds are also present in natural and synthetic polymers and in organic molecules that contain nitrogen, and other molecules with this type of bond are still being studied in the world of chemistry.

References

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5. Chemguide. (sf). ChemGuide. Retrieved from chemguide.co.uk