# When does hydrogen bonding happen?

What kinds of molecules will experience hydrogen bonding (inter and intra)? Why does it occur for some H atoms but not others?

What kinds of molecules will experience hydrogen bonding (inter and intra)?

Intramolecular hydrogen bonds:

• The hydrogen bond which is within the same molecules is called intramolecular hydrogen bond.

• It will form when 2 functional group of a molecule can form hydrogen bonds with each other.

• In order for this to happen, both a hydrogen donor an acceptor must be present within one molecule, and they must be within close proximity of each other in the molecule.

• EXAMPLE:

Intermolecular hydrogen bonds:

• The hydrogen bond which occurs between two separate molecules is caleed intermolecular hydrogen bond.

• They can occur between any number of like or unlike molecules as long as hydrogen donors and acceptors are present an in positions in which they can interact.

• EXAMPLE:

Why does it occur for some H atoms but not others?

Hydrogen bond depends on certain factors like electronegativity, viscosity, atomic size.

Viscosity:

Those substances which are capable of forming hydrogen bonds tend to have a higher viscosity than those that do not. Substances which have the possibility for multiple hydrogen bonds exhibit even higher viscosities.

electronegativity:

Hydrogen bonding cannot occur without significant electronegativity differences between hydrogen and the atom it is bonded to.

Thus, we see molecules such as PH3, which no not partake in hydrogen bonding.

Atomic size:

The size of donors and acceptors can also effect the ability to hydrogen bond. This can account for the relatively low ability of Cl to form hydrogen bonds. When the radii of two atoms differ greatly or are large, their nuclei cannot achieve close proximity when they interact, resulting in a weak interaction.

Source for detailed explanation

• Are lone pairs a necessity for an H bond to be formed? Check this link they advertise lone pairs as the means of salvation (for an H bond to form). I'm commenting because you haven't mentioned the role of lone pairs in your answer even though the question is "When does hydrogen bonding happen?" – Gaurang Tandon Jan 3 '18 at 16:09

Most people seems to think that hydrogen bonds are limited to $\ce{X-H\bond{~}Y}$ interactions, where $\ce{X}$ and $\ce{Y}$ are heteroatoms.

In the crystal, a $\ce{C-H}$ of an aromatic system or a $\ce{C=C}$ double bond can serve as a donor as well and interact with heteroatoms, such as oxygen atoms in $\ce{C=O}$ or $\ce{C_{arene}-O-C_{alkyl}}$.

These interactions can form complex networks of rings and strands.

We've examined and published that a while ago for a couple of different phthalimides, such as N-(4-methoxybenzyl)phthalimide or N-allyl­phthalimide, and some more.

While the articles are behind a paywall (they were published before Acta Cryst. E went open source), the CIF files can be downloaded and played with/examined.

A crucial parameter for hydrogen bonds is distance, if the acceptor is too far away, there is (obviously) no hydrogen bond.

As a rule of thumb, $2.3\,\mathrm{\mathring{A}}$ is a distance where hydrogen bonds can be expected.