Instantaneous Dipole Moment

[Sean Sugai 4E]

What is an instantaneous dipole moment and what does it have to do with London forces?

[Sebastian Lee 1L]

I believe this is referring to the fact that at any moment in time, electron distribution in a molecule can be slightly unequal. Electrons may be "hanging out" on one side of a molecule at an instant point in time. This creates a temporary/instantaneous dipole moment because of this unequal charge distribution. In London dispersion forces, it is this instantaneous dipole moment which induces a partial charge on the molecules. If one nonpolar molecule has a slight negative charge on one side at one instance, it can push away electrons in a neighboring molecule and cause a partial positive on the closer side. It is this type of intermolecular interaction that causes solid and liquid states in nonpolar molecules.

[Andrew Liang 1I]

Instantaneous dipole moment happens when a polar molecule distorts the electrons in a nonpolar molecule, causing an uneven distribution of electrons which led to a temporary dipole.

[Brittany Tran 3I]

Since the electrons in an electron cloud are constantly moving, an instantaneous dipole moment can occur if the electrons are concentrated in one region at any given time. This can result in an unequal charge distribution and a temporary dipole moment in the molecule, which can induce temporary dipole moments in surrounding molecules. these temporary attractions between molecules are LDFs

[Sion Hwang 4D]

Instantaneous dipole moments occur in any molecules.
This is because within a molecule, electrons are not fixed to a certain position or trajectory (as we have seen with the wave functions).
We really cannot tell where an electron will be, and its movement is "instantaneous."

Hence, at a random instant, a certain location within the molecule may have a higher concentration of electrons than other locations. This creates an overall, instantaneous negative charge at that location, since electrons are cluttered there. Then, a dipole moment from an end without many electrons at an instant can be formed (from that positively charged area to the negatively charged area).
Thus, this is the instantaneous dipole moment that can be present within a molecule.

This is important in dispersion forces, as London dispersion forces are intermolecular forces caused by an instantaneous dipole (where at some point in time, there is a dipole moment in one molecule and a dipole moment in another molecule that is attracted to each other). These random, instantaneous dipole moments can add up and make a greater impact when there are more of the molecules (since there is a higher chance of random dipole moments being formed and thus causing higher dispersion forces). They can also occur if the molecule is larger itself, as this larger surface area creates a higher chance (more electrons and more surfaces) of instantaneous dipoles to form.