Anharmonicity

Anharmonicity is when things that are supposed to vibrate at the same speed don't.

Imagine you have two friends swinging on swings next to each other. They start off swinging at the same rate, but one friend starts pushing themselves higher and higher while the other friend stays at the same height. Even though they started off doing the same thing, they're now doing something different.

This is what happens with anharmonicity. In chemistry, we often talk about molecules that vibrate like little springs. When we measure the vibrations of a molecule, we expect them to all vibrate at the same "frequency" (kind of like how we expect our friends on the swings to swing at the same rate). But in reality, some parts of the molecule might be vibrating faster or slower than others because of how the atoms are arranged. This is called anharmonicity.

Anharmonicity is important in chemistry because it affects how molecules absorb and emit light, and how they interact with other molecules. Scientists have to take anharmonicity into account when they study molecules and reactions, just like we have to take into account how our friends are swinging differently.