molecular vibration
/ muh-LEK-yuh-ler vy-BRAY-shun /
A molecular vibration is the constant tiny jiggling of the atoms in a molecule, even when the molecule looks perfectly still. Picture two balls joined by a spring: they can stretch apart and snap back, or, with a third ball, bend the angle between them like a closing scissors. Atoms held by chemical bonds do exactly this, ceaselessly.
More precisely, a molecular vibration is a coordinated, repetitive motion of a molecule's atoms about their average positions. Each mode — a stretch, a bend, a twist — happens at a fixed frequency set by the masses of the atoms and the stiffness of the bonds, much as a stiffer or shorter spring vibrates faster.
It matters because these vibration frequencies fall in the infrared range, so a molecule absorbs infrared light precisely where its vibrations resonate. That is the whole basis of infrared and Raman spectroscopy. The honest caveat is that not every vibration shows up in every method — a vibration appears in the infrared only if it changes the molecule's electric dipole.
A water molecule has three basic vibrations: the two O–H bonds stretch in step, stretch out of step, and the H–O–H angle bends. Each absorbs infrared light at its own frequency, giving water its characteristic infrared bands.
Stretches and bends each resonate at their own infrared frequency.
Heavier atoms and weaker bonds vibrate more slowly, so they absorb at lower wavenumbers; lighter atoms and stronger bonds vibrate faster and absorb higher. This is why O–H and C–H stretches sit at the high-frequency end of an infrared spectrum.