Treating the full internal nuclear-motion dynamics of a polyatomic molecule is complicated. It is conventional to examine the rotational movement of a hypothetical "rigid" molecule as well as the vibrational motion of a non-rotating molecule, and to then treat the rotation-vibration couplings using perturbation theory.
Thumbnail: A model visualizing molecular vibrations. Two atoms are connected by a spring to account for the flexibility of the bond. Image used with permission (CC SA-BY 3.0; Tby11)