Past exam of the chemistry A course of the University of Cambridge / 2025 / Paper 1 / A3 / c / ii

A nonlinear triatomic molecule has $3N-6=3$ vibrational normal modes. For water they are the symmetric O–H stretch, the H–O–H bend, and the antisymmetric O–H stretch, shown above. In the symmetric stretch both O–H lengths increase together; in the antisymmetric stretch one increases while the other decreases; the bend predominantly changes the bond angle. All three change a component of the electric dipole moment and are infrared-active. Every arrow reverses half a period later; motion of oxygen maintains the molecular centre of mass.