Normal modes of H₂O and CO₂ — IR active modes change the dipole moment
A molecule with N atoms has 3N−6 (nonlinear) or 3N−5 (linear) vibrational modes. For H₂O: ν₁=3657 cm⁻¹ (sym stretch, IR active), ν₂=1595 cm⁻¹ (bend, IR active), ν₃=3756 cm⁻¹ (asym stretch, IR active). CO₂ symmetric stretch is IR inactive (no dipole change) but Raman active — mutual exclusion rule for centrosymmetric molecules.