Symmetry and molecular vibrations A molecular vibration is IR active only if it results in a change in the dipole moment of the molecule A molecular vibration is <PERSON> active only if it results in a change in the polarizability of the molecule In group theory terms: A vibrational mode is IR active if it corresponds to an irreducible representation with the same symmetry of ax, y, z coordinate (or function) and it is <PERSON> active if the symmetry is the same as A quadratic function x 2, y 2, z 2, xy, xz, yz, x 2 -y 2 If the molecule has a center of inversion, no vibration can be both IR & <PERSON> active