Question

Assume you just synthesized for the first time, new a small ion with the formula F₃⁺ . You know it is a trimer but you don't know if it is linear or bent. Would an IR and/or Raman analysis provide evidence for either structure?

Answer 1

Infra red light is absorbed when a change in dipole moment occurs in the molecule.

If a molecule has a center of symmetry , then the vibrations are centrosymmetric and are IR inactive.

But Centrosymmetric molecules are Raman active. molecules are raman active when there is change of polarisability due to molecular vibration.

This is in accordance with Mutual Exclusion rule.

hence, If the ion (F₃^-) is linear then , it is centrosymmetric ,so it will be IR inactive but Raman active.

But If it is bent , the it is not centrosymmetric , hence it will be IR active but Raman inactive.

linear Raman active but IR inactive Bent Raman inactive but IR active
In centrosymmetric molecules ; IR active vibrations are Raman inactive and vice versa.