The vibrational modes of a triatomic molecule can be determined in specific cases.
Symmetric linear molecules
A symmetric linear molecule ABA can perform:
- Antisymmetric longitudinal vibrations with frequency
- Symmetric longitudinal vibrations with frequency
- Symmetric transversal vibrations with frequency
In the previous formulas, M is the total mass of the molecule, mA and mB are the masses of the elements A and B, k1 and k2 are the spring constants of the molecule along its axis and perpendicular to it.
Ozone, O3 is an example of a triatomic molecule with all atoms the same. Triatomic hydrogen, H3, is unstable and breaks up spontaneously. H3+, the trihydrogen cation is stable by itself and is symmetric.