Rotation of polyatomic molecules

D. Perturbative Treatment of Vibration-Rotation Coupling III. Rotation of Polyatomic Molecules... 

The basic theory of vibration-rotation of polyatomic molecules has been worked out for many years, including the form of the vibration-rotation Hamiltonian, the quantum mechanical solution of certain anharmonic oscillators," and the relationship, in terms of perturbation theory,between experimental spectroscopic data and higher than quadratic terms in the potential function. Still, prediction of harmonic... 

As stated earlier, in considering rotation of polyatomic molecules we can follow the approximation used for diatomic molecules, that the rotation can be treated as independent of vibration. Much of the stmcture of rotational spectra of polyatomic molecules can be understood by using as a model an aggregate of nuclear masses connected rigidly at their equilibrium positions. In contrast to diatomic molecules where only one axis of rotation is required, in polyatomic molecules we must consider rotation about any axis. 

---