
Examples: CH4, CCl4, SF6
For the case of I = IA = IB = IC the rotational energy is given by:

We obtain the respective quantum mechanical equation if we shift to the operators H and J:

The eigenvalues to J² are already 
known:     J²⋅Ψ = 
 J(J+1) h²⋅Ψ     with J = 
0, 1, 2, ... 

It has become common practice to write this equation in a form with the 
rotational constant B which has the unit of the wavenumber, 
cm-1. The product term hcB equals the fraction 
h2/2I. 

With c denoting the speed of light, the transformation between the moment of inertia I and rotational constant B is

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