Centrifugal distortion the semi-rigid rotor

Centrifugal distortion of a symmetrical diatomic molecule. Rotation at an angular velocity co generates a centrifugal force F on each atom of mass m, leading to an extension Sr of the bond. This generates a contrary force kt r, where k is the force constant. The moment of inertia increases from 2m r/2 f to 2m(r/2 + 8r/2f. 

Thus for a linear molecule the distortion is described by D, the centrifugal distortion constant, which is typically of the order of one millionth of B. 

For symmetric tops centrifugal distortion can arise from rotation about the top axis as well as from the overall molecular rotation, so three centrifugal distortion constants are needed  

Occasionally it is found necessary to include a sextic distortion constant H) as well, and the term 1)  

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