What is non-rigid rotator?

The rigid rotator is a mechanical model that is used to explain rotating systems. An arbitrary rigid rotator is a 3-dimensional rigid object, such as a top. A special rigid rotor is diatomic molecule which is an example of linear rotor.The fixed distance between the two masses and the values of the masses are the only characteristics of the rigid model. However, for many actual diatomic this model is too restrictive since distances are usually not completely fixed. To achieve a better and more practical form corrections on the rigid model has to be made to compensate for small variations in the distance which bought the model of non rigid rotator in the consideration. It gives the explanation that molecular bonds (and therefore the interatomic distance ) are not completely fixed; the bond between the atoms stretches out as the molecule rotates faster. This effect can be accounted for by introducing a new correction factor known as the centrifugal distortion constant.