Effects of nonrigidity

Because of the differing ways in which effects of nonrigidity enter, different experimental techniques do not yield exactly the same values for structural parameters. Furthermore, more than one set of structural parameters may be derivable from the same experimental data, depending on the way in which corrections are made for nonrigidity. As a result, structural parameters reported in the literature have a variety of meanings. In this paper various types of structural parameters commonly reported for gas-phase molecules will be defined and the interrelations among them discussed. A similar discussion has been given by Kuchitsu and Cyvin.1... 

In order to simplify the problem, one identifies from the beginning the well-bound molecules or complex ions in the crystal and treats them as rigid bodies. These rigid bodies will execute not only translational oscillations but also rotational oscillations. Coupling is allowed for between all the various units in the crystal and the resulting vibrations are called external vibrations. The effects of nonrigidity of the molecules or complex ions may be examined separately, thereby treating the effects of the crystal... 

The rigid-rotor treatment discussed in the previous section accounts for the general features of the rotational spectrum. These gross features are modified somewhat when the effects of nonrigidity, nuclear coupling, and so forth are taken into account. In this section, the effects of centrifugal distortion are considered. 

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