Nonrigid molecule energy level

Dalton, B. J. (1971), Nonrigid Molecule Effects on the Rovibronic Energy Levels and Spectra of Phosphorous Pentafluoride, J. Chem. Phys. 54,4745. de Shalit, A., and Talmi, 1. (1963), Nuclear Shell Theory, Academic Press, N.Y. 

Jensen, P. (1983), The Nonrigid Bender Hamiltonian for Calculating the Rotation-Vibration Energy Levels of a Triatomic Molecule, Comp. Phys. Rep. 1,1. 

For diatomic molecules, corrections can be made for the assumption used in the derivation of the rotational partition function that the rotational energy levels are so closely spaced that they can be considered to be continuous. The equations to be used in making these corrections are given in Appendix 6. Also given are the equations to use in correcting for vibrational anharmonicity and nonrigid rotator effects. These corrections are usually small.22... 

For a nonrigid symmetric top molecule there are three different centrifugal distortion constants 1, D", and ). The rotational energy levels of the vibrational level v of an oblate symmetric top can be represented by... 

For a diatomic molecule, the energy levels of the nonrigid rotor are given, to the order of perturbation considered here, by... 

In a nonrigid rotating diatomic molecule, centrifugal distortion will elongate the bond as the rotational frequency increases. To accurately fit the energy levels, a correction term must be added to Eq. (1.55)... 

In the previous section the rigid-rotor approximation has been applied, while in the following we account for the nonrigidity of the molecules, which means that the nuclear positions are no longer fixed at their equilibrium values. We first address the effect of the rotation itself on the energy levels (centrifugal distortion), and then the effect of molecular vibrations on the spectroscopic parameters is presented. 

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