Spectral Analysis of Formaldehyde Near Symmetric Rotor

SPECTRAL ANALYSIS OF FORMALDEHYDE [ ] (NEAR-SYMMETRIC ROTOR) 

The formaldehyde molecule (H2CO) can be assumed to have a structure in which the two hydrogen nuclei are located symmetrically 

It can be shown that H2CO is not a pyramidal molecule with one plane of symmetry by observing the 3 1 intensity alternation of the fine-line structure of some of its bands, which indicates that there are two planes of symmetry in the molecule. The 3 1 intensity alternation is due to the fact that the normal state energy levels with odd K have three times the statistical weight of states with even K. Therefore, subbands originating from even-K energy levels will be weak, while those from odd-K levels will be strong. 

Earlier, the concepts of planes of symmetry and reflection were described for various symmetry groups. If the model of H2CO is studied, it can be shown that the v, V2, and V3 vibrations are symmetric with respect to both symmetry planes in H2CO. The V4 and V5 vibrations are symmetric with respect to the plane of symmetry in which the molecule lies and antisymmetric with respect to the second plane. The Vg vibration is antisymmetric with respect to the first reflection plane and symmetric with respect to the second, so that its symmetry is opposite to that of V4 and V5. 

In the discussion that follows, it will be shown that V3 and V5 are coupled by a rotational resonance interaction which changes the rotational structure of V3 from that expected for an A band. The rotational structure of the V5 band is strongly perturbed by a rotational resonance interaction with the fundamental and so the line structure of V5 is determined by the interaction between V5 and v. This interaction between V5 and is of the Coriolis resonance type. 

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