1.2- Dichloroethene point group

Solution, (a) PF5 is nonpolar (trigonal bipyramidal, 031, point group), (b) Co(en)3 + is nonpolar (D3 point group), (c) c/s-Dichloroethene is polar (C2V point group), (d) The boat form of cyclohexane is polar (C2V point group). 

For more complex molecules in point groups with the inversion, centre vibrations that are IR active will also have irreducible representations that are ungerade while Raman-active modes will be gerade. Since different vibrations will usually occur at different frequencies, it is unlikely that bands in the two spectra will appear to be coincident. For example, rra 5-l,2-dichloroethene has an inversion centre as it belongs to the point group C2h, the IR and Raman spectra for this molecule are compared in Figure 6.13a. This molecule has six atoms, and so 3x6 — 6 = 12 vibrational modes. The total number of bands in the Raman and IR spectra is fewer than 12 because some vibrations are too low frequency to be detected in the range shown. However, it can be seen that the Raman and IR frequencies are indeed different to one another. 

This is not the case for molecules that belong to other point groups. For example, the cis isomer of 1,2-dichloroethene belongs to the C2V point group. Here, the character table... 

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