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Labham posted an Question
November 17, 2019 • 21:11 pm 15 points
  • IIT JAM
  • Chemistry (CY)

How CO2 and SF6 are both IR active and IR inactive molecules?

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    For a molecule with a center of symmetry, vibrational modes are either infrared active or Raman
    active, but not both. This restriction is called the Exclusion rule. Molecules with a center of symmetry have an inversion center. For example, O2, N2, CO2, C2H2, C6H6, PtCl42–, and SF6 have an inversion center, are therefore centrosymmetric, and the exclusion rule applies.
    However, N2O, H2O, SO2, SO3, CO32–, NH3, CH4, and H2C=CH2 do not have a center of symmetry, are not centrosymmetric, and are both infrared and Raman active. The selection rules for the normal modes for several types of small molecules are shown in Table 1.

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