CO2 is IR Active Explain | Vibrational Spectra of CO2 Molecule

Vibrational Spectra of CO₂ Molecule

Vibrational Spectra of CO₂ Molecule

CO₂ is IR Active, Explain.

Carbon dioxide is a linear molecule hence, number of vibrational modes is equal to four.
We know that Number of vibrational modes for linear molecule = 3n − 5
So, the number of vibrational modes of CO₂ Molecule = 3 x 3 − 5 = 4

CO₂ Molecule has zero dipole moment but during vibration, some change observed. Hence, CO₂ Molecule is IR active. Symmetrical stretching molecule is IR inactive because, during vibration, there is no change in dipole moment. Asymmetric stretching of CO₂ Molecule is IR active. Two bending modes having frequency (𝜈₂) are degenerate because they differ only in directions.
However, since there is change in dipole meoment during the bending vibrations, the molecule in this mode is IR active and appears at 667cm^-1. Thus out of four vibrational modes of CO₂ Molecule, only three is IR active and out of three two are degenerate. Hence, in IR, two fundamental bands at 2349cm^-1 and 667cm^-1 are observed.

Which of the following will give IR spectra and why?

H₂, Cl₂, HCl, H₂O, CO₂ and HBr
Hints: Molecules having permanent dipole moment whose vibrational mode is associated by change in dipole moment will show absorption in IR region. So, HCl, H₂O, CO₂ and HBr will show absorption in IR region because their dipole moment changed during vibration.

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