CO2 is IR Active Explain | Vibrational Spectra of CO2 Molecule

Vibrational Spectra of CO₂ Molecule

CO₂ is IR Active, Explain.

Carbon dioxide is a linear molecule hence, the total number of fundamental vibrational modes is equal to four.

Number of vibrational modes for linear molecule = 3n − 5
Modes of CO₂ Molecule = (3 × 3) − 5 = 4

The CO₂ molecule has an overall net zero dipole moment in its equilibrium ground state configuration, but during specific modes of vibration, a dynamic change in dipole moment is observed. Symmetrical stretching does not create a transient dipole moment change because the symmetric pulling cancels out, making this specific mode IR inactive.

In contrast, the asymmetric stretching of the CO₂ molecule is highly IR active. The remaining two bending modes sharing the same frequency (𝜈₂) are degenerate because they differ exclusively in their spatial orientation plane.

However, since there is a definitive displacement change in the dipole moment during these bending paths, the molecule in this specific mode acts dynamically IR active and displays an absorption peak at 667 cm⁻¹. Thus, out of the four total theoretical vibrational modes of the CO₂ molecule, only three are active, and out of those three, two are degenerate. Consequently, in an experimental IR analysis spectrum, two fundamental signature bands are clearly resolved at 2349 cm⁻¹ and 667 cm⁻¹.

Exercise Question: Which of the following will give an IR spectra and why?

H₂, Cl₂, HCl, H₂O, CO₂ and HBr

Solution / Hint: Only molecules possessing a permanent dipole moment, or whose internal core vibrational transitions are directly accompanied by a dynamic shift in net dipole distribution symmetry, can absorb radiant energy within the IR spectrum band region. Therefore, HCl, H₂O, CO₂, and HBr will readily show clear absorption bands inside an IR spectrum because their atomic dipole matrices change value positions during active structural vibrations.