Define whether the following vibrational modes of CO2 and NO2 are IR active or not. A weak signal is considered active: Symmetric stretch: 0=C=0 Asymmetric stretch: 0-C=0 In-plane bending: 0 Out-of-plane bending: 0
The Correct Answer and Explanation is:
To determine whether vibrational modes of CO₂ and NO₂⁻ are IR active, we apply the principle that a vibrational mode is IR active if it results in a change in the dipole moment of the molecule. Here’s the correct classification for each vibrational mode shown in the image:
CO₂ (Carbon Dioxide)
- Symmetric Stretch:IR Inactive
- In this mode, both O atoms move simultaneously toward and away from the C atom.
- Since CO₂ is a linear and symmetric molecule, this motion does not change the dipole moment (which remains zero), so it is IR inactive.
- Asymmetric Stretch:IR Active
- In this mode, one O atom moves toward the C while the other moves away.
- This creates a temporary dipole moment as the charge distribution becomes asymmetric. Therefore, it is IR active.
- In-Plane Bending:IR Active
- The O atoms move perpendicular to the linear axis of the molecule.
- This bending distorts the symmetry and creates a change in dipole moment, making it IR active.
NO₂⁻ (Nitrite Ion)
- Symmetric Stretch:IR Active (weak)
- Though the molecule is bent and not linear like CO₂, the symmetric stretch causes a slight dipole moment fluctuation.
- Hence, it is weakly IR active.
- Asymmetric Stretch:IR Active
- The two N–O bonds stretch unequally, leading to a considerable change in dipole moment.
- This motion is strongly IR active.
- In-Plane Bending:IR Active
- Similar to CO₂, the bending motion in NO₂⁻ changes the dipole moment, making it IR active.
Summary Table:
| Vibrational Mode | CO₂ IR Activity | NO₂⁻ IR Activity |
|---|---|---|
| Symmetric Stretch | No | Yes (weak) |
| Asymmetric Stretch | Yes | Yes |
| In-Plane Bending | Yes | Yes |
These assignments reflect how molecular symmetry and changes in dipole moment determine IR activity.
