Calculating $CO_2$ Vibrational Modes
Carbon dioxide ($CO_2$) is a molecule with 3 atoms (N=3).
As a linear molecule, the total number of fundamental vibrational modes is calculated using the formula:
$ \text{Total Modes} = 3N - 5 $
Substituting N=3:
$ \text{Total Modes} = 3(3) - 5 = 9 - 5 = 4 $
So, $CO_2$ has 4 fundamental vibrational modes.
Identifying IR Active Vibrations
A vibrational mode is considered Infrared (IR) active if it results in a change in the molecule's dipole moment during the vibration.
The $CO_2$ molecule (O=C=O) is linear and symmetric. Its permanent dipole moment is zero.
Let's analyze the 4 vibrational modes:
- Symmetric Stretch: Both C=O bonds stretch or compress simultaneously. The symmetry is maintained, and the dipole moment remains zero. This mode is IR inactive.
- Asymmetric Stretch: One C=O bond stretches while the other compresses. This breaks the symmetry and induces a change in the dipole moment. This mode is IR active.
- Bending (Degenerate): The molecule bends in the plane or out of the plane. This breaks the symmetry, inducing a change in the dipole moment. These two bending modes are degenerate (have the same frequency) and are both IR active.
Conclusion on IR Active Modes
Out of the 4 total vibrational modes, 3 modes cause a change in the dipole moment:
- Asymmetric Stretch
- In-plane Bending
- Out-of-plane Bending
Therefore, the number of IR active vibrational normal modes for $CO_2$ is 3.
Important Questions from IR Spectroscopy
In an IR spectra of 1-octyne and 4-octyne, the IR-spectra of 4-octyne does not have C ≡ C stretch absorption peak. The reason for this observation is that _________.
An IR-spectra is found to have a medium adsorption peak near 3400cm-1. This corresponds to which organic compound?
The number of CO bands for isomers from sets (i) and (ii) in their IR spectra
Set (i): Trigonal bipyramidal isomers, axial‐Fe(CO)4L (A)and equatorial‐Fe(CO) 4 L(B)
Set (ii): Octahedral isomers, fac‐Mo(CO)3L3 (C) and mer‐Mo(CO)3L3(D)
are
Match List I with List II
List I
List II
functional groups
respective approximate symmetric and asymmetric stretching frequencies
A.
N - H bonds of R - NH2
I.
1790 and 1810
B.
N - O bonds of R - NO2
II.
3300 and 3400
C.
C = O bonds of anhydride
III.
1350 and 1550
Choose the correct answer from the options given below:
The molecule that can absorb in the infra-red among the following is
