spectroscopy - Is carbon dioxide IR inactive?

I am told that carbon dioxide is IR inactive.

This somehow strikes me as untrue or at best oversimplified, because yes, it is overall symmetrical, but can't $\ce{CO2}$ be induced to show asymmetric stretching patterns?

So this leads me to the question: in a simplified presentation of IR, why might someone present symmetrical, non-polar molecules as IR "inactive"? Could we be relying on a simpler definition of IR inactive?

Answer

I am told that carbon dioxide is IR inactive.

You're right, that's not true. Since carbon dioxide is linear it has $3n-5 = 4$ vibrations and they are pictured below.

• The symmetric stretch does not result in a change (of the initially zero dipole moment), so it is ir-inactive.


• The asymmetric stretch does result in a change in dipole moment so it is ir-active.


• The bend also results in a change in dipole moment so it too is ir-active.

We expected 4 vibrations and I've only listed 3. This is because the "bend" (let's start by placing the molecule along the x-axis) can occur in the y direction and the z direction. But these two motions are the same, just deforming in different directions, the bend is said to be degenerate, accounting for the "fourth" vibration.

To sum up, carbon dioxide has 2 ir-active vibrations.

Edit - response to example added (question d) by OP

Question d is incorrect. Either the author 1) inadvertently switched the column headings (IR active, IR inactive) or 2) meant to use some molecule other than carbon dioxide.

The first 3 rules you learn for interpreting IR and Raman spectra are

• The number of molecular vibrational modes equals 3n-6 (3n-5 for linear molecules), where n is the number of atoms.


• An ir active band will be observed if a vibration results in a change of the dipole moment. The initial dipole moment in the molecule's equilibrium geometry can be zero; all you need is a change. The terms "polar" and "non-polar" can be confusing, they often mean different things to different people. Leave "polar" out of the criteria for ir activity and stick with dipole moment, it is a much better understood term.


• the rule of mutual exclusion, it states that, for centrosymmetric molecules (molecules with a center of symmetry, like carbon dioxide), vibrations that are IR active are Raman inactive, and vice versa. So for carbon dioxide there is 1 Raman band and two IR bands.

Here's a link to a recent SE Chem question: How can I deduce the linearity of XeF2 from the IR spectrum? where these rules were used to determine the structure of a molecule.