IR Spectra: Why is CO2 a doublet at 2350cm-1?

In summary, the presence of a doublet peak at 2350 cm-1 in CO2 in IR spectroscopy is due to a phenomenon called Fermi resonance. This occurs because CO2, despite being a linear molecule, has both IR-active symmetric and asymmetric stretches for its N-O bond. This is different from other molecules with linear bonds, which only have one IR-active stretch. This phenomenon is explained by chemistree as a result of symmetric and asymmetric stretching, and is a classical example of Fermi resonance.
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CO2 is supposed to create a "doublet" peak at 2350 cm-1 according to my IR correlation chart. Why is this doublet created? As DrDu explained in my previous post, two peaks are seen for the N-O bond because it has IR-active symmetric and asymmetric stretches. But CO2 is a linear molecule, and thus should not have an IR-active symmetric stretch.

I found another post on this forum in which chemistree explains the doublet as resulting from "symmetric and asymmetric" stretching, but that doesn't make sense to me considering the above. Does anyone know why the CO2 doublet exists? Thank you!
 
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FAQ: IR Spectra: Why is CO2 a doublet at 2350cm-1?

Why is CO2 a doublet at 2350cm-1?

CO2 is a doublet at 2350cm-1 because it has a linear molecular structure, meaning that the carbon atom is bonded to two oxygen atoms in a straight line. This results in two distinct vibrational modes for the molecule, known as symmetric and asymmetric stretching, which occur at slightly different frequencies. The doublet at 2350cm-1 corresponds to the asymmetric stretching mode.

What does the term "doublet" mean in IR spectra?

In IR spectra, a doublet refers to two closely spaced peaks that appear as a result of two distinct vibrational modes within a molecule. These modes can be caused by different types of molecular bonds or atomic motions, and their frequency difference is often very small (typically less than 10cm-1).

How does the doublet at 2350cm-1 in CO2 compare to other molecules?

The doublet at 2350cm-1 in CO2 is a unique feature that is not commonly seen in other molecules. This is because not many molecules have a linear molecular structure like CO2, which is required for the two distinct vibrational modes to occur. However, other linear molecules such as N2O and CS2 also exhibit similar doublets at similar frequencies.

Can the intensity of the doublet at 2350cm-1 be used to determine the concentration of CO2?

No, the intensity of the doublet at 2350cm-1 in CO2 cannot be used to accurately determine its concentration. While the intensity of IR bands can provide information about the concentration of a molecule in certain cases, the doublet at 2350cm-1 is not a strong enough feature for quantitative analysis.

Is the doublet at 2350cm-1 always present in the IR spectrum of CO2?

Yes, the doublet at 2350cm-1 is a characteristic feature of the IR spectrum of CO2 and will always be present. However, the intensity of the doublet may vary depending on factors such as the concentration of CO2 and the sensitivity of the IR instrument being used.

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