How To Read IR Spectroscopy (Example)

The chemical bonds between the individual atoms in molecules can be thought of as springs. These springs can absorb the energy from light which will cause them to move. By measuring what light gets absorbed by the sample we can infer what functional groups make up the sample. However, you need to combine IR Spectroscopy with other techniques such as NMR spectroscopy or mass spectrometry to gain the full picture!
The resulting IR spectrum looks like this. The transmittance, i.e. the percentage of light that passes through the sample is displayed on the Y-axis. The wavenumber, i.e. 1 divided by the wavelength of the light in centimetres is displayed on the X-axis. Each “dip”, also referred to as a peak, in the spectrum corresponds to a wavenumber at which light has been absorbed by the sample molecule as the light has not reached the detector which in turn causes this dip in the spectrum.
The region between 600-1400 cm-1 is called the fingerprint region, and tends to be complex with multiple bands that overlap each other. Above this region is where one can manually identify the presence of most functional groups. To do this, we need to use a correlation table. I’ve included a super basic one here so let’s use that for our first practice problem.
Here we can see an example of a IR spectrum. First thing we should do is cross out the fingerprint region to make our lives easier. Next we can look at where we find dips or peaks. The first peak is located around the 3000 mark. If we check that location from a correlation table, we observe that this corresponds to a CH group. This is immediately followed by a broad dip between 3200-3600 cm-1. If we look from the table this corresponds to an alcohol group. These 2 pieces of information alone are not enough to determine what molecule this is. But if we combine IR spectroscopy with NMR spectroscopy we can determine the functional groups in addition to its carbon skeleton and suddenly, determining the type of molecule we are dealing with becomes MUCH easier!
Sources:
Khan Academy: • IR spectra practice | ...
The Organic Chemistry Tutor: • IR Spectroscopy - Basi...
The Complete Guide to Everything: • Introduction to IR Spe...
ChemBAM: chembam.com/definitions/ir-sp...
LibreTexts: chem.libretexts.org/Ancillary...