IR spectroscopy

In a nutshell

Infrared (IR) spectroscopy is an analytical technique which is used to identify a compound. Each bond has a unique vibration when exposed to IR, and this can be used to deduce functional groups present in a molecule. Molecules have a unique section of their spectra, called a fingerprint region which can identify the exact molecule present. 

​​What is infrared spectroscopy?

Infrared (IR) spectroscopy can be used to identify compounds in a sample. The principles of IR are based on the vibrations of atoms. Atoms are joined by a chemical bond that is always vibrating. When exposed to the infrared region of the electromagnetic spectrum, every bond has a unique frequency that it vibrates. 

The IR spectrometer

Infrared radiation is passed through a sample and the bond absorbs energy from the radiation and vibrates more. The bond will absorb radiation that has the same frequency as its natural frequency. This means that the radiation emerging from the sample will be missing the frequencies of the bonds that are present, hence the emerging spectra will show the bonds present.

Infrared spectra

The infrared spectrometer will plot a graph called an IR spectrum. This graph shows the transmittance (intensity of radiation) against the frequency of radiation. The frequency of radiation is shown as a wavenumber ($$cm^{-1}$$). 

Example

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As mentioned earlier, each bond has a unique frequency that it vibrates at so each peak represents a particular bond. This can be used to identify functional groups which are present in a molecule. The table below shows the frequencies of different bonds. 

It's important to note that the position of a bond can change the frequency. 

Example

The table below shows two examples of $$O-H$$​ bonds, alcohol and carboxylic acid. 

Using the above information the following information can be deduced from the spectrum.  

Fingerprint region

The region on the IR spectrum between $$500 \medspace cm^{-1}$$ and $$1500 \medspace cm^{-1}$$ is called the fingerprint region. This is a region which has peaks that are unique to a particular compound, like how a fingerprint is unique to each person.

Example 

The fingerprint region expected to be seen for molecules is kept on databases which can be accessed to confirm the identities of a particular molecule.

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Following a reaction using infrared spectroscopy

Comparing the IR spectrum of a molecule before and after a reaction can be used to deduce which reaction has taken place. 

Example 

For example, if you have a compound which contains an alcohol group and you oxidise it to an aldehyde, you can use the spectra to see if the reaction was successful. 

The first spectrum would have a peak at $$1000 - 1300 \medspace cm^{-1}$$ to show the $$O-H$$ alcohol group. The spectrum after the reaction should show a peak at $$1680 - 1750 \medspace cm^{-1}$$ to signify the presence of the carbonyl ($$C=O$$)  in an aldehyde.​

Combining techniques

Analytical techniques can be combined to identify a molecule. Mass spectrometry can be used to determine the molecular mass of a molecule. The functional group identification from the IR spectra can be combined with the fragments found in the mass spectra to determine the final structure of the molecule.