IR Spectroscopy
Chemistry: Cambridge International A Level, topic 3.9.4
Infrared Spectroscopy
Infrared (IR) spectroscopy is used to identify certain bonds within a compound. This means it's pretty useful in distinguishing between different functional groups.
Bonds and infrared
- Infrared radiation is absorbed by any polar bond. Non-polar bonds do not absorb radiation.
- Polar bonds only absorb radiation of specific frequencies.
- The absorbed infrared radiation makes the bonds vibrate.
- The frequency absorbed is specific to each bond.
- Different bonds in different places can have different frequencies.
- E.g. An alcohol OH is different to a carboxylic acid OH.
Common frequencies
- The frequency is normally reported as a wavenumber.
- A wavenumber is , where λ is the wavelength.
- The image above shows the common frequencies of several different bonds. You don’t need to learn them, as they’ll be supplied in an exam.
IR spectrum
- Above is a graph showing the IR spectrum of ethanoic acid.
Interpreting spectra
- The section from 1,500 to 500 cm-1 is called the fingerprint region. This is usually full of small peaks and is specific to any molecule.
- There are large databases of fingerprint regions and you can search for a match to identify your compound.
- If you don’t have access to a database, most helpful bonds are outside of this region.
- E.g. The previous slide showed ethanoic acid, which has a peak at 3,000, because of a carboxylic acid OH, and one at 1,720, because of a CO double bond.
Test yourself on IR Spectroscopy
What happens when bonds absorb infrared radiation?
Amines and amides both contain a C-N bond. Do they absorb the same frequency of infrared radiation?
On a spectrum, what's the absorbed frequency actually reported as?