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Infrared Spectroscopy

Chemistry: AQA A Level, topic 5.6.4

Infrared Spectroscopy

Infrared 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

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

Common frequencies

  • The frequency is normally reported as a wavenumber.
  • A wavenumber is 1λ\frac{1}{\lambda}, where λ is the wavelength.
Graph showing the IR spectrum of acetic acid

Graph showing the IR spectrum of acetic acid

Interpreting spectra

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 acetic 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.

Absorption of Infrared Radiation and Global Warming

Absorption of infrared radiation is great for identifying functional groups. It's also great for warming the planet!

The greenhouse effect

The greenhouse effect

  • Infrared (IR) radiation is useful for helping us identify compounds because it is absorbed by polar bonds.
  • Molecules in the atmosphere containing polar bonds also absorb IR radiation emitted by the earth.
  • E.g. carbon dioxide, water vapour, methane and CFCs.
  • Once these molecules have absorbed IR radiation, they re-radiate it in all directions, including back towards Earth.
  • This is called the greenhouse effect.
Global warming

Global warming

  • The greenhouse effect can be positive - it helps keep the Earth at a temperature that is suitable for life.
  • But human activities are causing a rise in greenhouse gases, which increases the strength of the greenhouse effect.
  • This leads to a rise in global temperatures - global warming.
  • Global warming is a big threat and can cause extreme weather, and flooding in low-lying areas.

Test yourself on Infrared 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?

  • Which of the following is NOT a greenhouse gas?

  • What type of radiation is absorbed and re-radiated in the greenhouse effect?

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