Infrared Spectroscopy (AQA A Level Chemistry): Revision Note

Interpreting an IR spectrum

• Infrared (IR) spectroscopy is a technique used to identify compounds

  • It works by measuring how covalent bonds in molecules absorb infrared radiation

  • This absorption causes the bonds to vibrate in specific ways

• An IR spectrometer irradiates the sample with infrared waves

  • Then, it detects how much radiation is absorbed at each frequency

  • The resulting spectrum shows which types of bonds are present

Bond vibrations and absorption

• All covalent bonds behave like tiny springs rather than rigid bars

• Like springs, they can vibrate in different ways, such as stretching and bending

• These vibrations occur at specific frequencies in the infrared (IR) region of the electromagnetic spectrum

• When IR radiation matches a bond’s natural frequency, the bond absorbs energy and vibrates more strongly

• Each vibration mode (e.g., symmetric stretch, bending) has its own characteristic frequency

• These absorbed frequencies are expressed as wavenumbers (cm⁻¹), which are the reciprocal of wavelength

Absorption patterns and functional groups

• Each organic compound has a unique IR spectrum

  • Particularly in the region below 1500 cm⁻¹, known as the fingerprint region

• This region contains many small peaks caused by complex bond vibrations that are difficult to assign to specific groups

• However, comparing the fingerprint region to spectra from a database allows the exact identification of a compound

• This is useful, for example, for identifying a particular isomer in a homologous series

  • They will all have the same functional groups but different fingerprint regions

• Each bond type absorbs within a characteristic wavenumber range

• Absorptions vary in width (broad or sharp) and intensity (strong or weak)

• For example:

  • O–H bonds in alcohols and carboxylic acids give broad peaks due to hydrogen bonding

  • C=O bonds in carbonyl compounds give sharp, strong peaks.

• Comparing an unknown IR spectrum with data for known compounds helps identify functional groups

• To tell alcohol vs carboxylic acid vs aldehyde/ketone apart:

  • Carboxylic acid — broad O–H at 2500–3000 cm⁻¹ AND a C=O at 1680–1750 cm⁻¹

  • Alcohol — O–H at 3230–3550 cm⁻¹ and NO C=O

  • Aldehyde/ketone — C=O at 1680–1750 cm⁻¹ and NO O–H.

Typical IR absorption ranges (from AQA Data Sheet)

Additional uses of IR spectra

• Some absorption bands overlap, so IR spectroscopy is often combined with other techniques (like mass spectrometry) to confirm the identity of a chemical

• IR spectra can also reveal impurities

• Unexpected peaks in the spectrum may indicate:

  • Contamination

  • Unreacted materials

• For example, a pure compound showing only a C=O peak might also show an O–H peak

  • This suggests the presence of an alcohol impurity

• This is especially useful after synthesis or purification.

  • It can confirm the absence of contaminants, byproducts, and unreacted material

IR radiation and global warming

• The Earth absorbs short-wavelength ultraviolet radiation from the sun

• It then re-emits this energy as long-wavelength infrared radiation

• Greenhouse gases like carbon dioxide, methane, and water vapour absorb this infrared radiation

  • This traps heat in the atmosphere

• These gases absorb (and re-emit) IR energy and help maintain the Earth's temperature

  • This is known as the greenhouse effect.

• Without greenhouse gases, Earth would be too cold to support life

Bond vibrations and greenhouse gases

• Just like in IR spectroscopy, these gases absorb IR radiation through bond vibrations:

  • CO₂ has two C=O bonds that absorb IR

  • CH₄ has four C–H bonds that absorb IR

  • H₂O has two O–H bonds that absorb IR

• Human activity is increasing the levels of greenhouse gases

• The absorption of infrared radiation by these gases is a key cause of global warming

The greenhouse effect and global warming

Related topics

• Mass Spectrometry

• Identification of Functional Groups by Test-Tube Reactions