Infrared Spectroscopy (Cambridge (CIE) AS Chemistry): Revision Note

Interpreting IR Spectra

• 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

• A spectrophotometer 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 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

Absorption range of bonds table

• Due to some absorption bands overlapping each other, other analytical techniques such as mass spectroscopy should be used alongside IR spectroscopy to identify an unknown compound