Infrared Spectroscopy (Oxford AQA International A Level Chemistry)

Infrared Spectroscopy

• Covalent bonds, within molecules, act like springs and can vibrate in different ways

• Infrared spectroscopy identifies compounds based on changes in these vibrations

How does infrared spectroscopy work?

• A spectrophotometer passes infrared radiation (energy) through the sample

• Some energy is absorbed by the bonds

• Some energy is transmitted

• The transmitted energy is detected and produces a spectrum

  • The x-axis is wavenumbers, cm^-1, which relate to the frequency of the infrared energy absorbed and transmitted

  • The y-axis is transmittance, which can be expressed as a decimal or a percentage

Example infrared spectrum (ethanol)

• Particular bonds give characteristic peaks, for example:

  • O-H bonds in carboxylic acids and alcohols have a typical peak that appears as a wide / broad scoop

    • The position and shape can be used to distinguish between carboxylic acids and alcohols

  • C=O bonds in aldehydes and ketones have a typical peak that appears as a sharp / intense spike

Infrared data table

• Typically, it is not possible to identify an unknown compound using only infrared spectroscopy

• By referring to a data table, it can be used to:

  • Identify functional groups with molecules - this makes it possible to confirm the presence of a known compound in a sample

  • Detect impurities in samples - this is achieved by comparing a sample's spectrum with its known spectrum from a database

• To identify unknown compounds, infrared spectroscopy is used alongside other techniques such as:

  • Elemental analysis - to determine the empirical formula

  • Mass spectrometry - to determine the molecular mass and fragment ions from the whole molecule

  • (NMR spectroscopy is also included in the techniques but this is not covered as part of the AS course) 

Fingerprinting

• The region below about 1500 cm^-1 is called the fingerprint region and is unique to every molecule

• It has many peaks that can be difficult to assign

• These peaks represent the complex vibrational interactions that occur between different bonds within a molecule

• The value of the fingerprint region is in being able to compare the spectrum to a known compound from a database and coming up with an exact match

• This is particularly useful, for example, in identifying a specific member of a homologous series

  • All members of the series will show the same type of bonds present, but no two molecules will have the same fingerprint region

IR radiation and global warming

• Short wavelength ultraviolet radiation from the sun is absorbed by the Earth’s surface

• It is then re-emitted from the surface of the Earth as longer wavelength infrared radiation

• Much of this infrared radiation is trapped inside the Earth’s atmosphere by greenhouse gases such as carbon dioxide, methane and water vapour

• Greenhouse gases absorb and store the energy from this radiation to maintain the Earth's temperature

  • This is known as the greenhouse effect

  • Without these greenhouse gases, the temperature on Earth would not be able to support life

• Just like infrared spectroscopy, the bonds in carbon dioxide, methane and water are able to absorb infrared radiation

  • Carbon dioxide has two C=O bonds which can absorb infrared radiation

  • Methane has four C-H bonds which can absorb infrared radiation

  • Water has two O-H bonds which can absorb infrared radiation

• Human activities are increasing the levels of greenhouse gases in the atmosphere

• The bonds in greenhouse gases absorbing infrared radiation is one cause of global warming

The greenhouse effect and global warming