Infrared spectroscopy
Keywords: infrared, radiation,
absorption, spectroscopy
Learning Objectives
• State that absorption of infrared radiation causes
covalent bonds to vibrate.
• Identify absorption peaks in an infrared spectrum.
• State that modern breathalysers measure ethanol
levels by analysis using infrared spectroscopy.
All molecules absorb infrared radiation. This
absorbed energy makes bonds vibrate with
either a stretching or bending motion
Every bond vibrates at its own unique frequency
The amount of vibration depends on:
• The bond strength
• The bond length
• The mass of each atom involved in the bond
Most bonds vibrate at a frequency between 300
and 4000cm-1, in the infrared part of the
electromagnetic spectrum
The absorbed energies can be displayed as an IR
spectrum. By analysing this spectrum, we can
determine details about a compound’s chemical
structure. The spectrum indicates the presence of
functional groups in the compound.
A beam of infrared radiation is passed through a
sample of the material under investigation. The beam
contains the full range of frequencies present in the
infrared region. The molecule absorbs some of these
frequencies and the emerging beam is analysed to plot
a graph of transmittance against frequency. Frequency
is measured using wavenumbers with units cm-1
Schematic diagram of an infrared spectrometer
The molecule’s spectrum has a number of troughs
(called peaks). Each peak represents the absorbance of
energy from infrared radiation that causes the vibration
of a particular bond in the molecule under investigation
Applications of infrared spectroscopy
Infrared spectroscopy has many everyday uses:
• It is used extensively in forensic science (eg. To
analyse paint fragments from vehicles in hit-andrun offences)
• Monitoring the degree of unsaturation in polymers
• Quality control in perfume manufacture
• Drug analysis
• Testing the breath of suspected drunken drivers for
ethanol