Infrared Spectroscopy (CIE A Level Chemistry)

• Infrared (IR) spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies
• A spectrophotometer irradiates the sample with electromagnetic waves in the infrared region and then detects the intensity of the wavelength of IR radiation which goes through the sample
• All organic molecules absorb IR radiation and depending on which energies of radiation are absorbed, bonds between atoms will vibrate by stretching, bending and twisting
• The molecules will only vibrate at a specific frequency
• The resonance frequency is the specific frequency at which the molecules will vibrate to stimulate larger vibrations
• Depending on the rest of the molecule, each vibration will absorb specific wavelengths of IR radiation which are also shown as the reciprocal of the wavelength
  • This unit is called the wavenumber (cm^-1)
• Particular absorbances have characteristic widths (broad or sharp) and intensities (strong or weak)
  • For example, hydrogen bonds cause the O-H bonds in alcohols and carboxylic acids to be broad whereas the C-O bond in carbonyl (C=O) groups have a strong, sharp absorbance peak
• The energies absorbed by different functional groups are given as a range and an unknown compound can be identified by comparing its IR spectrum to the IR spectrum of a known compound

Absorption range of bonds table

Bond	Functional groups containing the bond	Characteristic infrared absorption range (in wavenumbers) / cm-1
C-O	Hydroxy, ester	1040 - 1300
C–C	Aromatic compound, alkene	1500 - 1680
C=O	Amide Carbonyl, carboxyl Ester	1640 - 1690 1670 - 1740 1710 - 1750
CN	Nitrile	2200 - 2250
C–H	Alkane	2850 - 2950
N–H	Amine, amide	3300 - 3500
O–H	Carboxyl Hydroxyl	2500 - 3000 3200 - 3600

• 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