• 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 stimular 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 absorbance 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

• 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

Worked Example: Analysing IR spectra

Answer

IR spectrum?A?is?propanone?and spectrum?B?is?propan-2-ol.

In IR spectrum?A?the presence of a strong, sharp absorption around 1710 cm^-1?corresponds to the characteristic C=O, carbonyl, group in a ketone.

In spectrum?B?the presence of a strong, broad absorption around 3200-3500 cm^-1?suggests that there is an alcohol group present, which corresponds to the -OH group in propan-2-ol.