Infra+red+Spectroscopy

=Infrared Spectroscopy (IR)=

One of the first scientists to observe infrared radiation was William Herschel in the early 19th century. He noticed that when he attempted to record the temperature of each colour in visible light, the area just beyond red light gave a marked increase in temperature compared to the visible colours. However it was only in the early 20th century that chemists started to take an interest in how infrared radiation interacted with matter and the first commercial infrared spectrometers were manufactured in the USA during the 1940s.

Interaction with matter
The bonds within molecules all vibrate at temperatures above absolute zero. There are several types of vibrations that cause absorptions in the infrared region. Probably the most simple to visualise are bending and stretching, examples of which are illustrated below using a molecule of water. If the vibration of these bonds result in the change of the molecule’s dipole moment then the molecule will absorb infrared energy at a frequency corresponding to the frequency of the bond’s natural vibration. This absorption of energy resulting in an increase in the amplitude of the vibrations is known as resonance.

An IR spectrometer detects how the absorption of a sample varies with wavenumber, cm-1, which is the reciprocal of the wavelength in cm (1/wavelength). The wavenumber is proportional to the energy or frequency of the vibration of the bonds in the molecule.

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Infrared Spectrometers
Infrared spectrometers work in a variety of ways but all of them pass infrared radiation across the full IR range through a sample. The sample can be a thin film of liquid between two plates, a solution held in special cells, a mull (or paste) between two plates or a solid mixed with potassium bromide and compressed into a disc. The cells or plates are made of sodium or potassium halides, as these do not absorb infrared radiation.

However the IR spectrometer ‘in the suitcase’ can use a technique called attenuated total reflectance (ATR), which allows IR spectra to be run on solid and liquid samples without any additional preparation.

Interpreting a spectrum
As molecules often contain a number of bonds, with many possible vibrations, an IR spectrum can have many absorptions. This can be helpful as it results in each molecule’s spectrum being unique. If the spectrum of a molecule has already been recorded on a database, any spectrum produced can be compared to that database to help identify the molecule. The spectrum can also be used to highlight specific bonds and hence functional groups within a molecule, to help determine its structure.

Look at this spectrum of ethanol, CH3CH2OH, to see which bonds are responsible for particular absorptions. Although IR is not able to provide enough information to find the exact structure of a ‘new’ molecule, in conjunction with other spectroscopic tools, such as NMR and mass spectrometry, IR can help provide valuable information to help piece together the overall structure.



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