Ir Spectroscopy

IR spectroscopy Infra-red region of electromagnetic wave is used to analyze organic compound. Hence it is called infrared spectroscopy. The visible light is made up of a continuous range of different electromagnetic frequencies - each frequency can be seen as a different color. Infra-red radiation also consists of a continuous range of frequencies - but our eyes can't detect them. If infra-red frequencies radiations are incident one at a time on a sample of an organic compound, some frequencies get absorbed by the compound. A detector on the other side of the compound would show that some frequencies pass through the compound, but other frequencies are strongly absorbed.

The spectrum shows the % of transmittance which means % of energy which gets through the compound. A percentage transmittance of 100 would mean that all of that frequency passed straight through the compound without any being absorbed. In practice, that never happens - there is always some small loss, giving a transmittance of perhaps 95% as the best you can achieve.

Frequency is shown on the horizontal axis in the form of Wavenumber which is defined as Wave number =

1/ frequency in cms

The infrared radiation carries energy. Absorption of frequency means absorption of energy. The energy absorbed is transmitted to compound. The range of energy carries by infrared radiation is same as covalent bond energy. Hence infra red energy absorbed by the compound directly reaches to covalent bond of the compound. When the bond receives energy the atoms forming the bond start vibrating. The vibration is possible side ways, vertically, horizontally, angularly axially or in any other manner

The energy involved in this vibration depends upon length of the bond, angle of the bond, mass of the atoms and other bond characteristics. That means that each different bond will vibrate in a different way, involving different amounts of energy and so each different bond will absorb a different frequency (and hence energy) of infra-red radiation. The infra-red spectrum of propan-1-ol, CH3CH2CH2OH is shown

The spectrum shows different pattern of absorption by different bond. In the diagram, three sample absorptions are picked out to show the bond vibrations which produced them. Notice that bond stretching and bending produce different troughs in the spectrum. Thus spectrum can be used to detect the type of bond present in the compound. It also can be used to detect different function group present in the compound. If a bond does not vibrate at all in the given range of frequency no energy will be absorbed and no spectrum is produced but this is less likely as some covalent character is always is present in the compound.

Some of the function group spectrum is given below for ready reference.

An IR spectrum will help in identifying the function group present in the compound but does not give shape size, or molecular mass of the compound. However it does a unique spectrum. Two compounds will never have the same IR spectrum. IR spectrum of an unknown compound can be always matched to spectrum available in data bank and unknown compound can be detected. Therefore IR spectrum is also called finger print of the compound. The most of the compounds are detected in the frequency region of 600 to 1400 cm -1. This region is therefore is also called as finger print region of IR spectrum. *******************************************************************************