Principle of IR Spectroscopy

• IR spectroscopy detects the absorption of light by a compound in the IR region of the electromagnetic radiation (spectroscopy).
• To absorb light a molecule must have a bond within its structure that can exhibit dipole moment which means electrons within a bond are not shared equally.

                                                     

• Electrons within the O-H bond are not equally shared meaning that the electrons are more closely attracted to the oxygen than the hydrogen. As the electrons are negatively charged, this leads to the O bond bearing a slight negative charge and H a slight positive charge (as it is seen in the above structure).                                                                                                                                              Due to this charge difference IR light is absorbed by this particular bond, and as a result we get a peak within an IR spectrum.
• Functional groups have different bonds which possess dipole moment , absorbs at different areas of the IR spectrum.
• IR spectrometer consist of a lamp or heated rod that will emit light in IR region. Spectrometer have detector which collects all wavelengths of IR radiation that have passed through the sample and converts these to wavenumbers. Each chemically distinct molecule will have a different absorption pattern made up from the number and different types of bond presence of differing functional groups. 
• In IR spectroscopy, a sample is irradiated with infrared light and the transmitted or reflected light is measured, allowing structural analysis and quantification. Absorbance analysis involves measuring the amount of absorption of light by the molecules in a sample, which is wavelength dependent.
• In a molecule, interatomic chemical bonds can stretch as if they were springs. The energy associated with such stretching vibrations is close to that of infrared light, so that molecules absorb infrared light and vibrate. 
• The vibrations that can be induced by IR light are limited to those involving a change in dipole moment.                                                                                                                                                       Example- Carbon dioxide (CO2) which is a linear molecule, does not absorb infrared light because does not have dipole moment while H2O (a non-linear molecule) possess dipole moment so it absorbs infrared light in IR region.