2.1 INTRODUCTION 
Infrared spectroscopy: intarection between ELECTRO MAGNETIC IR LIGHT WITH MATTER IS CALLED IR SPECTROSCOPY 




➤ IR spectroscopy is one of the most widely useful tools for chemist for the detection of functional group 
in a molecule.


➤ IR radiation is type of electromagnetic radiation which is between UV-visible and microwave region of 
EMR spectrum.
It can be divided in three parts: 
Near-infrared = 14000-4000 cm-1
Mid-infrared = 4000 -400 cm-1
Far-infrared= 400 -10 cm-1
When any molecule absorbs IR radiation, it undergoes vibrational and rotational change. 
IR spectra are usually recorded as graph of wavelength or wave number vs % transmittance.

2.2 ORIGIN (THEORY OR PRINCIPLE) OF IR SPECTRA
When any molecule absorbs IR radiation, it undergoes vibrational and rotational change.
➤ Transitions in vibrational and rotational energy levels give IR spectrum. 

➤ When a molecule absorbs IR radiation below 100 cm-1,  transitions in its rotational energy levels occur. When a molecule absorbs IR radiation in the range 100-10,000 cm-1,  transitions in its rotational energy levels occur. These energy levels are also quantized, so we get vibratonal spectra of bands.

A single transition in vibrational energy levels is accompanied by a large number of transitions in
rotational energy levels. 



➤ So we get the spectra of vibrational-rotational bands instead of lines. Study of bands in the middle IR region of 4000-400 cm-1

(2.5-15 μ) is important.


➤ If we consider different atoms in a molecule as balls of different masses and if we consider the
covalent bonds between them as weightless tiny springs, then atoms in a molecule are not fixed but they vibrate.