FT IR Near IR IR Far IR 12500 4000 667 50 PRINCIPLE The molecules absorption of infra red radiation to excitation of lower to higher vibrational level Each vibrations level contains number of closed vibrational levels So infrared is called as vibrational rotational spectra All the bonds not capable of absorb the IR region the molecule containing the dipole moment change that molecule only absorb the IR light EXAMPLE C O N H OH Etc V2 r r r v1 V0 THEORY OF MOLECULAR VIBRATIONS 1 Absorption in the IR region is due to the changes in vibrational and rotational levels 2 Operating frequency is less than 100cm 1 are absorbed molecules containing rotational level only The absorption is quantized a separate lives are formed due to molecular rotation 3 Apply the more energy 10 4 10 2 cm 1 passed through the sample it quantized the clear strong single vibrational energy changes change large number of rotational energy changes These vibrational spectra appear as vibrational rotational bands 4 Vibrational energy is depends upon i ii iii Mass of the atom present in a molecule Strength of the bond Arrangement of the atoms within the molecules MOLECULAR VIBRATIONS Stretching bending Scissoring Symmetric asymmetric rocking Wagging Twisting STRETCHING Distance between the two atoms is increases or decreases but the bond axis is not changed Two types of stretching are there 1 Symmetrical stretching 2 Asymmetrical stretching Symmetrical stretching Movement of the atoms with respect to a particular atom in the same direction Bond length Increases Bond length Decreases Asymmetric stretching In this vibration one atom moves to the central atom and another one is opposite to first one Bond length Bond length Bond length Bond length Decreases Increases Increases Decreases TYPES OF BENDING VIBRATIONS DEFORMATIONS 1 Scissoring 2 Rocking 3 Wagging 4 Twisting Two atoms are approaches each other Scissoring Rocking Two atoms are move to the same directions Or Wagging Two atoms are move up forward and down backward to the plane with respect to the central atom Twisting One atom moves up the plane another one another moves down the plane with respect to the central atom Bending vibrations are required lesser energy higher wavelength than stretching vibrations EXPRESSION FOR VIBRATIONAL FREQUENCY The value of the stretching frequency of a bond is calculated by Hooke s law 1 2 1 1 2 2 1 2 Where K force constant of the bond C velocity of radiation m1 m2 Mass of the concerned atom g SELECTION RULES ACTIVE FORBIDDEN VIBRATIONS The molecule containing atoms are dipolar movement is changed its active in Infrared spectrum Some fundamental vibrations are IR active and some are inactive The molecule has center of symmetry its vibrations are centrosymmetric its IR inactive and Raman active Example A C A The molecules has non centrosymmetric its IR active and Raman Inactive Example A C A In CO2 molecules Stretching modes O C O O C O Symmetrical stretching Asymmetric stretching Dipole moment is 0 Dipole is change IR inactive IR active Raman active Raman active 2350 cm 1 O C O O C O Bending modes Bending vibrations 667 cm 1 FACTORS AFFECTING VIBRATIONAL FREQUENCY Wave number of absorption is calculated by using Hooke s law The calculated frequency is never exactly equal to the experimental value The change is due to the structure of the molecules i Coupled vibrations and fermi resonance One stretching absorption frequency for an isolated C H bonds but in methylene CH2 group two absorption is observed its corresponding to symmetrical and asymmetrical stretching Symmetric Asymmetric Asymmetric stretching vibrations always higher wavenumber than symmetrical vibrations These vibrations occur at different frequencies than required C H stretching Similarly the coupled vibrations of CH3 group takes place at different frequency compared to CH2 group H H H H H H H H Symmetric asymmetric Example Acid anhydride two 0 stretching absorption between 1850 1800 and 1790 2745 cm 1 This can be explained due to the symmetrical and asymmetrical stretching FERMI RESONANCE In IR spectrum the molecule transfers the energy from fundamental to overtone level and its back again It s called as Fermi resonance Overtone level Absorb Release Photon Photon Fundamental level ii electronic effects The absorption frequency is shifts to another wavelength due to the electronic effects like inductive effects Mesomeric effects field effect etc In this effect change the force constant or bond strength its absorption frequency is shifts from a normal frequency value Example Introduction of I effect inducing alkyl group the bond length increases i e Weakening the force constant K decreases the absorption is decreases HCHO CH3CHO CH3COCH3 1750 cm 1 1745 cm 1 1715 cm 1 From left to right I effect increases Bond length increases Bond strength decreases Force constant decreases Absorption is Decreases Example 2 Introduction of I effect inducing halogen group the bond length decreases i e Strengthening the force constant increases the absorption is increases CH3COCH3 CH3COCH2Cl CH3COCHCl2 ClCH2COCHCl2 1715 cm 1 725 cm 1 1740 cm 1 1750 cm 1 From left to right I effect increases Bond length decreases Bond strength increases Force constant increases Absorption is Increases iv Hydrogen bonding Hydrogen bond is present in this molecule it affects the absorption frequency Stronger hydrogen bond is there the molecule has higher absorption occupy the lower number Intermolecular hydrogen bonding Intermolecular hydrogen bonds between the molecules are rise to broad peak Dilution of the solution its affection Example H F H F Intermolecular hydrogen bond Intramolecular hydrogen bonding Intramolecular within the molecule hydrogen bond is appeared as sharp and well defined Its is not affected by dilution F H O Example N H stretching 3500 cm 1 Dilute solution 3300 cm 1 H bonded O H group 3600 Free O H group 3350 iv Bond angle Less bond angle compounds are occupied in higher wave number C CO C O Bond angle 120 120 Low frequency high frequency INSTRUMENTATION OF IR SPECTRUM Motor spectrum A AMPLIFIER E F Detector D M1 Source sample M2 beam SAMPLE monochromator Source Organic compound analyzing IR instrument using Nernst glower it s a mixture of zirconium yttrium and erbium It s heated to 1500 to produce IR radiation Silicon carbide rod also electrically heated to produce IR Monochromator Optical prism grating quartz Sample container Sodium chloride or certain alkali metal halides are