INFRARED SPECTROSCOPY Dr. R. P. Chavan Head, Department of Chemistry

CONTENTS INTRODUCTION FUNDAMENTAL VIBRATIONS OVERTONE COMBINATION BAND VIBRATIONAL COUPLING IMPORTANT GROUP FREQUENCIES FOR THE COMMON FUNCTIONAL GROUPS REFERENCES

INTRODUCTION An infrared spectroscopy(IR spectroscopy or Vibrational spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals.

For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an infrared spectrometer or spectrophotometer to produce an infrared spectrum. A basic IR spectrum is essentially a graph of infrared light absorbance or transmittance on the vertical axis v/s frequency or wavelength on the horizontal axis. Typical units of frequency used in IR spectra are reciprocal centimeters are also called wave numbers

FUNDAMENTAL VIBRATIONS The fundamental vibrations correspond in quantum treatment to the first vibrational transition i.e from the zeroth vibrational level to the first vₒ → v₁ Two kinds of fundamental vibrations are : Streching vibrations Bending vibrations

Streching vibrations : In this type, the vibrations between the two atoms increases or decreases but the atom remains in the same bond axis. Two types of streching vibrations : Symmetrical streching : In this type the movement of the atoms with respect to a particular atom in a molecule is in the same direction. Asymmetrical streching : In these vibrations one atom approaches the central atom while the other departs from it.

2) Bending vibrations : In this type of vibrations the positions of the atoms change with respect to the original bond axis. We know that more energy is required to strech a spring than that required to bend it. Thus, we can say that streching absorptions of a band appear at high frequencies (higher energy) as compared to the bending absorption to the same band.

Four types of bending vibrations : Scissoring : In this type two atoms approaches each other. Rocking : In this type, the movement of the atoms take place in the same direction. Wagging : Two atoms move up to below the plane with respect to the central atom. Twisting : In this type, one of the atoms move up the plane while the other moves down the plane with respect to the central.

The IR Spectroscopic Process There are two types of bond vibration: IR Spectroscopy Introduction The IR Spectroscopic Process There are two types of bond vibration: Stretch – Vibration or oscillation along the line of the bond Bend – Vibration or oscillation not along the line of the bond H C H C symmetric asymmetric H C H C H C H C twist wag scissor rock in plane out of plane

The IR Spectroscopic Process Infrared Spectroscopy The IR Spectroscopic Process As a covalent bond oscillates – due to the oscillation of the dipole of the molecule – a varying electromagnetic field is produced The greater the dipole moment change through the vibration, the more intense the EM field that is generated

No of fundamental vibration A)For nonlinear molecules (3n-6) Example CH4 = 3*5-6=9 so, no of fundamental vibrations are 9 C6H6 =3*12-6=30 so, no of fundamental vibrations are 30 B) For linear molecules (3n-5) Example CO2 = 3*3-5=4 so, no of fundamental vibrations are 4 CS2 =3*3-5=4 so, no of fundamental vibrations are 4

The region below 1500 cm-1 is known as finger print region Fermi Resonance It is an interaction which can occurs between overtone band and the fundamental vibration and it is known as Fermi resonance Example If fundamental frequency of –OH is 3300 cm-1 and the first overtone of carbonyl is 3400 cm-1 then these bands may overlap with one another and the net result is that the resonance intensity of two bands increases or decrease Finger Print Region The region below 1500 cm-1 is known as finger print region Approx. 1200 cm-1 to 667 cm-1 This region is important for the identification of two compound If the finger print region of two compound are identical then the compound must be the same

IMPORTANT GROUP FREQUENCIES FOR THE COMMON FUNCTIONAL GROUPS Alkanes : Methyl and methylene (-CH₂) group generally give strong streching absorption band at 2850- 3000 cm⁻¹. Symmetrical bending vibration band for –CH₃ groups occur near 1370 - 1380 cm⁻¹. Asymmetrical bending vibration of -CH₃ groups and also –CH₂ gives at 1450 – 1470 cm⁻¹.

2) Alkenes : ≡ C – H bending vibration band at 700–1000cm⁻¹ ≡ C – H streching vibration band at 3020-3240cm⁻¹ 3) Alkyl halides : C - F at 1100 – 1000 cm⁻¹ C – Cl at 750 – 700 cm⁻¹ C – Br at 600 – 500 cm⁻¹ C – I at 500 cm⁻¹

4) Alcohol : -OH Streching vibration strong band at 3200- 3600cm⁻¹. C-O streching vibration band at 1000 - 1200cm⁻¹. 5) Aldehydes and ketones : A strong absorption band due to C=O streching absorption in the region around 1100 cm⁻¹ which indicates presence of aldehyde, ketone, carboxylic acid, ester, amide, anhydride. Cyclic ketone, absorption region is 1725- 1700cm⁻¹.

THANK YOU