M.Sc Organic Chemistry,Infra-red Spectroscopy

, IR SPECTROSCOPY

INTRODUCTION

—Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is utilized to confirm  the functional groups.

—The absorption of Infra-red radiation causes the various band in a molecule to stretch and bend with respect to one another

—The infrared region of the spectrum extends radiation with wave numbers ranging from about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200µ.

—Infrared region lies between visible and microwave region.

PRINCIPLE OF IR SPECTROSCOPY

•All the bonds in the molecule are not capable of absorbing Infra-red energy but only those bonds which are accompanied by change in dipole moment will absorb in the infra red region.

•Vibrational transitions which are accompanied by a change in dipole

 moment  of the molecule are called infrared active transitions. Thus, these are responsible for absorption of energy in the infra-red region  eg. Vibrational transitions of C=O, N-H, O-H etc.

•Transitions in Carbon-Carbon  bonds in symmetrical alkenes and alkynes are not accompanied by the by change in dipole moment and hence do not absorb in the infra-red region 

—A molecule can vibrate in many ways, and each way is called a vibrational mode.

—If a molecule contains ‘N’ atoms,

   then total number of vibrational modes

ØFor linear molecule it is (3N-5)

ØFor non linear molecule it is (3N-6)

—Eg: H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

MOLECULAR VIBRATIONS

—There are 2 types of vibrations.

1)Stretching vibrations

2)Bending vibrations

1)Stretching vibrations:  In these type of vibration,the distance between the two atoms increases or decreases with respect to central atom but  the atoms remain in the same bond axis.

a) symmetrical stretching:

Two bonds increase or decrease in length with respect to particular (central) atom  in the same direction.

b) Asymmetrical stretching: 
    In this one bond length is increased and other is decreased. i.e. one atom approaches the central atom 
    while the other departs from it   
 2)Bending vibrations: During the vibration the distance between remain constant but position of atoms changes relative to the original bond axis. In this case bond angle is altered.

•These are also called as deformations.

•These are of 2 types

•a) In plane bending→ scissoring, rocking

•b) Out plane bending→ wagging, twisting

—Scissoring: —This is an in plane bending. In this case bond angles are decreased. Two atoms approach each

                 other with respect to central atom.

Rocking: In this movement of the two atoms takes place in same direction with respect to central atom.

Wagging:—It is an out of plane bending. In this two atoms move to one side of the plane.They move up and 

              down the plane with respect to central atom.

Twisting: In this one atom moves above the plane and the other atom moves below the plane with respect to 

              central atom.