, 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.
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.
