3. INTRODUCTION
An important method to determine the structure of
compound.
Its spectrum contains a large number of absorption
bands .
IR spectrum of a chemical substance is a fingerprint
for its identification.
Very near IR region 1.2-2.5µ or 1.2-2.5×10-4 cm
Near IR region 2.5-25µ or 2.5-25×10-4 cm
Far IR region 25-300µ or 25-300×10-4 cm
4. •IR spectra is plotted as percentage transmittance against wave
number
5. •Quite useful in predicting the presence of certain
functional groups
EXAMPLE:-
-OH group - 3600-3200 cm-1
carbonyl group of ketone - 1710 cm-1
6. PRINCIPLE OF IR
Molecule having electric dipole only absorb IR
radiation.
Opposite charges will exert force in opposite direction
thus tend to decrease separation . when these atoms
vibrate ,they absorb IR radiations.
The frequency or wavelength of absorption depends
on the relative masses of the atoms , the force
constants of the bonds & the geometry of the atoms.
The vibratory motion of the nuclei of diatomic
molecule may be similar to the vibration of a linear
harmonic oscillator.
7. • Hooke’s law is applicable:
m1 d2 r1 = -k (r –re ) (i)
dt2
m2 d2 r2 = -k ((r –re ) (ii)
dt2
k - proportionality constant or force constant & used to
measure stiffness of the bond.
r1 & r2 - positions of atoms 1 & 2 relative to the centre of gravity of
the molecule.
now, r1 =m2 r (iii)
m1 + m2
r2 = m1 r (iv)
m1 +m2
m1 & m2 - masses of 2 atoms of a vibrating diatomic molecule.
8. • on substituting eq.(iii) in (i)
m1 m2 d2 r = -k(r- re )
m1 +m2 dt2
re is a constant , its differentiation w.r.t. time will be zero
d2 r = d2 (r- re ) (v-a)
dt2 dt2
therefore, m1 m2 d2 (r- re ) = -k(r-re ) (vi)
m1 + m 2 dt2
put (r –re ) =x & m1 m2 = µ (vii)
m1 + m2
on substituting (vii) in(vi) , we get µ d2 x =-k x (viii)
dt2
or d2 x + k x = o or d2 x + w 2 x =0 (ix)
dt2 µ dt2
9. Where w2 =k or w= √k ; υ =ω = 1 √k
x x 2п 2п µ
therefore
υ = 1 √ k
2пc (m1 m 2/ m 1+m2)
WHERE υ = the vibrational frequency (cm-1 )
c = velocity of light (cms-1 )
k = force constant of bond(dyne cm-1 )
So, by the help of k & masses of different atoms ,the
vibrational frequency can be calculated .
Different values of vibrational frequency tells us different
absorption bands & different modes of vibration in a
molecule.
11. NORMAL MODES OF VIBRATION
Vibrations are of two types:-
[1] Stretching Vibrations:- further they are of two types
(a) Symmetric stretching vibration &
(b) Asymmetric stretching vibration
[2]Bending Vibrations:- These are of 4 types
(a) Scissoring
(b) Rocking
(c) Wagging &
(d) Twisting
12. SELECTION RULES
IR radiation is absorbed when the molecule have dipole
moment while Raman effect occurs when the molecule is
polarized therefore different selection rules.
In a molecule( with a centre of symmetry) those vibrations
which are symmetrical about the centre of symmetry are
inactive in IR but active in Raman spectra.
While those vibrations which are not centrosymmetric are
active in IR and usually inactive in Raman.
15. • RADIATION SOURCE:- Usually nearnst glower or globar at 1273-
2073 K is used as a source. It is non conducting at ambient
temperature therefore nichrome wire, carbon arc or mercury arc can
be used.
SAMPLE HEATING :- compound can be in vapour phase, as
pure liquids & in solid state. But these have to be treated
differently. The only common point to the sampling of
different phases is that the material containing the
sample must be transparent to IR radiation.
MONOCHROMATOR & OPTICAL MATERIAL :- The
separation of desired frequencies can be achieved by
means of monochromator. prisms & gratings are used for
this purpose.
16. • DETECTOR:-They change thermal radiant energy into
electrical energy.
Detectors are of two types:-
Selective and Non Selective
e.g. Photocells , Photographic cells , thermocouple ,
bolometer , pneumatic cells.
AMPLIFIER:- The radiant power is low for infrared region , it
means that detector signal will also be low. Therefore it is
essential to use AC amplifier.
17. APPLICATIONS OF IR
IDENTIFICATION OF ORGANIC COMPOUND:
established from its finger print region(1400-900cm-1 )
DISTINCTION BETWEEN TWO TYPES OF
HYDROGEN BONDING.
STUDY OF CHEMICAL REACTION.
STUDY OF KETO – ENOL TAUTOMERISM.
STUDY OF COMPLEX MOLECULES.
ROTATIONAL ISOMERISM.
DETECTION OF IMPURITY IN A COMPOUND.
