INFRARED SPECTROSCOPY (Conditions, Force Constant)

1. Prepared By
Shanta Majumder
Session: 2016-17
Department of Chemistry
Comilla University
1/6/20191

2. Presentation On
IR Spectroscopy
1/6/20192

3. Presented Topics
• Conditions for IR
Spectroscopy
• Bond Stretching
Frequency
• Variation of Force
Constant Values
1/6/20193

4. 1/6/20194
Conditions for IR Spectroscopy
• Molecule must be polar
• Molecule must be asymmetric
• Molecules must have stable or changeable
dipole moment

5. 1/6/20195
Bond Stretching Frequency
The value of stretching vibrational frequency of a bond can be calculated
by the application of Hooke’s law which may be represented as:
ῡ =
1
2𝜋𝑐
𝑘
𝜇
Here,
𝑘= Force Constant
𝑐= Velocity of Light= 3× 108
cm/sec
𝜇= Reduced Mass=
𝑚1 𝑚2
𝑚1+𝑚2
; 𝑚1 & 𝑚2 are the masses of atoms in grams.

6. 1/6/20196
Calculation of Bond Stretching Frequency
By using the masses in amu units and simplifying the equation of ῡ, we get:
ῡ=4.12 𝑘
𝜇
From the above equation, the bond frequencies for different types of bonds. Some
examples are given below:
Bonds Values ῡ=𝟒. 𝟏𝟐 𝒌
𝝁 (𝒄𝒎−𝟏
)
C-H Bond 𝑘=5 × 105
(for single bonds)
𝜇= 0.923
3032 𝑐𝑚−1
(Calculated)
3000 𝑐𝑚−1
(Experimental)
C-D Bond 𝑘=5 × 105
(for single bonds)
𝜇= 1.71
2228 𝑐𝑚−1
(Calculated)
2206 𝑐𝑚−1
(Experimental)
C=C Bond 𝑘=10 × 105
(for single bonds)
𝜇= 6
1682 𝑐𝑚−1
(Calculated)
1650 𝑐𝑚−1
(Experimental)
C=O Bond 𝑘=10 × 105
(for single bonds)
𝜇= 6.857
1573 𝑐𝑚−1
(Calculated)

7. 1/6/20197
Variation of Force Constant Values
Force constant varies with:
• Strength of the bond
• Mass of the atoms
• Hybridization
• Resonance of the bonds

8. 1/6/20198
Strength of the Bonds
Strong
Bond
Larger k
values
Higher ῡ
values
C=C C=C C−C
2150 𝑐𝑚−1 1650 𝑐𝑚−1
1200 𝑐𝑚−1
Increasing k Values

9. 1/6/20199
Mass of the Atoms Bonded to Carbon
Increased
Mass
Reduced
Mass
Increases
k Value
Decreases
Lower ῡ
Values
C-H C-C C-O C-Cl C-Br C-I
3000𝑐𝑚−1
1200𝑐𝑚−1
1100𝑐𝑚−1
750𝑐𝑚−1
600𝑐𝑚−1
500𝑐𝑚−1
Increasing k Values

10. 1/6/201910
Hybridization
s-character
increases
Bond
strength
increases
Higher k
values
𝑠𝑝 𝑠𝑝2
𝑠𝑝3
=C−H =C−H −C−H
3300𝑐𝑚−1 3100𝑐𝑚−1
2900𝑐𝑚−1
Increasing k Values
Higher ῡ
values

11. 1/6/201911
Resonance
Resonance
increases
Bond
strength
decreases
k value
decreases
ῡ values
also
decreases
1715𝑐𝑚−1
1675 𝑐𝑚−1

12. 1/6/201912
THANKS
TO
ALL