This presentation discuss about he factors affecting IR vibrational frequencies. It discuss about various factors such as Bond Order, Fermi resonance, Inductive effect etc..

2. FACTORS
AFFECTING
VIBRATIONAL
FREQUENCIES
BOND ORDER
INDUCTIVE AND
RESONANCE
EFFECT
HYDROGEN
BONDING
FERMI
RESONANCE
BOND ANGLE

3. BOND ORDER
 When Bond Order increases, IR vibration frequency
increases.
C ≡ C C = C C − C
Frequency 2150 cm⁻¹ 1650 cm⁻¹ 1200 cm⁻¹
( ῡ )
 When Bond Order increases, Bond Strength increases.
Force constant.
It measures Strength of the bond.

4. INDUCTIVE EFFECT AND RESONANCE EFFECT
INDUCTIVE EFFECT
 +I effect weaken the bond, hence frequency decreases.
O
‖
C
H H
O
‖
C
CH₃ H
O
‖
C
CH₃ CH₃
ῡ (C=O) 1750 cm⁻¹ 1745 cm⁻¹ 1715 cm⁻¹
 -I effect strengthen the bond, hence frequency increases.
O
‖
C
CH₃ CH₃
O
‖
C
CH₃ Cl
ῡ (C=O) 1715 cm⁻¹ 1800 cm⁻¹

5. RESONANCE EFFECT
 This effect causes weakening of a bond, hence absorption frequency
decreases.
Here the nitrogen atom is less electronegative than the oxygen
atom, Amide undergo conjugation in greater extent.
Alcohol Carboxylic acid
ῡ (C-O) - Pure single bond ῡ (C-O) – Partial double bond

6.  In most of the cases, resonance effect works along with inductive effect and
cannot be ignored.
 Only inductive effect is considered when the compound in meta
Substituted.
 In para substitution, both inductive and resonance effects become
important and the domination of one over the other will decide the
wavenumber of the absorption.
I & III – Resonance effect
II & IV – Inductive effect dominates over Resonance effect.

7. HYDROGEN BONDING
 Hydrogen bonding changes the position and shape of an infrared
absorption band.
 X-H stretching - Low Frequency (broad and high intense)
X-H bending - High Frequency
 Hydrogen Bonding – two types
a. Intermolecular
b. Intramolecular
 Intermolecular Hydrogen Bonding
i. Give rise to Broad band
ii. Concentration dependent.
 Intramolecular Hydrogen Bonding
i. Give rise to Sharp band
ii. Concentration independent.

8. The O-H stretching of a concentrated (hydrogen bonded) solution
of an alcohol occurs at about 3550 cm-1, whereas the O-H stretching
band of a dilute solution (with little or no hydrogen bonding) appears
at 3650 cm-1.

9. FERMI RESONANCE
This is resonance between fundamental band and adjacent
overtone. So that both are shifted from their original position. One
goes to higher frequency while other shifts to lower frequency. Their
intensity is almost same.
For eg,
CO₂
Symm Stretching - 1337 cm⁻¹
Bending Vib. - 667.3 cm⁻¹
It’s overtone = 2 . 667.3
= 1334.6 cm⁻¹
One fundamental vib. & other is
overtone shows resonance.
As a result, Symm stretching = 1285 cm⁻¹
Overtone = 1386 cm⁻¹
Intensity almost same
 As they have same frequency, they can resonate & exchange energy.

10. BOND ANGLES
 Increasing ring strain, increase the C=O stretching frequency.
 Decreasing ring strain , decreases the stretching frequency.