Introduction, Coupling vibration, Requirements for effective coupling, References. coupling occurs in IR by stretching and bending vibration, symmetrical and asymmetrical stretching vibration.

1. Coupling Vibration in IR and its Significance
Department of Pharmaceutical Sciences Dr. Harisingh Gour Vishwavidyalaya,
Sagar M.P.
Presented by -
DauRam Chandravanshi
M. Pharm I sem.

2. Contents-
• Introduction
• Coupling vibration
• Requirements for effective coupling
• References

3. Introduction
• Strong coupling of stretching vibrations occurs when there is a common atom
between the two vibrating bonds. Coupling of bending vibrations occurs when there
is a common bond between vibrating groups.
• When two bond oscillators share a common atom, they rarely behave as individual
oscillators unless the individuals oscillation frequencies are widely different. This is
because mechanical coupling interaction between the oscillators. For example of CO2

4. Coupling vibrations
• An isolated C-H bond has only one stretching vibrational frequency where as
methylene group shows two stretching vibrations, symmetrical and
asymmetrical.
• Asymmetric vibrations always occur at higher frequencies or wavenumbers
than symmetric stretching vibrations. These are known as coupled vibrations.

5. • Since these vibrations occur at different frequencies than that required
for an isolated C-H stretching similarly, coupled vibrations of –CH3
group occur at different frequencies compared to -CH2 group.

6. • Sometimes it happens that two different vibrations levels have almost the
same energy. If such vibrations belongs to the same species (i.e. –CH2 or
–CH3 group), then a mutual perturbation of energy may occur, resulting
in the shift of one towards lower frequency while the other towards
higher frequency. It is also accompanied by a substantial increase in the
intensity of the respective bands.

7. The Requirements for effective coupling
interaction
1. The vibrations must be of the same symmetry species if interaction is to
occur.
OO C
• If no coupling occurs the absorption should be at = 1725 - 1705 cm-1
• The high absorption of C=O in CO2 comparing with C=O in carbonyl
compounds (C=O stretching vibrations in aliphatic ketone: 1725 - 1705 cm-1)
because of coupling between the two C=O bonds
' = 2330 cm-1

8. 2. Coupling between bending and stretching vibrations can occur the
stretching band forms one side of the changing angle that varies in the
bending vibrations.
bond
c
a bbond
• Coupling may occurs between c, a and b bending and c-a or b-a
stretching vibrations.

9. 3. Interaction is maximum when the coupled group absorb near the same
frequency.
4. Strong coupling between stretching vibration requires a common atom
between the groups.
5. A common bond necessary for coupling of bending vibrations.
bondbond
common bond

10. 5. If the groups are separated by the two or more bonds, little or no
interaction occur.
a
b
c
d
e
f
• No coupling between a-b and e-f. Coupling requires that the vibrations
be of the same symmetry species

11. Identification of compounds due to effect of coupling:
• The position of an absorption peak corresponding to a given organic
functional group varies due to coupling.
• These variations result from a coupling of the CO stretching with
adjacent CC stretching or C-H vibrations.
H
H
H
O H H
H
H
H
H
O H H
H
H
H
H
H
OH
H
H
HC C C C C C C
1034 cm-1 1053 cm-1 1105 cm-1

12. References
• Silverstein R. M., Webster F. X., Kiemle D. J. and Bryce D. L.
Spectrometric identification of organic compounds 8th edition 2016,
published by Willey India Pvt. Ltd. 74-75.
• Kaur H. Spectroscopy, publisher Pragati Prakashan 14th edition 2018,
139-140.
• Pavia Donald L. Lampman Gary M., Kriz George S. and Vyvyan James
R. Cengage Learning 5th edition 2015,