Types of Molecular Vibrations & Factors Influencing Vibrational Frequency in I.R. Spectroscopy

1. Types of Molecular Vibrations & Factors
Influencing Vibrational Frequency in
I.R. Spectroscopy
Presented by,
Mr. Ranjit Thavare
Roll No. 04
Class: First Year of M. Pharmacy
Department: Pharmaceutical Chemistry
Dattakala College of Pharmacy,
Chincholi, Daund.
INFRARED SPECTROSCOPY
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2. CONTENTS
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01. Types of Molecular Vibrations
02. Stretcing and Bending Vibrations
03. Stretching Vibrations
04. Bending Vibrations
05. Hook’s Law for Frequency Calculation
06. Factors Influencing Vibrational Frequency
07. References
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3. 01.Types of Molecular Vibrations
Molecular
Vibrations
Fundamental
Vibrations
Stretching
Vibrations
Asymmetric Symmetric
Bending
Vibrations
In Plane Bending
Scissoring Rocking
Out of Plane
Bending
Wagging Twisting
Non Fundamental
Vibrations
Overtones, Fermi
Resonance
Combination
Tones
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4. Stretching
Vibrations
Stretching vibration involves a
continuous change in the inter-
atomic distance along the axis
of the bond between two atoms.
In requires more energy so
appear at shorter wavelength
Bending
Vibrations
Bending vibrations are
characterized by a change in
the angle between two bonds
It requires less energy so
appear at longer
wavelength
02. Stretching and Bending Vibrations Vibrations
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5. Stretching
Vibrations
Asymmetric
Inter atomic
distance between
two atoms is
opposite
alternate/opposite
Symmetric
Inter atomic
distance between
two atoms is
increase/decreases
03. Stretching Vibrations Vibrations
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6. 04. Bending Vibrations
Bending
Vibrations
In Plane
Bending
Scissoring Rocking
Out of
Plane
Bending
Wagging Twisting
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7. 04. Bending Vibrations Cont.
❖ These are characterized by continuously changing bond angle and axis with common atom.
❖ These are of various types as (+ve indicates movement out of plane above and -ve indicates
movement back of plane)
❑ Scissoring vibrations occur when two atoms move back and forth towards each other.
❑ Rocking vibration occurs due to oscillation of atoms and forth out of equilibrium plane.
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8. 04. Bending Vibrations Cont.
❑ Wagging vibrations result when the unit oscillates in equilibrium plane-formed by the atoms.
❑ Twisting vibrations occur when structural unit rotates around the bond which joins to the molecule
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9. 05. Hook’s Law for Frequency Calculation.
❑ The probable frequency of absorption can be calculated by the Hook's law.
❑ Hook’s law: -
𝜗 =
1
2𝜋𝑐
𝑓
𝑀𝑥 𝑀𝑦 / 𝑀𝑥 +𝑀𝑦
Where,
𝜗 = The vibrational frequency (cm-1)
𝑐 = Velocity of light (cm/s)
𝑓 = force constant of the bond (dyne/cm)
𝑀𝑥 & 𝑀𝑦 = mass (g) of atom 𝑥 and atom 𝑦 respectively.
❑ However, it has been observed that the calculated value of frequency of absorption is not exactly
equal to the experimental value.
❑ There are many factors which are responsible for shifts in vibrational frequencies are as follows;
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10. 06. Factors Influencing Vibrational Frequency.
❑ Coupled vibrations
❑ Fermi-resonance
❑ Hydrogen bonding
❑ Electronic effects
❑ Coupled vibrations:
✓ An isolated C-H bond has only one stretching vibrational frequency where as
methylene group shows two stretching vibrations, symmetrical and asymmetrical.
✓ Because of mechanical coupling or interaction between C-H stretching vibrations in
the CH2 group.
✓ Asymmetric vibrations occur at higher frequencies or wave numbers than symmetric
stretching vibrations.
✓ These are known as coupled vibrations because these vibrations occur at different
frequencies than that required for an isolated C-H stretching.
✓ A strong vibrational coupling is present in carboxylic acid anhydrides in which
symmetrical and asymmetrical stretching vibrations appear in the region 1720-
1825 cm-1. 10

11. 06. Factors Influencing Vibrational Frequency Cont.
✓ The interaction is very effective probably because of the partial double bond
character in the carbonyl oxygen bonds due to resonance which also keeps the
system planar for effective coupling.
✓ For interaction to occur, the vibrations must be of same symmetry species.
✓ There must be a common atom between the groups for strong coupling between
stretching vibrations.
✓ For coupling of bending vibrations, a common bond is necessary.
✓ Interaction is greatest when coupled groups absorb, individually, near
the same frequency.
❑ Fermi-resonance:
➢ A vibration of large amplitude produced by a relatively small vibration.
➢ Coupling of two fundamental vibration modes produces two new modes of vibration,
with frequencies higher and lower than that observed in absence of interaction.
Interaction can also take place between fundamental vibrations and overtones or
combination tone vibrations and such interactions are known as Fermi Resonance. 11

12. 06. Factors Influencing Vibrational Frequency Cont.
✓ For eg. symmetrical stretching vibration of CO2 in Raman spectrum shows band at
1337 cm-1.
✓ The two bending vibrations are equivalent and absorb at the same frequency of
667.3 cm-1.
✓ The first overtone of this is 2 X 667.3= 1334.6 cm-1.
✓ Fermi resonance occurs.
✓ There is mixing of 1337 cm-1 and 1334.6 cm-1 to give two bands at 1285.5 cm-1 and
at 1388.3 cm-1 with intensity ratio 1:0.9 respectively.
❑ Hydrogen bonding:
o It occurs in any system containing a proton donor group (X-H) and a proton acceptor.
if the s-orbital of the proton can effectively overlap the P or 𝜋 orbital of the acceptor
group.
o The stronger the hydrogen bond, the longer the O-H bond, the lower the vibration
frequency and broader and more intense will be the absorption band.
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13. 06. Factors Influencing Vibrational Frequency Cont.
o The N-H stretching frequencies of amines are also affected by hydrogen bonding as
that of the hydroxyl group but frequency shifts for amines are lesser than that for
hydroxyl compounds.
o Because nitrogen is less electronegative than oxygen so the hydrogen bonding in
amines is weaker than that in hydroxyl compounds.
o Intermolecular hydrogen bonds gives rise to broad bands, while intramolecular
hydrogen bonds give sharp and well defined bands.
o The inter and intramolecular hydrogen bonding can be distinguished by dilution.
o Intramolecular hydrogen bonding remains unaffected on dilution and as a result the
absorption band also remains unaffected where as in intermolecular, bonds are
broken on dilution and as a result there is a decrease in the bonded O-H absorption.
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14. 06. Factors Influencing Vibrational Frequency Cont.
❑ Electronic effects :
• Changes in the absorption frequencies for a particular group take place when the
substituents in the neighborhood of that particular group are changed.
• It includes;
1. Inductive effect
2. Mesomeric effect
3. Field effect
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15. 07. References.
1. ‘PHARMACEUTICAL ANALYSIS’ Volume-II ‘Instrumental Methods’ by, Dr. A. V.
Kasture, etal. Page No. 207-221.
2. Sandorfy, C., R. Buchet, and G. Lachenal. "Principles of molecular vibrations for
near-infrared spectroscopy." Near-Infrared Spectroscopy in Food Science and
Technology; Ozaki, Y., McClure, WF, Christy, AA, Eds (2007): 11-46.
3. Czarnecki, M.A., Morisawa, Y., Futami, Y. and Ozaki, Y., 2015. Advances in
molecular structure and interaction studies using near-infrared spectroscopy.
Chemical reviews, 115(18), pp.9707-9744.
4. Wilson, E.B., Decius, J.C. and Cross, P.C., 1980. Molecular vibrations: the theory of
infrared and Raman vibrational spectra. Courier Corporation.
5. Hsu, C.P.S., 1997. Infrared spectroscopy. Handbook of instrumental techniques for
analytical chemistry, 249.
6. Noda, I., 1989. Two-dimensional infrared spectroscopy. Journal of the American
Chemical Society, 111(21), pp.8116-8118.
7. Gütlich, P., Goodwin, H.A., Tuchagues, J.P., Bousseksou, A., Molnár, G., McGarvey,
J.J. and Varret, F., 2004. The role of molecular vibrations in the spin crossover
phenomenon. Spin Crossover in Transition Metal Compounds III, pp.84-103. 15

16. THANK YOU ☺
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