Infrared spectroscopy, also known as vibrational-rotational spectroscopy, involves the absorption of infrared radiation by molecules, leading to increased vibrational and rotational energy. It is used qualitatively to determine molecular structures and identify specific functional groups, with the spectrum acting as a chemical fingerprint unique to each molecule. The theory behind infrared absorption is rooted in molecular vibrations and the modification from harmonic to anharmonic oscillators, which explains the presence of overtones and combination bands in the spectra.

1. INFRARED SPECTROSCOPY
Ravish Yadav
VIBRATIONAL ROTATIONAL SPECTROSCOPY

2. • MOLECULAR ABSORPTION SPECTROSCOPY
• MATTER : MOLECULE
• EMR: INFRA RED RADIATION
• TYPE OF INTERACTION: ABSORPTION OF RADIATION
• EFFECT ON THE MOLECULE: AS A RESULT OF ABSORPTION OF
INFRARED RADIA TION THE VIBRATIONAL AND ROTATIONAL ENERGY
OF THE MOLECULE INCREASES. MOLECULE BECOMES VIBRATIONALLY
EXCITED.
MH + IR ʎ MH* (VIBRATIONALLY EXCITED MOLECULE)
•HENCE IR SPECTROSCOPY IS ALSO KNOWN AS VIBRATIONAL –
ROTATIONAL SPECTROSCOPY
FEATURES

3. FEATURES
• APPLICATION: QUALITATIVE.
• HEPLS IN DETERMINATION OF STRUCTURE OF A MOLECULE.

4. • Infrared light lies between the visible and microwave portions of the
electromagnetic spectrum.
IR REGION IS FURTHER DIVIDED IN TO:
1. NEAR IR 2. MID IR 3. FAR IR

5. OVERTONE REGION
VIBRATIONAL REGION
THERMAL WAVES
IR REGION IS FURTHER DIVIDED IN TO:

6. MID IR
RANGE: 4000-667 CM-1
FUNCTIONAL
GROUP REGION
RANGE: 4000-1400 CM-1
HELPS IN DETECTION OF
SPECIFIC FUNCTIONAL
GROUPS IN THE MOLECULE
e.g –OH,C=O , double
bonds, triple bonds etc
FINGER PRINT &
SINGLE BOND
REGION
RANGE: 1400 - 667 CM-1
IR SPECTRUM OF A MOLECULE
= CHEMICAL FINGERPRINT OF
MOLECULE. BANDS IN THIS
REGION IS UNIQUE TO A
MOLECULE. HENCE THIS
REGION IS USED FOR
IDENTIFICATION OF
MOLECULE

8. THEORY & PRINCIPLE OF IR

10. MOLECULAR VIBRATIONS: DIFFERENT
TYPES OF VIBRATIONS IN MOLECULES

11. Stretching: SYMMETRIC STRECTING

12. Stretching: ASYMMETRIC STRECTING

13. BENDING (INPLANE): SCISSORING

14. BENDING (INPLANE): ROCKING

15. BENDING (OUT OF PLANE - OOP):
WAGGING

16. BENDING (OUT OF PLANE - OOP):
TWISTING

17. PRINCIPLE OF IR

18. IR ACTIVE VIBRATIONS:

19. IR ACTIVE VIBRATIONS:

20. CONDITION FOR
ABSORPTION OF IR by
BONDS
Molecular vibrations
must produce a
change in dipole
moment of bond
The frequency of IR
radiation = Frequency
of bond vibration
CONDTION FOR A MOLECULAR VIBRATION
(BOND) TO ABSORB IR RADIATION

21. CONDTION FOR ABSORPTION OF IR

22. MOLECULAR VIBRATION MUST PRODUCE A
CHANGE IN DIPOLE MOMENT OF THE BOND
CONDTION FOR ABSORPTION OF IR

23. CONDTION FOR ABSORPTION OF IR

24. CONDTION FOR ABSORPTION OF IR

25. H2 I2
MM =2 g/mole
MM =254 g/mole

26. RECORDING IR SPECTRUM OF A COMPD

35. IR ABSROPTION ENERGY LEVEL & TYPES OF
PEAKS OBTAINED IN IR SPECTRUM

36. POTENTIAL ENERGY LEVEL DIAGRAM

37. In the case of the anharmonic
oscillator, the vibrational
transitions no longer only
obey the selection rule n =
1. This type of vibrational
transition is called
fundamental vibration.
Vibrational transitions with n
= 2, 3, ... are also possible,
and are termed overtones.
Called first, second, and so
on, overtones.
Fundamentals and Overtones

38. The Harmonic Oscillator
Consider a diatomic molecule as two spherical masses (m1 and m2)
connected with a spring with a given force constant (k). Vibration of
such a molecule can be described by HARMONIC OSCILLATOR.
 The P.E of the spring and the mass is almost zero when the mass
is at rest ( equilibrium position)
 As the bonds vibrates , the spring is compressed or stretched The
bond length and PE also Changes with vibrations. The average
bond length increases due to vibration . Increase Vibration =
Increase bond length =increases in potential energy. (PE)
 The plot of PE vs equilibrium bond length is called as Potential
Energy Curve . Its parabolic in shape and symmetrical about
equilibrium bond length. This curve has many vibrational energy
levels. These vibrational energy levels are
1.Equidistant &
2.Transitions are only allowed between neighboring energy levels
with:
ΔE = ±1
Thus only V0 to V1 transitions are possible. And only fundamental
bands are obtained in the IR spectrum. But in reality overtones and
combination band are obtained . Harmonic oscillator doesnot explain
formation of overtones hence it sis considered to be wrong model for ir
absorption process
IR Absorption & Potential Energy level curve : HARMONIC OSCILLATOR

40. Harmonic oscillator model has limitations and is not the true mode as it does not
consider:
• Repulsive forces between the vibrating atoms. When the bond is compressed the two
atoms around the bond will come close. This will lead to repulsion b/n electron clouds
of two atoms and hence this repulsive force will also act similar to restoring force.
Harmonic oscillator assumes that restoring force is the only force acting on the system.
Hence bonds do not follow harmonic oscillation
• As the vibrational energy of the bond increases the bonds may dissociate . The
possibility of dissociation is not considered in Harmonic oscillator.
Thus energy level diagram is modified to include the effects of these and a new model
was proposed by MORSE called ANHORMONIC POTENTIAL ENERGY LEVEL CUVE
OR MORSE”S POTENTIAL ENERGY LEVEL CURVE
ANHARMONIC OSCILLATOR

41. ANHARMONIC OSCILLATOR
Unlike the harmonic oscillator, energy
levels are no longer equidistant .
Anharmonic Potential Energy level
curve explains formation of
overtones. As the gap between the
higher energy levels is less hence IR
transitions like V0 to V2 or V0 to V3
are possible. The resulting
bands/peaks are called as overtones
and their intensity is less than the
fundamental bands.

42. Anharmonicity leads to deviations of two kinds:
1) OVERTONE BANDS: At higher energy levels, E becomes smaller and the
selection rule is not followed. Higher energy levels are closely placed. As a
result, weaker transitions called overtones are sometimes observed. These
transitions correspond to  =  2 or  3.
• The frequencies of such overtone transitions are approximately two or three
times that of the fundamental frequency
• The intensities are lower than that of the fundamental frequency.
2) COMBINATION BANDS: Different vibrations in a molecule can interact to give
absorption peaks with frequencies that are approximately sums or differences
of their fundamental frequencies (combination frequencies)
• The Intensities of overtone, combination and difference peaks are lower than
that of the fundamental frequency.
ANHARMOINIC OSCILLATOR

43. Fundamentals and Overtones

44. Fundamentals and Overtones

45. Fundamentals and Overtones

46. Fundamentals and Overtones
1864/2=932

47.  The frequency of a combination is approx. the sum of the frequencies of the individual
bands.
 Combinations of fundamentals with overtones are possible as well as well as
fundamentals involving two or more vibrations.
 The vibrations must involve the same functional group and have the same symmetry.
Combination bands for water speciation in
hydrated Na2O·6SiO2 (NS6) glasses
Shigeru Yamashita, Harald Behrens, Burkhard C. Schmidt, Ray
Dupree. Water speciation in sodium silicate glasses based on NIR
and NMR spectroscopy. Chemical Geology 256 (2008) 231–241.
COMBINATION BANDS

48. COMBINATION BANDS

49. FERMI RESONANCE