1. Infrared Spectroscopy
Provides information about the vibrations of functional groups in a molecule
Therefore, the functional groups present in a molecule can be deduced from an IR spectrum 1

2. Two important parameters in all IR spectra:
The frequency of the signal, 
The intensity of the signal, I
What structural features of a molecule do  and I depend on? 1

3. The functional group concept of organic chemistry
IR spectroscopy can identify functional groups!

5. n-Hexane: CH3CH2CH2CH2CH2CH3?
1650 cm-1
3100 cm-1
1-Hexene: CH2=CHCH2CH2CH2CH3

6. n-Hexane: CH3CH2CH2CH2CH2CH3
3-heptanone:

7. n-Hexane: CH3CH2CH2CH2CH2CH3
1-Hexanol: CH3CH2CH2CH2CH2CH2OH

8. What in the world is this peak due to?
Where are
the saturated
C-H stretches?

9. Where are
the saturated
C-H stretches?
2,3-dimethyl butane

10. 1-hexene Where are the C-H stretches? Where are the =C-H stretches?
What’s this?
Where are
the C-H stretches?
1-hexene
Where are
the =C-H stretches?
Why are the =C-H stretches of lower intensity than the saturated C-H stretches?
Where is
the C=C stretch?

11. Where is the
the stretch?

12. Where is the
stretch?
Where is the C-H
stretch of ?

13. Where is the
stretch?

14. Why is the C-H stretch so small compared to the other spectra?
Where is the
stretch?

15. What in the world is this peak due to?
Where is the
stretch?

16. Where is the
OH stretch?
Where is the
C=O stretch?

17. Where is the
OH stretch?
Where is the
stretch?

18. IR: Masses, Atoms and Springs
A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds).
As a first approximation use Hooke’s Law
F = -kx
F = force, restoring back to equilibrium position
k = characteristic stretching constant
x = displacement from the equilibrium position

19. IR Stretching Frequencies of two bonded atoms:
What Does the Frequency, , Depend On?
= frequency
k = spring strength (bond stiffness)
mr = reduced mass (~ mass of largest atom)

20. Vibrations, potential energy and motion

21. IR Stretching Frequencies: What Do they Depend On?
Directly on the strength of the bonding between the two atoms ( ~ k)
Inversely on the reduced mass of the two atoms (v ~ 1/m)
Expect:  will increase with increasing bond strength (bond order) and decreasing mass

22. Examples of stretching frequencies and correlations with bond strengths (bond order)
350
600
840
Bond order 1 2 3 
2200 cm-1
1600 cm-1
1000 cm-1
C-C
C=C
For same reduced mass!
*In kJ/mol

23. Only the natural frequency will be absorbed
Quantum mechanics: The frequency () depends on the energy gap between vibrational levels
E = h= hc/(cm-1)
Only the natural frequency will be absorbed
The natural frequency (8.67 x 1013 s-1) is absorbed selectively

24. What does the absorption intensity depend on?
The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency  can be transferred to the atoms involved in the vibration
The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon

25. Dipole Moment Must Change during for a vibration to be “IR active”!
In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole moment changes.
Which of the vibrations are “IR” active”?

26. The are not “Greenhouse” gases
What is the intensity of an IR signal of:
O2 or N2 or H2?
Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of the molecule
O2, N2 and H2 DO NOT ABSORB IR LIGHT!
The are not “Greenhouse” gases

27. Does O=C=O absorb IR light?
Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light
vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light
No dipole
generated
Dipole
generated

28. Which should have a higher stretching frequency, CO, CO+, or CO-? Why?
Ans: The higher the bond order, the stronger the bond and the higher the frequency for the IR stretch.
CO = ()2()4 CO+ = ()2()3 CO- = ()2()4()1
Bond order: CO = 3, CO+ = 5/2, CO- = 5/2
CO will have the higher stretching frequency
CO+ and CO- will have similar, lower frequencies

29. IR Stretching Frequencies of two bonded atoms:
What Does the Frequency, , Depend On?
= frequency
k = spring strength (bond stiffness)
mr = reduced mass (mass of largest atom)

30. The influence of reduced mass on the stretching frequency 
For any X-H bond the reduced mass is approximately equal to the mass of the very light hydrogen atom = 1
For a C-C bond the reduced mass is approximately equal to the mass of a carbon atom = 12
From the formula for , compute the differences in  for a X-H and C-C bond of equal strengths.

31. Infrared Spectroscopy: further review
region of infrared that is most useful lies between mm ( cm-1)
depends on transitions between vibrational energy states
stretching
bending 2

32. Stretching Vibrations of a CH2 Group
Symmetric
Antisymmetric 2

33. Bending Vibrations of a CH2 Group
In plane
In plane 2

34. Bending Vibrations of a CH2 Group
Out of plane
Out of plane 2

35. Infrared Spectrum of Hexane
bending
C—H stretching
bending
bending
CH3CH2CH2CH2CH2CH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

36. Infrared Spectrum of 1-Hexene
C=C
H—C
C=C—H
H2C=C
H2C=CHCH2CH2CH2CH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

37. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H
sp2 C—H
sp3 C—H
sp2 C—O 1200
sp3 C—O 8

38. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (multiple bonds) C —C C— —C N 8

39. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (carbonyl groups)
Aldehydes and ketones
Carboxylic acids
Acid anhydrides and
Esters
Amides 8

40. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Bending vibrations of alkenes
CH2
RCH
CH2
R2C
890
CHR'
cis-RCH
CHR'
trans-RCH
CHR'
R2C 8

41. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Bending vibrations of derivatives of benzene
Monosubstituted and
Ortho-disubstituted
Meta-disubstituted and
Para-disubstituted 8

42. Infrared Spectrum of tert-butylbenzene
Ar—H
C6H5C(CH3)3
H—C
Monsubstituted benzene
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

43. Infrared Absorption Frequencies: functional groups
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols)
O—H (carboxylic acids)
N—H 8

44. Infrared Spectrum of 2-Hexanol
H—C
O—H OH
CH3CH2CH2CH2CHCH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8

45. Infrared Spectrum of 2-Hexanone
CH3CH2CH2CH2CCH3
H—C
C=O
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 8