Infrared spectroscopy measures the absorption of infrared light by chemical bonds in molecules. It is used to identify functional groups and determine molecular structure. The infrared spectrum is measured in wavenumbers from 4000-400 cm-1, which corresponds to wavelengths from 2.5-25 micrometers. There are two main regions - the functional group region from 4000-1400 cm-1 and the fingerprint region from 1400-600 cm-1. Common functional groups like alcohols, amines, carbonyls, and acids each absorb at different wavenumbers depending on their structure. Examples of spectra are shown for various compounds containing these functional groups.

1. INFRARED
SPECTROSCOPY

3. Electromagnetic Spectrum

4. Bond Stretching

5. High Frequency Stretching
Low Frequency Bending

6. Frequency of Absorption
Measured in cm-1 (Wavenumbers)
Wavelength units typically microns (
m)
10,000m = 1 cmor 1m = 10-4 cm
Frequency units typically measured incm-1
= # waves or cycles in 1 cm
(wavenumbers)
Mid-Infrared frequency range is
4000 - 400 cm-1
(2.5 - 25m)

7. High Frequency to Low Frequency
Functional Groups Region
4000 - 1400 cm-1
Fingerprint Region
1400-600 cm-1

8. General Trends:
i) Stretching frequencies are higher than
corresponding bending frequencies. (It is easier
to bend a bond than to stretch or compress it.)
ii) Bonds to hydrogen have higher stretching
frequencies than those to heavier atoms.
iii) Triple bonds have higher stretching
frequencies than corresponding double bonds,
which in turn have higher frequencies than single
bonds. (Except for bonds to hydrogen).

11. IR Spectrum of Octane

12. Decane

13. 2,2,4-Trimethylpentane

14. Hybridization of Carbon
Affects n of Absorption

15. Hexane and Hexene

16. 1-Pentene

17. 1-Dodecene

18. 2-Methyl-1,3-butadiene
(Isoprene)

19. Terminal Alkynes are Readily
Identifiable with IR

20. 1-Heptyne

21. Aromatics
• C-H str (sp2) at 3050-3100 cm-1
• C-C str (breathing) 1600 & 1500 cm-1
H

22. Toluene

23. o-Xylene

24. Alcohols
O-H str at 3600-3200 cm-1
C-O str at 1050-1150 cm-1

25. Cyclohexanol

26. O-H Stretch

27. 2-Methyl-1-butanol

28. Benzyl alcohol

29. 1o Amines

30. 2-Methylpentanediamine

31. 2o Amine

32. Dipropylamine

33. Carbonyl Compounds

34. Acetone

35. Cycloheptanone

36. Acetophenone

37. Conjugation Lowers n of
Absorption

39. Ketone and Aldehyde

40. Octanal

41. Carboxylic Acid

42. Hexanoic Acid

43. Phenyl Acetate, an Ester

44. Ethyl Butanoate

45. Methyl Benzoate

46. Benzoic Anhydride, an
Acid Anhydride

47. Butanamide, an Amide

48. O

49. CH3CH2OCCH 3
O

50. CH2CH
O

51. C N

52. CH2=CHCH2CH2CH3

53. HC CCH2CH2OH

54. CH3CH2CH2NH2

55. CH2CH2CH2CH2CO2H