1. Spectroscopy
Infrared Spectra

2. Infrared spectra in this presentation are taken by permission from the SDBS web site:

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4. Spectroscopy “seeing the unseeable”
Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

5. electromagnetic relationships: λυ = c λ µ 1/υ E = hυ E µ υ
  E = hc/λ E µ 1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant λ c

6. Two oscillators will strongly interact when their energies are equal.
E1 = E2
λ1 = λ2
υ1 = υ2
If the energies are different, they will not strongly interact!
We can use electromagnetic radiation to probe atoms and molecules to find what energies they contain.

7. some electromagnetic radiation ranges
Approx. freq. range Approx. wavelengths
Hz (cycle/sec) meters
Radio waves x x10-4
Infrared (heat) x x x10-7
Visible light x x x x10-7
Ultraviolet x x x
X rays x x
Gamma rays > 3x < 10-11

8. Infrared radiation
λ = 2.5 to 17 μm
υ = 4000 to 600 cm-1
 These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.
IR is used to tell:
1. what type of bonds are present
2. some structural information

9. IR source è sample è prism è detector
graph of % transmission vs. frequency
=> IR spectrum
100 %T
v (cm-1)

10. toluene

11. Some characteristic infrared absorption frequencies
BOND COMPOUND TYPE FREQUENCY RANGE, cm-1
C-H alkanes and
alkenes (m) and
RCH=CH and
R2C=CH
cis-RCH=CHR (v)
trans-RCH=CHR
aromatic rings (m) and
monosubst and
ortho-disubst
meta-disubst and (m)
para-disubst (m)
alkynes
O-H alcohols or phenols (b)
C=C alkenes (v)
aromatic rings and 1600 (v)
C≡C alkynes (v)
C-O primary alcohols (b)
secondary alcohols (b)
tertiary alcohols (b)
phenols (b)
alkyl ethers
aryl ethers (b) and (m)
all abs. strong unless marked: m, moderate; v, variable; b, broad

12. IR spectra of ALKANES
C—H bond “saturated”
(sp3) cm-1
cm-1
-CH
-CH “ and 1375
-CH(CH3) “ and 1370, 1385
-C(CH3) “ and 1370(s), 1395 (m)

13. n-pentane 2850-2960 cm-1 CH3CH2CH2CH2CH3 sat’d C-H 3000 cm-1

14. n-hexane
CH3CH2CH2CH2CH2CH3

15. 2-methylbutane (isopentane)

16. 2,3-dimethylbutane

17. cyclohexane
no 1375 cm-1
no –CH3

18. IR of ALKENES
=C—H bond, “unsaturated” vinyl
(sp2) cm-1
RCH=CH &
R2C=CH
cis-RCH=CHR (v)
trans-RCH=CHR
C=C bond cm-1 (v)

19. 1-decene 3020-3080 cm-1 C=C 1640-1680 unsat’d C-H
& RCH=CH2
C=C

20. 4-methyl-1-pentene
& RCH=CH2

21. 2-methyl-1-butene
R2C=CH2

22. 2,3-dimethyl-1-butene
R2C=CH2

23. IR spectra BENZENEs
=C—H bond, “unsaturated” “aryl”
(sp2) cm-1
mono-substituted ,
ortho-disubstituted
meta-disubstituted , (m)
para-disubstituted (m)
C=C bond 1500, 1600 cm-1

24. ethylbenzene 3000-3100 cm-1 Unsat’d C-H 690-710, 730-770 mono-
1500 & 1600
Benzene ring
, mono-

25. o-xylene
ortho

26. p-xylene
(m)
para

27. m-xylene
meta
, (m)

28. styrene
no sat’d C-H
1640
C=C
&
RCH=CH2
mono

29. 2-phenylpropene
Sat’d C-H
R2C=CH2
mono

30. p-methylstyrene
para

31. IR spectra ALCOHOLS & ETHERS
C—O bond (b) cm-1
1o ROH 1050
2o ROH 1100
3o ROH 1150
ethers
O—H bond (b) 

32. 1-butanol
(b) O-H
C-O 1o
CH3CH2CH2CH2-OH

33. 2-butanol
O-H
C-O 2o

34. tert-butyl alcohol
O-H
C-O 3o

35. methyl n-propyl ether
no O--H
C-O ether

36. 2-butanone
 C=O
~1700 (s)

37. C9H12 1500 & 1600 benzene C-H unsat’d & sat’d mono
C9H12 – C6H5 = -C3H7
isopropylbenzene
n-propylbenzene?

38. n-propylbenzene

39. isopropylbenzene
isopropyl split

40. C8H6 C-H unsat’d 1500, 1600 benzene 3300 C8H6 – C6H5 = C2H C-H mono
phenylacetylene

41. C4H8
Unst’d
C=C
R2C=CH2
isobutylene CH3
CH3C=CH2

42. Which compound is this?
2-pentanone
1-pentanol
1-bromopentane
2-methylpentane
1-pentanol

43. What is the compound?
1-bromopentane
1-pentanol
2-pentanone
2-methylpentane
2-pentanone

45. In a “matching” problem, do not try to fully analyze each spectrum
In a “matching” problem, do not try to fully analyze each spectrum. Look for differences in the possible compounds that will show up in an infrared spectrum.

46. 1

47. 2

48. 3

49. 4

50. 5

51. 6