1. Infrared spectroscopy Ultraviolet-Visible spectroscopy
Chapter 13 Spectroscopy
Infrared spectroscopy
Ultraviolet-Visible spectroscopy
Nuclear magnetic resonance spectroscopy
Mass Spectrometry
Dr. Wolf's CHM 201 & 202 1

2. Principles of Molecular Spectroscopy: Electromagnetic Radiation
Dr. Wolf's CHM 201 & 202 1

3. Electromagnetic Radiation
is propagated at the speed of light
has properties of particles and waves
the energy of a photon is proportional to its frequency
Dr. Wolf's CHM 201 & 202 2

4. Figure 13.1: The Electromagnetic Spectrum
Longer Wavelength ()
Shorter Wavelength ()
400 nm
750 nm
Visible Light
Higher Frequency ()
Lower Frequency ()
Higher Energy (E)
Lower Energy (E)
Dr. Wolf's CHM 201 & 202 6

5. Figure 13.1: The Electromagnetic Spectrum
Longer Wavelength ()
Shorter Wavelength ()
Higher Frequency ()
Lower Frequency ()
Higher Energy (E)
Lower Energy (E)
Ultraviolet
Infrared
Dr. Wolf's CHM 201 & 202 6

6. Figure 13.1: The Electromagnetic Spectrum
Cosmic rays
 Rays
X-rays
Ultraviolet light
Visible light
Infrared radiation
Microwaves
Radio waves
Energy
Dr. Wolf's CHM 201 & 202 4

7. Principles of Molecular Spectroscopy: Quantized Energy States
Dr. Wolf's CHM 201 & 202 5

8. E = h
Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states.
Dr. Wolf's CHM 201 & 202 6

9. UV-Vis infrared microwave radiofrequency What Kind of States?
electronic
vibrational
rotational
nuclear spin
UV-Vis
infrared
microwave
radiofrequency
Dr. Wolf's CHM 201 & 202 7

10. Infrared Spectroscopy
Gives information about the functional groups in a molecule
Dr. Wolf's CHM 201 & 202 1

11. Infrared Spectroscopy
region of infrared that is most useful lies between m ( cm-1)
depends on transitions between vibrational energy states
stretching
bending
Dr. Wolf's CHM 201 & 202 2

12. Stretching Vibrations of a CH2 Group
Symmetric
Antisymmetric
Dr. Wolf's CHM 201 & 202 2

13. Bending Vibrations of a CH2 Group
In plane
In plane
Dr. Wolf's CHM 201 & 202 2

14. Bending Vibrations of a CH2 Group
Out of plane
Out of plane
Dr. Wolf's CHM 201 & 202 2

15. Figure 13.31: Infrared Spectrum of Hexane
bending
C—H stretching
bending
bending
CH3CH2CH2CH2CH2CH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Dr. Wolf's CHM 201 & 202
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 8

16. Figure 13.32: 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
Dr. Wolf's CHM 201 & 202
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 8

17. 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
Dr. Wolf's CHM 201 & 202 8

18. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (multiple bonds) C —C C— —C N
Dr. Wolf's CHM 201 & 202 8

19. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (carbonyl groups)
Aldehydes and ketones
Carboxylic acids
Acid anhydrides and
Esters
Amides
Dr. Wolf's CHM 201 & 202 8

20. 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
Dr. Wolf's CHM 201 & 202 8

21. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Bending vibrations of derivatives of benzene
Monosubstituted and
Ortho-disubstituted
Meta-disubstituted and
Para-disubstituted
Dr. Wolf's CHM 201 & 202 8

22. Figure 13.33: 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
Dr. Wolf's CHM 201 & 202
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 8

23. Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols)
O—H (carboxylic acids)
N—H
Dr. Wolf's CHM 201 & 202 8

24. Figure 13.34: Infrared Spectrum of 2-Hexanol
H—C
O—H OH
CH3CH2CH2CH2CHCH3
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Dr. Wolf's CHM 201 & 202
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 8

25. Figure 13.35: Infrared Spectrum of 2-Hexanone
CH3CH2CH2CH2CCH3
H—C
C=O
2000
3500
3000
2500
1000
1500
500
Wave number, cm-1
Dr. Wolf's CHM 201 & 202
Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved. 8

26. Ultraviolet-Visible (UV-VIS) Spectroscopy
Gives information about conjugated  electron systems
Dr. Wolf's CHM 201 & 202 1

27. Transitions between electron energy states
gaps between electron energy levels are greater than those between vibrational levels
gap corresponds to wavelengths between 200 and 800 nm
E = h
Dr. Wolf's CHM 201 & 202 2

28. X-axis is wavelength in nm (high energy at left, low energy at right)
Conventions in UV-VIS
X-axis is wavelength in nm (high energy at left, low energy at right)
max is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially  electron system
Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity ()
Dr. Wolf's CHM 201 & 202 3

29. UV Spectrum of cis,trans-1,3-cyclooctadiene
2000
Molar absorptivity ()
max 230 nm
max 2630
1000
Wavelength, nm
Dr. Wolf's CHM 201 & 202 4

30. * Transition in cis,trans-1,3-cyclooctadiene
  


LUMO

E = h 
HOMO 
Most stable -electron configuration
-Electron configuration of excited state
Dr. Wolf's CHM 201 & 202 5

31. * Transition in Alkenes
HOMO-LUMO energy gap is affected by substituents on double bond
as HOMO-LUMO energy difference decreases (smaller E), max shifts to longer wavelengths
Dr. Wolf's CHM 201 & 202 5

32. Methyl groups on double bond cause max to shift to longer wavelengths
CH3 C C H
CH3 H H
max 170 nm
max 188 nm
Dr. Wolf's CHM 201 & 202 5

33. Extending conjugation has a larger effect on max; shift is again to longer wavelengths
max 170 nm
max 217 nm
Dr. Wolf's CHM 201 & 202 5

34. max 217 nm (conjugated diene)H C
max 217 nm (conjugated diene) C H
CH3
H3C
max 263 nm conjugated triene plus two methyl groups
Dr. Wolf's CHM 201 & 202 5

35. orange-red pigment in tomatoes
Lycopene
orange-red pigment in tomatoes
max 505 nm
Dr. Wolf's CHM 201 & 202 5

36. Mass Spectrometry
Dr. Wolf's CHM 201 & 202 1

37. Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron e–
Dr. Wolf's CHM 201 & 202 2

38. Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron e–
electron is deflected but transfers much of its energy to the molecule
Dr. Wolf's CHM 201 & 202 2

39. Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron e–
electron is deflected but transfers much of its energy to the molecule
Dr. Wolf's CHM 201 & 202 2

40. Principles of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron.
Dr. Wolf's CHM 201 & 202 2

41. Principles of Electron-Impact Mass Spectrometry
This energy-rich species ejects an electron. e–
+ •
forming a positively charged, odd-electron species called the molecular ion
Dr. Wolf's CHM 201 & 202 2

42. Principles of Electron-Impact Mass Spectrometry
Molecular ion passes between poles of a magnet and is deflected by magnetic field
amount of deflection depends on mass-to-charge ratio
highest m/z deflected least
lowest m/z deflected most
+ •
Dr. Wolf's CHM 201 & 202 5

43. Principles of Electron-Impact Mass Spectrometry
If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose.
However, the molecular ion often fragments to a mixture of species of lower m/z.
Dr. Wolf's CHM 201 & 202 6

44. Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation and a radical.
+ •
Dr. Wolf's CHM 201 & 202 2

45. Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation and a radical. + •
Usually several fragmentation pathways are available and a mixture of ions is produced.
Dr. Wolf's CHM 201 & 202 2

46. Principles of Electron-Impact Mass Spectrometry
mixture of ions of different mass gives separate peak for each m/z
intensity of peak proportional to percentage of each ion of different mass in mixture
separation of peaks depends on relative mass + + + + + +
Dr. Wolf's CHM 201 & 202 6

47. Principles of Electron-Impact Mass Spectrometry
mixture of ions of different mass gives separate peak for each m/z
intensity of peak proportional to percentage of each atom of different mass in mixture
separation of peaks depends on relative mass + + + + + +
Dr. Wolf's CHM 201 & 202 6

48. Some molecules undergo very little fragmentation
Benzene is an example. The major peak corresponds to the molecular ion.
Relative intensity
100 80 60 40 20
m/z = 78
m/z
Dr. Wolf's CHM 201 & 202 9

49. Isotopic Clusters H H H 79 79 78 93.4% 6.5% 0.1%
all H are 1H and all C are 12C
one C is 13C
one H is 2H
Dr. Wolf's CHM 201 & 202 10

50. Isotopic Clusters in Chlorobenzene
visible in peaks for molecular ion
Relative intensity
100 80 60 40 20
112
114
m/z
Dr. Wolf's CHM 201 & 202 11

51. Isotopic Clusters in Chlorobenzene+
no m/z 77, 79 pair; therefore ion responsible for m/z 77 peak does not contain Cl
Relative intensity
100 80 60 40 20 77
m/z
Dr. Wolf's CHM 201 & 202 11

52. Alkanes undergo extensive fragmentation
CH3—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH3
Relative intensity 43 57
100 80 60 40 20
Decane 71 85 99
142
m/z
Dr. Wolf's CHM 201 & 202 13

53. Propylbenzene fragments mostly at the benzylic position
Relative intensity
100 80 60 40 20 91
CH2—CH2CH3
120
m/z
Dr. Wolf's CHM 201 & 202 14

54. Molecular Formula as a Clue to Structure
Dr. Wolf's CHM 201 & 202 1

55. Molecular Weights
One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula.
However, we can gain some information even from the molecular weight. Mass spectrometry makes it relatively easy to determine molecular weights.
Dr. Wolf's CHM 201 & 202 6

56. The Nitrogen Rule
A molecule with an odd number of nitrogens has an odd molecular weight.
A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight.
NH2 93
138
NH2
O2N
183
NH2
O2N
NO2
Dr. Wolf's CHM 201 & 202 6

57. Exact Molecular Weights
CH3CO O
CH3(CH2)5CH3
Heptane
Cyclopropyl acetate
Molecular formula
C7H16
C5H8O2
Molecular weight
100
100
Exact mass
Mass spectrometry can measure exact masses. Therefore, mass spectrometry can give molecular formulas.
Dr. Wolf's CHM 201 & 202 6

58. Molecular Formulas
Knowing that the molecular formula of a substance is C7H16 tells us immediately that is an alkane because it corresponds to CnH2n+2
C7H14 lacks two hydrogens of an alkane, therefore contains either a ring or a double bond
Dr. Wolf's CHM 201 & 202 6

59. Index of Hydrogen Deficiency
relates molecular formulas to multiple bonds and rings
index of hydrogen deficiency = 1
(molecular formula of alkane – molecular formula of compound) 2
Dr. Wolf's CHM 201 & 202 6

60. index of hydrogen deficiency
Example 1
C7H14
index of hydrogen deficiency 1 2
(molecular formula of alkane – molecular formula of compound) = 1 2
(C7H16 – C7H14) = 1 2
(2) = 1 =
Therefore, one ring or one double bond.
Dr. Wolf's CHM 201 & 202 6

61. Example 2 C7H12 1 = (C7H16 – C7H12) 2 (4) = 2
(4) = 2
Therefore, two rings, one triple bond, two double bonds, or one double bond + one ring.
Dr. Wolf's CHM 201 & 202 6

62. index of hydrogen deficiency =
Oxygen has no effect
CH3(CH2)5CH2OH (1-heptanol, C7H16O) has same number of H atoms as heptane
index of hydrogen deficiency = 1 2
(C7H16 – C7H16O)
= 0
no rings or double bonds
Dr. Wolf's CHM 201 & 202 6

63. index of hydrogen deficiency =
Oxygen has no effect
CH3CO O
Cyclopropyl acetate
index of hydrogen deficiency = 1
(C5H12 – C5H8O2)
= 2 2
one ring plus one double bond
Dr. Wolf's CHM 201 & 202 6

64. Treat a halogen as if it were hydrogen.
If halogen is present
Treat a halogen as if it were hydrogen. H Cl
C3H5Cl C
same index of hydrogen deficiency as for C3H6 H
CH3
Dr. Wolf's CHM 201 & 202 6

65. Rings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum of rings plus multiple bonds.
Catalytic hydrogenation tells us how many multiple bonds there are.
Dr. Wolf's CHM 201 & 202 6

66. Introduction to 1H NMR Spectroscopy
Dr. Wolf's CHM 201 & 202 8

67. The nuclei that are most useful to organic chemists are:
1H and 13C
both have spin = ±1/2
1H is 99% at natural abundance
13C is 1.1% at natural abundance
Dr. Wolf's CHM 201 & 202 9

68. Nuclear Spin + +
A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.
Dr. Wolf's CHM 201 & 202 10

69. The distribution of nuclear spins is random in the absence of an external magnetic field.+ + + + +
Dr. Wolf's CHM 201 & 202 11

70. An external magnetic field causes nuclear magnetic moments to align parallel and antiparallel to applied field. + + + H0 + +
Dr. Wolf's CHM 201 & 202 11

71. There is a slight excess of nuclear magnetic moments aligned parallel to the applied field.+ + + H0 + +
Dr. Wolf's CHM 201 & 202 11

72. Energy Differences Between Nuclear Spin States+ +
increasing field strength
no difference in absence of magnetic field
proportional to strength of external magnetic field
Dr. Wolf's CHM 201 & 202 12

73. Some important relationships in NMR
Units Hz
kJ/mol (kcal/mol)
tesla (T)
The frequency of absorbed electromagnetic radiation is proportional to
the energy difference between two nuclear spin states which is proportional to
the applied magnetic field
Dr. Wolf's CHM 201 & 202 6

74. Some important relationships in NMR
The frequency of absorbed electromagnetic radiation is different for different elements, and for different isotopes of the same element.
For a field strength of 4.7 T: 1H absorbs radiation having a frequency of 200 MHz (200 x 106 s-1) 13C absorbs radiation having a frequency of 50.4 MHz (50.4 x 106 s-1)
Dr. Wolf's CHM 201 & 202 6

75. Some important relationships in NMR
The frequency of absorbed electromagnetic radiation for a particular nucleus (such as 1H) depends on its molecular environment. This is why NMR is such a useful tool for structure determination.
Dr. Wolf's CHM 201 & 202 6

76. Nuclear Shielding and 1H Chemical Shifts
What do we mean by "shielding?"
What do we mean by "chemical shift?"
Dr. Wolf's CHM 201 & 202 14

77. Shielding
An external magnetic field affects the motion of the electrons in a molecule, inducing a magnetic field within the molecule.
The direction of the induced magnetic field is opposite to that of the applied field. C H
H 0
Dr. Wolf's CHM 201 & 202 15

78. Shielding
The induced field shields the nuclei (in this case, C and H) from the applied field.
A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. C H
H 0
Dr. Wolf's CHM 201 & 202 15

79. Chemical Shift
Chemical shift is a measure of the degree to which a nucleus in a molecule is shielded.
Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. C H
H 0
Dr. Wolf's CHM 201 & 202 15

80. Chemical Shift
Chemical shifts (d) are measured relative to the protons in tetramethylsilane (TMS) as a standard. Si
CH3
H3C
d =
position of signal - position of TMS peak
spectrometer frequency
x 106
Dr. Wolf's CHM 201 & 202 15

81. Downfield Decreased shielding Upfield Increased shielding
(CH3)4Si (TMS)
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm) measured relative to TMS
Dr. Wolf's CHM 201 & 202 1

82. Chemical Shift
Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at a spectrometer frequency of 200 MHz.
d =
position of signal - position of TMS peak
spectrometer frequency
x 106
d =
1456 Hz - 0 Hz
200 x 106 Hx
x 106
d =
Dr. Wolf's CHM 201 & 202 15

83. H C Cl  7.28 ppm Chemical shift (, ppm) 1 1.0 2.0 3.0 4.0 5.0 6.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
Dr. Wolf's CHM 201 & 202 1

84. Effects of Molecular Structure on 1H Chemical Shifts
protons in different environments experience different degrees of shielding and have different chemical shifts
Dr. Wolf's CHM 201 & 202 17

85. Electronegative substituents decrease the shielding of methyl groups
least shielded H
most shielded H
CH3F
CH3OCH3
(CH3)3N
CH3CH3
(CH3)4Si
d 4.3
d 3.2
d 2.2
d 0.9
d 0.0
Dr. Wolf's CHM 201 & 202 21

86. Electronegative substituents decrease shielding
H3C—CH2—CH3
d 4.3
d 2.0
d 1.0
O2N—CH2—CH2—CH3
Dr. Wolf's CHM 201 & 202 21

87. Effect is cumulative CHCl3  7.3 CH2Cl2  5.3 CH3Cl  3.1 21
Dr. Wolf's CHM 201 & 202 21

88. Methyl, Methylene, and Methine
CH3 more shielded than CH2 ; CH2 more shielded than CH
H3C C
CH3 H
d 0.9
d 1.6
d 0.8
CH2
d 1.2
Dr. Wolf's CHM 201 & 202 21

89. Protons attached to sp2 hybridized carbon are less shielded than those attached to sp3 hybridized carbon H C H
CH3CH3
 7.3
 5.3
 0.9
Dr. Wolf's CHM 201 & 202 21

90. But protons attached to sp hybridized carbon are more shielded than those attached to sp2 hybridized carbon C H
 5.3
 2.4
CH2OCH3 C H
Dr. Wolf's CHM 201 & 202 21

91. Protons attached to benzylic and allylic carbons are somewhat less shielded than usual
 1.5
 0.8
H3C
CH3
d 0.9
d 1.3
H3C—CH2—CH3
 1.2
H3C
CH2
 2.6
Dr. Wolf's CHM 201 & 202 21

92. Proton attached to C=O of aldehyde is most deshielded C—H
H3C
Dr. Wolf's CHM 201 & 202 21

93. C H R C H N 2.1-2.3 0.9-1.8 C H C H 2.5 1.5-2.6 C H O C H Ar 2.3-2.8
Type of proton
Chemical shift (), ppm
Type of proton
Chemical shift (), ppm C H R C H N C H C H
2.5 C H O C H Ar
Dr. Wolf's CHM 201 & 202 25

94. C H NR 2.2-2.9 C H 4.5-6.5 C H Cl 3.1-4.1 C H Br H Ar 6.5-8.5 2.7-4.1
Type of proton
Chemical shift (), ppm
Type of proton
Chemical shift (), ppm C H NR C H C H Cl C H Br H Ar C O H C H O
9-10
Dr. Wolf's CHM 201 & 202 25

95. H NR 1-3 H OR 0.5-5 H OAr 6-8 C O HO 10-13 Type of proton
Chemical shift (), ppm H NR
1-3 H OR
0.5-5 H
OAr
6-8 C O HO
10-13
Dr. Wolf's CHM 201 & 202 25

96. Interpreting Proton NMR Spectra
Dr. Wolf's CHM 201 & 202 1

97. Information contained in an NMR spectrum includes:
1. number of signals
2. their intensity (as measured by area under peak)
3. splitting pattern (multiplicity)
Dr. Wolf's CHM 201 & 202 2

98. exist in different molecular environment
Number of Signals
protons that have different chemical shifts are chemically nonequivalent
exist in different molecular environment
Dr. Wolf's CHM 201 & 202 3

99. N CCH2OCH3 OCH3 NCCH2O Chemical shift (, ppm) 1 1.0 2.0 3.0 4.0 5.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
Dr. Wolf's CHM 201 & 202 1

100. Chemically equivalent protons
are in identical environments
have same chemical shift
replacement test: replacement by some arbitrary "test group" generates same compound
H3CCH2CH3
chemically equivalent
Dr. Wolf's CHM 201 & 202 7

101. Chemically equivalent protons
Replacing protons at C-1 and C-3 gives same compound (1-chloropropane)
C-1 and C-3 protons are chemically equivalent and have the same chemical shift
ClCH2CH2CH3
CH3CH2CH2Cl
H3CCH2CH3
chemically equivalent
Dr. Wolf's CHM 201 & 202 7

102. Diastereotopic protons
replacement by some arbitrary test group generates diastereomers
diastereotopic protons can have different chemical shifts C Br
H3C H
 5.3 ppm
 5.5 ppm
Dr. Wolf's CHM 201 & 202 9

103. Spin-Spin Splitting in NMR Spectroscopy
not all peaks are singlets
signals can be split by coupling of nuclear spins
Dr. Wolf's CHM 201 & 202 12

104. Cl2CHCH3 4 lines; quartet 2 lines; doublet CH3 CH
Figure (page 536)
Cl2CHCH3
4 lines;
quartet
2 lines;
doublet
CH3 CH
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
Dr. Wolf's CHM 201 & 202 1

105. Two-bond and three-bond coupling
protons separated by two bonds (geminal relationship)
protons separated by three bonds (vicinal relationship)
Dr. Wolf's CHM 201 & 202 14

106. Two-bond and three-bond coupling
in order to observe splitting, protons cannot have same chemical shift
coupling constant (2J or 3J) is independent of field strength
Dr. Wolf's CHM 201 & 202 14

107. coupled protons are vicinal (three-bond coupling)
Figure (page 536)
Cl2CHCH3
4 lines;
quartet
2 lines;
doublet
CH3 CH
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
coupled protons are vicinal (three-bond coupling)
CH splits CH3 into a doublet, CH3 splits CH into a quartet
Chemical shift (, ppm) 1

108. Why do the methyl protons of 1,1-dichloroethane appear as a doublet?Cl
signal for methyl protons is split into a doublet
To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1
Dr. Wolf's CHM 201 & 202 19

109. Why do the methyl protons of 1,1-dichloroethane appear as a doublet?Cl
signal for methyl protons is split into a doublet
There are two orientations of the nuclear spin for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C-2 protons.
Dr. Wolf's CHM 201 & 202 19

110. Why do the methyl protons of 1,1-dichloroethane appear as a doublet?Cl
signal for methyl protons is split into a doublet
The protons at C-2 "feel" the effect of both the applied magnetic field and the local field resulting from the spin of the C-1 proton.
Dr. Wolf's CHM 201 & 202 19

111. Why do the methyl protons of 1,1-dichloroethane appear as a doublet?Cl
"true" chemical shift of methyl protons (no coupling)
this line corresponds to molecules in which the nuclear spin of the proton at C-1 reinforces the applied field
this line corresponds to molecules in which the nuclear spin of the proton at C-1 opposes the applied field
Dr. Wolf's CHM 201 & 202 19

112. Why does the methine proton of 1,1-dichloroethane appear as a quartet?Cl
signal for methine proton is split into a quartet
The proton at C-1 "feels" the effect of the applied magnetic field and the local fields resulting from the spin states of the three methyl protons. The possible combinations are shown on the next slide.
Dr. Wolf's CHM 201 & 202 19

113. Why does the methine proton of 1,1-dichloroethane appear as a quartet?
There are eight combinations of nuclear spins for the three methyl protons.
These 8 combinations split the signal into a 1:3:3:1 quartet. C H Cl
Dr. Wolf's CHM 201 & 202 20

114. The splitting rule for 1H NMR
For simple cases, the multiplicity of a signal for a particular proton is equal to the number of equivalent vicinal protons + 1.
Dr. Wolf's CHM 201 & 202 22

115. Splitting Patterns: The Ethyl Group
CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet)
Dr. Wolf's CHM 201 & 202 24

116. BrCH2CH3 4 lines; quartet 3 lines; triplet CH3 CH2
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
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117. Splitting Patterns of Common Multiplets
Table 13.2 (page 540)
Splitting Patterns of Common Multiplets
Number of equivalent Appearance Intensities of lines protons to which H of multiplet in multiplet is coupled Doublet 1:1
2 Triplet 1:2:1
3 Quartet 1:3:3:1
4 Pentet 1:4:6:4:1
5 Sextet 1:5:10:10:5:1
6 Septet 1:6:15:20:15:6:1
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118. Splitting Patterns: The Isopropyl Group
(CH3)2CHX is characterized by a doublet-septet pattern (septet at lower field than the doublet)
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119. BrCH(CH3)2 2 lines; doublet 7 lines; septet CH3 CH
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
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120. 13C NMR Spectroscopy
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121. 1H and 13C NMR compared:
both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons)
both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.)
it is convenient to use FT-NMR techniques for 1H; it is standard practice for 13C NMR
Dr. Wolf's CHM 201 & 202 6

122. 1H and 13C NMR compared:
13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom
a signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and
at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)
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123. 1H and 13C NMR compared:
13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei
Figure (a) shows the 1H NMR spectrum of 1-chloropentane; Figure (b) shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum.
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124. 1H
ClCH2
CH3
ClCH2CH2CH2CH2CH3
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Chemical shift (, ppm)
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125. 13C
ClCH2CH2CH2CH2CH3
a separate, distinct peak appears for each of the 5 carbons
CDCl3 20 40 60 80
100
120
140
160
180
200
Chemical shift (, ppm)
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126. are measured in ppm () from the carbons of TMS
13C Chemical Shifts
are measured in ppm () from the carbons of TMS
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127. 13C Chemical shifts are most affected by:
electronegativity of groups attached to carbon
hybridization state of carbon
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128. Electronegativity Effects
Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts.
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129. 1H 13C Types of Carbons Classification Chemical shift, d (CH3)3CH CH4
0.2 -2 8 16 25 28
primary
secondary
tertiary
quaternary
0.9
1.3
1.7
Replacing H by C (more electronegative) deshields C to which it is attached.
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130. Electronegativity effects on CH3
Chemical shift, d 1H
0.2
2.5
3.4
4.3
13C -2 27 50 75
CH4
CH3NH2
CH3OH
CH3F
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131. Electronegativity effects and chain lengthCl
CH2
CH3
Chemical
shift, d 45 33 29 22 14
Deshielding effect of Cl decreases as number of bonds between Cl and C increases.
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132. 13C Chemical shifts are most affected by:
electronegativity of groups attached to carbon
hybridization state of carbon
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133. Hybridization effects
114
138 36
sp3 hybridized carbon is more shielded than sp2
sp hybridized carbon is more shielded than sp2, but less shielded than sp3
CH3 H C
CH2 68 84 22 20 13
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134. Carbonyl carbons are especially deshielded
CH2 C O
CH2
CH3 41
171 61 14
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135. RCH3 0-35 CR RC 65-90 R2CH2 15-40 CR2 R2C 100-150 R3CH 25-50 110-175
Table 13.3 (p 549)
Type of carbon
Chemical shift (), ppm
Type of carbon
Chemical shift (), ppm
RCH3
0-35 CR RC
65-90
R2CH2
15-40
CR2
R2C
R3CH
25-50
R4C
30-40
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136. RCH2Br 20-40 RC N 110-125 O RCH2Cl 25-50 RCOR 160-185 RCH2NH2 35-50
Table 13.3 (p 549)
Type of carbon
Chemical shift (), ppm
Type of carbon
Chemical shift (), ppm
RCH2Br
20-40 RC N O
RCH2Cl
25-50
RCOR
RCH2NH2
35-50
RCH2OH
50-65 O
RCH2OR
50-65
RCR
Dr. Wolf's CHM 201 & 202 25

137. 13C NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be deceptive.
Dr. Wolf's CHM 201 & 202 3

138. CH3 OH 7 carbons give 7 signals, but intensities are not equal
Figure (page 551)
CH3 OH
7 carbons give 7 signals, but intensities are not equal 20 40 60 80
100
120
140
160
180
200
Chemical shift (, ppm)
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139. End of Chapter 13
Dr. Wolf's CHM 201 & 202