1. Nuclear Magnetic Resonance Spectroscopy
Organic Chemistry
Second Edition
David Klein
Chapter 16
Nuclear Magnetic Resonance Spectroscopy
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Klein, Organic Chemistry 2e

2. 16.1 Intro to NMR Spectroscopy
What is spectroscopy?
Nuclear Magnetic Resonance (NMR) spectroscopy may be the most powerful method of gaining structural information about organic compounds
NMR involves an interaction between electromagnetic radiation (light) and the nucleus of an atom
We will focus on C and H nuclei. WHY?
The structure (connectivity) of a molecule affects how the radiation interacts with each nucleus in the molecule
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Klein, Organic Chemistry 2e

3. 16.1 Intro to NMR Spectroscopy
Protons and neutrons in a nucleus behave as if they are spinning
If the total number of neutrons and protons is an ODD number, the atoms will have net nuclear spin
Examples:
The spinning charge in the nucleus creates a magnetic moment
We saw in Chapter 15 how a dipole moment creates an electric field
What does a magnetic moment create?
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Klein, Organic Chemistry 2e

4. 16.1 Intro to NMR Spectroscopy
Like a bar magnet, a magnetic moment exists perpendicular to the axis of nuclear spin
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Klein, Organic Chemistry 2e

5. 16.1 Intro to NMR Spectroscopy
If the normally disordered magnetic moments of atoms are exposed to an external magnetic field, their magnetic moments will align
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Klein, Organic Chemistry 2e

6. 16.1 Intro to NMR Spectroscopy
The aligned magnetic moments can be either with or against the external magnetic field
The α and β spin states are not equal in energy. WHY?
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Klein, Organic Chemistry 2e

7. 16.1 Intro to NMR Spectroscopy
When an atom with an α spin state is exposed to radio waves of just the right energy, it can be promoted to a β spin state
The stronger the magnetic field, the greater the energy gap
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Klein, Organic Chemistry 2e

8. 16.1 Intro to NMR Spectroscopy
The magnetic moment of the electrons generally reduces the affect of the external field
The more shielded a nucleus is with electron density, the smaller the α  β energy gap. WHY?
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Klein, Organic Chemistry 2e

9. 16.1 Intro to NMR Spectroscopy
The amount of radio wave energy necessary for the α  β energy transition depends on the electronic environment for the atom
When the α spins are flipped to β spins, the atoms are said to be in resonance
The use of the term, “resonance” here is totally different from when we are talking about electrons in molecular orbitals
How does NMR spectroscopy tell us about molecular structure?
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Klein, Organic Chemistry 2e

10. 16.2 Acquiring a 1H NMR Spectrum
NMR requires a strong magnetic field and radio wave energy
The strength of the magnetic field affects the energy gap
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Klein, Organic Chemistry 2e

11. 16.2 Acquiring a 1H NMR Spectrum
The strong magnetic field is created when a high current is passed through a superconducting material at extremely low temperature (≈4 Kelvin)
The greater the current, the greater the magnetic field
In most current NMR instruments, a brief pulse of radio energy (all relevant wavelengths) is used to excite the sample
Each of the atoms is excited and then relaxes, emitting energy
The emitted energy is recorded as a free induction decay (FID)
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Klein, Organic Chemistry 2e

12. 16.2 Acquiring a 1H NMR Spectrum
The FID contains all of the information for each atom
A mathematical treatment called a Fourier-transform separates the signals so an individual signal can be observed for each atom that was excited
Such an instrument is called an FT-NMR
Often multiple FIDs are taken and averaged together
Before analysis, NMR samples must be prepared neat or in a liquid solution and placed in a small NMR tube
The sample is placed into the magnetic field and the tube is spun at a high rate to average magnetic
field variations or tube imperfections
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Klein, Organic Chemistry 2e

13. 16.2 Acquiring a 1H NMR Spectrum
Solvents are used such as chloroform-d. WHY?
The magnet is super-cooled, but the sample is generally at room temp
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Klein, Organic Chemistry 2e

14. 16.3 Characteristics of a 1H NMR Spectrum
NMR spectra contain a lot of structural information
Number of signals
Signal location – shift
Signal area – integration
Signal shape – splitting pattern
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Klein, Organic Chemistry 2e

15. 16.4 Number of Signals
Protons with different electronic environments will give different signals
Protons that are homotopic will have perfectly overlapping signals
Protons are homotopic if the molecule has an axis of rotational symmetry that allows one proton to be rotated onto the other without changing the molecule
Find the rotational axis of symmetry in each molecule below
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Klein, Organic Chemistry 2e

16. 16.4 Number of Signals
Another test for homotopic protons is to replace the protons one at a time with another atom
If the resulting compounds are identical, then the protons that you replaced are homotopic
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Klein, Organic Chemistry 2e

17. 16.4 Number of Signals
Protons that are enantiotopic will also have perfectly overlapping signals
Protons are enantiotopic if the molecule has a plane of reflection that makes one proton the mirror image of the other
Find the mirror plane that splits each molecule below
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Klein, Organic Chemistry 2e

18. 16.4 Number of Signals The replacement test is universal
It will work to identify any equivalents protons whether they are homotopic or enantiotopic
If the resulting compounds are enantiomers, then the protons that you replaced are enantiotopic
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Klein, Organic Chemistry 2e

19. 16.4 Number of Signals
If the protons are neither homotopic nor enantiotopic, then the are NOT chemically equivalent
Perform the replacement test on the protons shown in the molecule below
How would you describe the relationship between the protons shown?
Practice with SkillBuilder 16.1
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Klein, Organic Chemistry 2e

20. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

21. 16.4 Number of Signals
There are some shortcuts you can take to identify how many signals you should see in the 1H NMR
The 2 protons on a CH2 group will be equivalent if there are NO chirality centers in the molecule
The 2 protons on a CH2 group will NOT be equivalent if there is a chirality center in the molecule
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Klein, Organic Chemistry 2e

22. 16.4 Number of Signals
There are some shortcuts you can take to identify how many signals you should see in the 1H NMR
The 3 protons on any methyl group will always be equivalent to each other
Multiple protons are equivalent if they can be interchanged through either a rotation or mirror plane
Practice with SkillBuilder 16.2
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Klein, Organic Chemistry 2e

23. 16.4 Number of Signals
Identify all the groups of equivalent protons in the molecules below and describe their relationships
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Klein, Organic Chemistry 2e

24. 16.4 Number of Signals
Recall that cyclohexane chairs have 6 equitorial and 6 axial protons
Do the axial and equitorial protons have different electronic environments?
Try the replacement test for an axial/equitorial pair
How many signals should you see for the molecule in the 1H NMR?
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Klein, Organic Chemistry 2e

25. 16.4 Number of Signals
At room temperature, the chair interconversion occurs rapidly
The NMR is not fast enough to see the individual structures, so the average is observed (1 signal)
What might you expect to see if the temperature of the NMR sample were brought down to -100 C?
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Klein, Organic Chemistry 2e

26. 16.5 Chemical Shifts
Tetramethylsilane (TMS) is used as the standard for NMR chemical shift
In many NMR solvents, 1% TMS is added as an internal standard
The shift for a proton signal is calculated as a comparison to TMS
For benzene on a
300 MHz instrument
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Klein, Organic Chemistry 2e

27. 16.5 Chemical Shifts
The shift for a proton signal is calculated as a comparison to TMS
For benzene on a 60 MHz instrument
The Hz of the signal is different in different instruments, but the shift relative to TMS (δ) is constant
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Klein, Organic Chemistry 2e

28. 16.5 Chemical Shifts
The shift for a proton signal is calculated as a comparison to TMS
The shift relative to TMS (δ) is a dimensionless number, because the Hz units cancel out
Units for δ are often given as ppm (parts per million), which simply indicates that signals are reported as a fraction of the operating frequency of the spectrometer
Most 1H signals appear between 0-10 ppm
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Klein, Organic Chemistry 2e

29. 16.5 Chemical Shifts
Early NMRs analyzed samples at a constant energy over a range of magnetic field strengths from low field strength = downfield to high field strength = upfield
Shielded protons required a stronger external magnetic field to be excited at the same energy as deshielded protons. WHY?
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Klein, Organic Chemistry 2e

30. 16.5 Chemical Shifts
Current NMRs analyze samples at a constant field strength over a range of energies
Shielded protons have a smaller magnetic force acting on them, so they have smaller energy gaps and absorb lower energy radio waves
Higher Energy
Lower Energy
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Klein, Organic Chemistry 2e

31. 16.5 Chemical Shifts
Alkane protons generally give signals around 1-2 ppm
Protons can be shifted downfield when nearby electronegative atoms cause deshielding. HOW?
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Klein, Organic Chemistry 2e

32. 16.5 Chemical Shifts
To predict chemical shifts, start with the standard ppm for the type of proton (methyl, methylene, or methine)
Use table 16.1 to adjust the ppm depending on proximity to certain function groups
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Klein, Organic Chemistry 2e

33. 16.5 Chemical Shifts Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

34. 16.5 Chemical Shifts
Handbooks can be used for functional groups beyond table 16.1
Practice with SkillBuilder 16.3
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Klein, Organic Chemistry 2e

35. 16.5 Chemical Shifts
Predict chemical shifts for all of the protons in the molecule below
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Klein, Organic Chemistry 2e

36. 16.5 Chemical Shifts
Predict chemical shifts for all of the protons in the molecule below
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Klein, Organic Chemistry 2e

37. 16.5 Chemical Shifts
When the electrons in a pi system are subjected to an external magnetic field, they circulate a great deal causing diamagnetic anisotropy
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Klein, Organic Chemistry 2e

38. 16.5 Chemical Shifts
Diamagnetic anisotropy means that different regions in space will have different magnetic strengths
Why are some regions more shielded than others?
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Klein, Organic Chemistry 2e

39. 16.5 Chemical Shifts
The result of the diamagnetic anisotropy effect is similar to deshielding for aromatic protons. What about the other protons?
Why does it appear that only one signal is given for all of the aromatic protons?
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Klein, Organic Chemistry 2e

40. 16.5 Chemical Shifts Explain all of the shifts in table 16.2
Practice conceptual checkpoints 16.10
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Klein, Organic Chemistry 2e

41. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

42. 16.6 Integration
The integration or area under the peak quantifies the relative number of protons giving rise to a signal
A computer will calculate the area of each peak representing that area with a step-curve
The curve height represents the integration
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Klein, Organic Chemistry 2e

43. 16.6 Integration
The computer operator sets one of the peaks to a whole number to let it represent a number of protons
The computer uses the integration ratios to set the values for the other peaks
1.48
1.56
1.00
1.05
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Klein, Organic Chemistry 2e

44. 16.6 Integration
Integrations represent numbers of protons, so you must adjust the values to whole numbers
If the integration of the first peak is doubled, the computer will adjust the others according to the ratio
2.96
3.12
2.00
2.10
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Klein, Organic Chemistry 2e

45. 16.6 Integration
The integrations are relative quantities rather than an absolute count of the number of protons
Predict the 1H shifts and integrations for tert-butyl methyl ether
Symmetry can also affect integrations
Predict the 1H shifts and integrations for 3-pentanone
Practice with SkillBuilder 16.4
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Klein, Organic Chemistry 2e

46. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

47. 16.7 Multiplicity
When a signal is observed in the 1H NMR, often it is split into multiple peaks
Multiplicity or a splitting patterns results
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Klein, Organic Chemistry 2e

48. 16.7 Multiplicity
Multiplicity results from magnetic affects that protons have on each other
Consider protons Ha and Hb
We already saw that protons align with or against the external magnetic field
Hb will be aligned with the magnetic field in some molecules. Other molecules in the sample will have Hb aligned against the magnetic field
Some Hb atoms have a slight shielding affect on Ha and others have a slight deshielding affect
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Klein, Organic Chemistry 2e

49. 16.7 Multiplicity
The resulting multiplicity or splitting pattern for Ha is a doublet
A doublet generally results when a proton is split by only one other proton on an adjacent carbon
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Klein, Organic Chemistry 2e

50. 16.7 Multiplicity
Consider an example where there are two protons on the adjacent carbon
There are three possible affects the Hb protons have on Ha
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Klein, Organic Chemistry 2e

51. 16.7 Multiplicity
Half of the Ha atoms will not experience a signal shift. WHY?
¼ of the Ha atoms will be shielded and ¼ deshielded
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Klein, Organic Chemistry 2e

52. 16.7 Multiplicity Ha appears as a triplet WHY?
The three peaks in the triplet have an integration ratio of 1:2:1
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Klein, Organic Chemistry 2e

53. 16.7 Multiplicity
Consider a scenario where Ha has three equivalent Hb atoms splitting it
Explain how the magnetic fields cause shielding
or deshielding
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Klein, Organic Chemistry 2e

54. 16.7 Multiplicity Ha appears as a quartet
What should the integration ratios be for the 4 peaks of the quartet?
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Klein, Organic Chemistry 2e

55. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

56. 16.7 Multiplicity
Table 16.3 shows how the multiplicity trend continues
By analyzing the splitting pattern of a signal in the 1H NMR, you can determine the number of equivalent protons on adjacent carbons
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Klein, Organic Chemistry 2e

57. 16.7 Multiplicity
The trend in table 16.3 also allows us to predict splitting patterns
Explain how the n+1 rule is used
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Klein, Organic Chemistry 2e

58. 16.7 Multiplicity Remember three key rules
Equivalent protons can not split one another
Predict the splitting patterns observed for
1,2-dichloroethane
To split each other, protons must be within a 2 or 3 bond distance
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Klein, Organic Chemistry 2e

59. 16.7 Multiplicity Remember three key rules
The n+1 rule only applies to protons that are all equivalent
The splitting pattern observed for the proton shown below will be more complex than a simple triplet
Complex splitting will be discussed later in this section
Practice with SkillBuilder 16.5
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Klein, Organic Chemistry 2e

60. 16.7 Multiplicity
Predict splitting patterns for all of the protons in the molecule below
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Klein, Organic Chemistry 2e

61. 16.7 Multiplicity
The degree to which a neighboring proton will shield or deshield its neighbor is called a coupling constant
The coupling constant or J value is the distance between peaks of a splitting pattern measured in units of Hz
When protons split each other, their coupling constants will be equal
Jab = Jba
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Klein, Organic Chemistry 2e

62. 16.7 Multiplicity
The coupling constant will be constant even if an NMR instrument with a stronger or weaker magnetic field is used
Higher field strength instruments will give better resolution between peaks,
because the coupling constant is a smaller percentage of the overall Hz available
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Klein, Organic Chemistry 2e

63. 16.7 Multiplicity
Sometimes recognizable splitting patterns will stand out in a spectrum
An isolated ethyl group gives a triplet and a quartet
Note the integrations
The triplet and quartet must have the same coupling constant if they are splitting each other
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Klein, Organic Chemistry 2e

64. 16.7 Multiplicity
A peak with an integration equal to 9 suggests the presence of a tert-butyl group
An isolated isopropyl group gives a doublet and a septet
Note the integrations
Practice with conceptual checkpoint 16.17
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Klein, Organic Chemistry 2e

65. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

66. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

67. 16.7 Multiplicity
Complex splitting results when a proton is split by NONequivalent neighboring protons
If Jab is much greater than Jbc, the signal will appear as a quartet of triplets
In the molecule shown, Hb is split into a quartet by Ha and into a triplet by Hc
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Klein, Organic Chemistry 2e

68. 16.7 Multiplicity
Complex splitting results when a proton is split by NONequivalent neighboring protons
If Jbc is much greater than Jab, the signal will appear as a triplet of quartets
If Jbc is similar to Jab, the signal will appear as a multiplet
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Klein, Organic Chemistry 2e

69. 16.7 Multiplicity
Complex splitting results when a proton is split by NONequivalent neighboring protons
Predict the splitting patterns for (S)-pent-2-en-4-ol
If Jbc is equal to Jab, what type of patterns will be observed?
Practice with conceptual checkpoint 16.18
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Klein, Organic Chemistry 2e

70. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

71. 16.7 Multiplicity
Splitting is not observed for some protons. Consider ethanol
The protons bonded to carbon split each other, but the hydroxyl proton is not split
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Klein, Organic Chemistry 2e

72. 16.7 Multiplicity
The hydroxyl proton and other labile or exchangeable protons undergo rapid exchange with trace amounts of acid. Show a reasonable mechanism
Such exchange blurs the shielding/deshielding affect of the neighboring protons giving a singlet that is often broadened
If ethanol is rigorously purified to remove traces of acid, then hydroxyl proton splitting is generally observed
Aldehyde protons also often appear as singlet because their coupling constants are sometimes too small to cause observable splitting
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Klein, Organic Chemistry 2e

73. 16.7 Multiplicity
Signals for exchangeable protons such as those shown below disappear completely when the 1H NMR sample is prepared for analysis in a deuterated solvent such as chloroform-d. WHY?
Protic compounds have exchangeable protons
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Klein, Organic Chemistry 2e

74. 16.8 Predicting Expected 1H Spectra for a Compound
Predict the chemical shift, integration, and splitting patterns for all of the protons in the following molecule
Draw a spectrum for the molecule
Practice with SkillBuilder 16.6
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Klein, Organic Chemistry 2e

75. 16.9 Using 1H Spectra to Distinguish Between Compounds
The three molecules below might be difficult to distinguish by IR or MS. WHY?
Explain how 1H NMR could distinguish between them
Practice with SkillBuilder 16.7
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Klein, Organic Chemistry 2e

76. 16.9 Using 1H Spectra to Distinguish Between Compounds
Explain how 1H NMR could be used to distinguish between the two molecules below
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Klein, Organic Chemistry 2e

77. 16.10 Analyzing a 1H NMR Spectrum
With a given formula and 1H NMR spectrum, you can determine a molecule’s structure by a 4-step process
Calculate the degree or unsaturation or hydrogen deficiency index (HDI). What does the HDI tell you?
Consider the number of NMR signals and integration to look for symmetry in the molecule
Analyze each signal, and draw molecular fragments that match the shift, integration, and multiplicity
Assemble the fragments into a complete structure like puzzle pieces
Practice with SkillBuilder 16.8
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Klein, Organic Chemistry 2e

78. 16.10 Analyzing a 1H NMR Spectrum
Consider the data below, and propose a structure for the molecule
The formula is C7H13Cl
1H NMR data: δ 5.3 (dq 1H); 5.1 (d 1H); 3.4 (s 2H); 2.0 (d 3H); 1.0 (s 6H)
Calculate HDI
Consider the number of NMR signals and integration to look for symmetry in the molecule
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Klein, Organic Chemistry 2e

79. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

80. 16.11 Acquiring a 13C NMR Spectrum
Because 1H is by far the most abundant isotope of hydrogen, 1H NMR signals are generally strong
13C only accounts for about 1% of carbon atoms in nature, so a sensitive receiver coil and/or concentrated NMR sample is needed
In 1H NMR, shift, splitting, and integration are important
In 13C NMR, only the number of signals and the shift will be considered
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Klein, Organic Chemistry 2e

81. 16.11 Acquiring a 13C NMR Spectrum
In 13C NMR, the 1H-13C splitting is often so complex that the spectrum is unreadable
To elucidate the 13C spectrum and make it easier to determine the total number of 13C signals, 13C NMR are generally decoupled
In the vast majority of 13C spectra, all of the signals are singlets
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Klein, Organic Chemistry 2e

82. 16.12 Chemical Shifts in 13C NMR Spectra
Compared to 1H, 13C atoms require a different frequency of energy to excite (resonate)
Compared to the standard TMS, 13C NMR signals generally appear between 220 and 0 ppm
Each signal on the 13C spectra represents a carbon with a unique electronic environment
Planes and axes of symmetry can cause carbon signals to overlap if their electronic surroundings are equivalent
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Klein, Organic Chemistry 2e

83. 16.12 Chemical Shifts in 13C NMR Spectra
Note how symmetry affect the number of signals for the molecules above
How many 13C signals should be observed for the molecule below
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Klein, Organic Chemistry 2e

84. 16.12 Chemical Shifts in 13C NMR Spectra
Like 1H signals, chemical shifts for 13C signals are affected by shielding or deshielding
Practice with SkillBuilder 16.9
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Klein, Organic Chemistry 2e

85. 16.12 Chemical Shifts in 13C NMR Spectra
Predict the number of signals and chemical shifts in the 13C NMR spectrum for the molecule below
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Klein, Organic Chemistry 2e

86. 16.13 Medically Speaking
MRI (magnetic resonance imaging) instruments are essentially 1H NMR spectrometers
The body is analyzed rather than a sample in an NMR tube
Different tissues have different concentrations of protons. WHY?
The MRI gives a 3D image of different tissues. HOW?
Would you expect there to be side-effects from exposure to either radio waves or a magnetic field?
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Klein, Organic Chemistry 2e

87. Additional Practice Problems
Explain why a deuterated solvent is used in NMR experiments rather than a protonated solvent. Is such a solvent necessary when analyzing 13C NMR?
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Klein, Organic Chemistry 2e

88. Additional Practice Problems
Identify all the groups of equivalent protons in the molecule below and explain WHY enantiotopic protons are equivalent while diastereotopic protons are not.
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Klein, Organic Chemistry 2e

89. Additional Practice Problems
Predict the chemical shift, integration, and splitting patterns for all of the protons in the following molecule
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Klein, Organic Chemistry 2e