1. Nuclear Magnetic Resonance Spectroscopy
Organic Chemistry
Second Edition
David Klein
Chapter 16
Nuclear Magnetic Resonance Spectroscopy
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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
WHAT if the total number of neutrons and protons is an EVEN number?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

8. 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

9. 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

10. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

11. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

12. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

13. 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 symmetry that makes one proton the mirror image of the other
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

14. 16.4 Number of Signals The replacement test is universal
These equivalents protons are enantiotopic
If the resulting compounds are enantiomers, then the protons that you replaced are enantiotopic
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

15. 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
Practice with SkillBuilder 16.1
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

16. 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
Diastereotopic
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

17. 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: Homotopic
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

18. 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 NOT be equivalent if there is a chirality center in the molecule: Diastereotopic
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

19. 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

20. 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

21. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

22. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

23. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

24. 16.5 Chemical Shifts
Alkane protons generally give signals around 1-2 ppm
Protons can be shifted downfield when nearby electronegative atoms cause deshielding.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

25. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

26. 16.5 Chemical Shifts Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

27. 16.5 Chemical Shifts
Predict chemical shifts for all of the protons in the molecule below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

28. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

29. 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?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

30. 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

31. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

32. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

33. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

34. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

35. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

36. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

37. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

38. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

39. 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

40. 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

41. 16.7 Multiplicity Ha appears as a quartet
What should the integration ratios be for the 4 peaks of the quartet?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

42. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

43. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

44. 16.7 Multiplicity Remember three key rules
To split each other, protons must be within a 2 or 3 bond distance
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

45. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

46. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

47. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

48. 16.7 Multiplicity of OH
The hydroxyl proton and other labile or exchangeable protons undergo rapid exchange with trace amounts of acid.
Such exchange blurs the shielding/deshielding affect of the neighboring protons giving a singlet that is often broadened
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

49. 16.9 Using 1H Spectra to Distinguish Between Compounds
The three molecules below might be difficult to distinguish by IR of MS.
1H NMR could distinguish between them
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

50. 16.9 Using 1H Spectra to Distinguish Between Compounds
Explain how 1H NMR could be used to distinguish between the two molecules below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

51. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

52. 16.11 Acquiring a 13C NMR Spectrum
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Klein, Organic Chemistry 2e

53. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

54. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

55. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

56. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

57. 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

58. 16.13 DEPT 13C NMR Spectra
Distortionless Enhancement by Polarization Transfer (DEPT) 13C NMR provides information the number of hydrogen atoms attached to each carbon
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

59. 16.13 DEPT 13C NMR Spectra
Full decoupled 13C spectrum: shows all carbon peaks
DEPT-90: Only CH signals appear
DEPT-135: CH3 and CH = (+) signals, CH2 = (-) signals
Practice with SkillBuilder 16.10
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

60. 16.13 DEPT 13C NMR Spectra
Explain how DEPT 13C spectra could be used to distinguish between the two molecules below
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Klein, Organic Chemistry 2e

61. Additional Practice Problems
Predict the number of signals and chemical shifts in the 13C NMR spectrum and DEPT spectrum for the molecule below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e