1. Nuclear Magnetic Resonance Spectroscopy
Organic Chemistry
Second Edition
David Klein
Chapter 15
Nuclear Magnetic Resonance Spectroscopy
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

2. 15.1 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: 1H, 13C, 15N, 19F, 31P
The spinning charge in the nucleus creates a magnetic moment
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

3. 15.1 Intro to NMR Spectroscopy
If the normally disordered magnetic moments of atoms are exposed to an external magnetic field, their magnetic moments will align
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

4. 15.1 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 3e

5. 15.1 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 3e

6. 15.2 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 3e

7. 15.2 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

8. 15.3 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 3e

9. 15.4 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 3e

10. 15.4 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 3e

11. 15.4 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

12. 15.4 Number of Signals
Identify all the groups of equivalent H in the molecules below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

13. 15.5 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 3e

14. 15.5 Chemical Shifts
Current NMRs analyze samples at a constant field strength over a range of energies
Shielded protons (electron shielding) 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 3e

15. From where does up/downfield come?
Early NMRs analyzed samples at a constant energy (ΔE) over a range of magnetic field (Bo) strengths from low field strength = downfield to high field strength = upfield
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

16. 15.5 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 3e

17. 15.5 Chemical Shifts
To predict chemical shifts, start with the standard ppm for the type of proton (methyl, methylene, or methine)
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

18. 15.6 Integration
Integrations represent numbers of protons, so you must adjust the values to whole numbers (C4H10O)
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 3e

19. 15.6 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

20. 15.7 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 3e

21. 15.7 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 3e

22. 15.7 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 3e

23. 15.7 Multiplicity Ha appears as a triplet WHY?
The three peaks in the triplet have an integration ratio of 1:2:1
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

24. 15.7 Multiplicity
Consider a scenario where Ha has three equivalent Hb atoms splitting it
Explain how the magnetic fields cause shielding
or deshielding Hb
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

25. 15.7 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 3e

26. 15.7 Multiplicity
Table 15.3 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 3e

27. 15.7 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 3e

28. 15.7 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 3e

29. 15.7 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 15.17
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

30. 15.7 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 3e

31. Stop Here Klein, Organic Chemistry 3e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

32. 15.5 Chemical Shifts
The result of the diamagnetic anisotropy effect is similar to deshielding for aromatic protons. What about the other protons?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

33. 15.9 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 3e

34. Klein, Organic Chemistry 3e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

35. 15.9 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 3e

36. 15.10 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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

37. 15.11 Acquiring a 13C NMR Spectrum
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

38. 15.11 Acquiring a 13C NMR Spectrum
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 3e

39. 15.12 Chemical Shifts in 13C NMR Spectra
13C NMR signals generally appear between 220 and 0 ppm
Each signal on the 13C spectra represents a unique carbon
Planes and axes of symmetry can cause carbon signals to overlap
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

40. 15.12 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 3e

41. 15.12 Chemical Shifts in 13C NMR Spectra
Like 1H signals, chemical shifts for 13C signals are affected by shielding or deshielding
Practice with SkillBuilder 15.9
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

42. 15.12 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 3e

43. 15.13 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 3e

44. 15.13 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 15.10
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

45. 15.13 DEPT 13C NMR Spectra
Explain how DEPT 13C spectra could be used to distinguish between the two molecules below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

46. 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 3e

47. 15.5 Chemical Shifts
The result of the diamagnetic anisotropy effect is similar to deshielding for aromatic protons.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e