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
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
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. 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

14. 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

15. 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

16. 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

17. 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

18. 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

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

20. 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

21. 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

22. 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

23. 15.7 Multiplicity The multiplicity trend continues
The splitting pattern can tell you the number of equivalent protons on adjacent carbons
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

24. 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

25. 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

26. 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

27. 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