1. 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 2012 John Wiley & Sons, Inc.

2. 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?
Copyright 2012 John Wiley & Sons, Inc.

3. 16.1 Intro to NMR Spectroscopy
Like a bar magnet, a magnetic moment exists perpendicular to the axis of nuclear spin
Copyright 2012 John Wiley & Sons, Inc.

4. 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 2012 John Wiley & Sons, Inc.

5. 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?
Copyright 2012 John Wiley & Sons, Inc.

6. 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 2012 John Wiley & Sons, Inc.

7. 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?
Copyright 2012 John Wiley & Sons, Inc.

8. 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?
Copyright 2012 John Wiley & Sons, Inc.

9. 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 2012 John Wiley & Sons, Inc.

10. 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)
Copyright 2012 John Wiley & Sons, Inc.

11. 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
Copyright 2012 John Wiley & Sons, Inc.

12. 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
Copyright 2012 John Wiley & Sons, Inc.

13. 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 2012 John Wiley & Sons, Inc.

14. 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 2012 John Wiley & Sons, Inc.

15. 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 2012 John Wiley & Sons, Inc.

16. 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
Copyright 2012 John Wiley & Sons, Inc.

17. 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
Copyright 2012 John Wiley & Sons, Inc.

18. 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
Copyright 2012 John Wiley & Sons, Inc.

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 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
Copyright 2012 John Wiley & Sons, Inc.

20. 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
Copyright 2012 John Wiley & Sons, Inc.

21. 16.4 Number of Signals
Identify all the groups of equivalent protons in the molecules below and describe their relationships
Copyright 2012 John Wiley & Sons, Inc.

22. 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?
Copyright 2012 John Wiley & Sons, Inc.

23. 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?
Copyright 2012 John Wiley & Sons, Inc.

24. 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
Copyright 2012 John Wiley & Sons, Inc.

25. 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
Copyright 2012 John Wiley & Sons, Inc.

26. 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
Copyright 2012 John Wiley & Sons, Inc.

27. 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?
Copyright 2012 John Wiley & Sons, Inc.

28. 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 2012 John Wiley & Sons, Inc.

29. 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?
Copyright 2012 John Wiley & Sons, Inc.

30. 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 2012 John Wiley & Sons, Inc.

31. 16.5 Chemical Shifts
Copyright 2012 John Wiley & Sons, Inc.

32. 16.5 Chemical Shifts
Handbooks can be used for functional groups beyond table 16.1
Practice with SkillBuilder 16.3
Copyright 2012 John Wiley & Sons, Inc.

33. 16.5 Chemical Shifts
Predict chemical shifts for all of the protons in the molecule below
Copyright 2012 John Wiley & Sons, Inc.

34. 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 2012 John Wiley & Sons, Inc.

35. 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?
Copyright 2012 John Wiley & Sons, Inc.

36. 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 2012 John Wiley & Sons, Inc.

37. 16.5 Chemical Shifts
The result of the diamagnetic anisotropy effect is similar to shielding for protons that extend into the pi system
Some of the protons in [14] Annulene appear at 8 ppm while others appear at -1 ppm. Which are which?
Copyright 2012 John Wiley & Sons, Inc.

38. 16.5 Chemical Shifts Explain all of the shifts in table 16.2
Practice conceptual checkpoints 16.10
Copyright 2012 John Wiley & Sons, Inc.

39. Study Guide for sections 16.1-16.5
DAY 22, Terms to know:
Sections Nuclear magnetic resonance, magnetic moment, alpha spin state, beta spin state, shielded, deshielded, resonance (in regard to spin states), free induction decay (FID), Fourier-transform (FT), chemical shift
DAY 22, Specific outcomes and skills that may be tested on exam 3:
Sections
Be able to explain what happens to magnets and also the spin states of nuclei when atoms are placed in a magnetic field.
Be able to explain what happens to the spin states when radio waves of the correct frequency excite nuclei in a magnetic field.
Be able to explain why shielded atoms have smaller energy gaps between alpha and beta spin states.
Be able to explain the relationship between the strength of the magnetic field the size of the energy gap between spin states.
Be able to explain how modern NMR instruments are used to obtain a FID.
Be able to explain how and why NMR samples are prepared including state of matter, solvent, spinning.
Be able to determine when H atoms are equivalent or non equivalent.
Be able to identify homotopic, enantiotopic, and diastereotopic protons and explain how such protons affect the number of signals and splitting patterns in the proton NMR.
Be able to explain why TMS is added to many NMR samples.
Be able to explain and use the terms upfield, downfield, shielded, deshielded, higher and lower energy and frequency and explain the relationships such as upfield is the area where shielded nuclei signals appear, etc.
Be able to explain how electronegative atoms affect shielding and shifts of proton signals in NMR.
Be able to give approximate predictions for where on the ppm scale proton signals will appear given the structure of a molecule.
Be able to explain how aromatic rings affect the shifts of protons nearby.
Klein, Organic Chemistry 2e

40. Practice Problems for sections 16.1-16.5
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period
Klein, Organic Chemistry 2e

41. Prep for Day 23 Must Watch videos: Other helpful videos:
(1H NMR integration)
(n+1 rule)
(coupling constants)
(complex splitting)
(1H NMR practice)
Other helpful videos:
(splitting)
and (more 1H NMR practice)
(lecture 19)
Read sections
Klein, Organic Chemistry 2e