1. Nuclear Magnetic Resonance Spectroscopy
(NMR)

2. Introduction
NMR spectroscopy dates from the ’s and in the space of some 40 years has revolutionized organic chemistry as tool for structural elucidation, kinetic studies, and quality control.
If you have attended an Edmonton Eskimo game or read reports on injured Oilers, you will have heard of magnetic-resonance imaging, a technique which monitors the 1H magnetic resonance signals from water in the body. This application of NMR science enables physicians to image body organs without recourse to X-rays.

3. Introduction
We have seen that IR spectroscopy is a vital tool for the identification of the functional groups present in organic molecules.
NMR provides an image of the molecule’s hydrocarbon skeleton.
NMR spectra result from the absorption of radio frequency radiation when certain atomic nuclei are placed in magnetic fields. Why?

4. Nuclear magnetic resonance
The nuclei of the elements fall into two categories: those which have a spin and those which do not.
The 1H, 13C, 19F nuclei and many others posses a spin.
As they are positively charged, the spinning nucleus generates a tiny magnetic moment - it behaves like a small bar magnet.

5. The effect of the magnetic field
In the absence of an applied magnetic field, these “little magnets” are randomly oriented.
When placed between the poles of a strong magnet, they are either aligned with (parallel) or against (antiparallel) the field.
The parallel spin state is slightly more stable than the antiparallel state. For every 1,000,000 protons, some 500, ,010 have the parallel state! Not a large difference but it is the basis of NMR!

6. The effect of the magnetic field
When a molecule is placed in a magnetic field and is subjected to electromagnetic radiation (radio frequency), the nuclear spin can flip from the parallel to the antiparallel state - the nuclei are said to be in resonance. h
spin aligned with
the applied field
antiparallel spin
magnetic field

7. The effect of the magnetic field
The difference in energy, DE, between the two spin states depends on the strength of the applied field, H0. The stronger the field, the larger the value of DE:

8. Schematic representation of the NMR spectrometer
sample N S
radio frequency detector
radio wave generator

9. 1H NMR spectrometers
The sample is placed in a glass tube which is then placed between the two poles of a very strong magnet.
It is then irradiated using radiation of a constant low frequency.
The magnetic field is varied. When it reaches the correct strength, the nuclei absorb energy and resonance occurs.
The absorption causes a tiny electrical current to flow in a receiver coil surrounding the sample which is displayed as a peak.

10. Fourier Transform NMR Spectrometers
The instrument irradiates the sample with a short pulse of RF radiation for about 10-5 s.
This pulse excites all the nuclei at once.
A computer performs a mathematical calculation known as a Fourier transform and a spectrum is produced.

11. CH3CH2OH

12. Homotopic hydrogen atoms
Such atoms can be identified by replacing each in turn with some other atom such as a bromine. If one gets the same compound, the atoms being replaced are chemically equivalent or homotopic.
These protons are not homotopic!

13. Homotopic hydrogen atoms
Such atoms can be identified by replacing each in turn with some other atom such as a bromine. If one gets the same compound, the atoms being replaced are chemically equivalent or homotopic.
These protons are homotopic!

14. Identify the number of groups of peaks in the 1H NMR spectra of the following:

15. Shielding effects
The position of an NMR peak is controlled by the shielding or deshielding of the nucleus (by electrons).
The free proton is a nucleus free of any influence by exterior, electronic factors.
However organic molecules contain covalently bonded nuclei, not free protons......

16. Shielding effects
Protons in organic molecules are surrounded by electrons.
The electron density about the protons varies according to several factors:
bond polarity
the hybridization state of the atom to which a given hydrogen is bonded
the presence of electron attracting or donating groups

17. Shielding effects
When a nucleus is placed in a magnetic field, the electrons surrounding it are in motion about the nucleus and create a small, localized magnetic field which opposes the applied field.
Thus the electrons generate a small magnetic field that shields the proton from the external field.

18. Shielding effects
The result is that one observes a decrease in the intensity of the total field near to the nucleus. The nucleus is said to be shielded.
Thus the position of an NMR absorption depends on the electron density about the hydrogen.

19. Chemical shifts
The position of an NMR absorption is called the chemical shift.
An internal standard is added and the positions of the peaks are measured relative to that of the internal standard, tetramethylsilane:

20. Tetramethylsilane
The protons in TMS are shielded and resonate at a position far removed from the spectral zone in which one usually finds proton absorption.
The chemical shifts are measured (in Hz) relative to this reference.

21. Chemical shifts
The absorptions are measured in relation to their distance from the TMS peak.
These distances vary according to the strength of the applied magnetic field.
Peaks separated by 54Hz at 60MHz are separated by 72Hz at 80MHz, 270Hz at 300MHz and by 540Hz at 600MHz.

22. Chemical shifts
This complication is eliminated by dividing the distance between the peak and that of TMS (in Hz) by the radio frequency of the spectrometer (in MHz):
distance between peak and that of (CH3)4Si in Hz
 =
ppm
frequency of the spectrometer in MHz

23. Typical chemical shifts
Type of hydrogen Chemical shift,  (ppm)
primary alkyl RCH
secondary alkyl RCH2R’
tertiary alkyl R3CH
allylic C=C-CH
benzylic ArCH2R

24. Typical chemical shifts
Type of hydrogen Chemical shift,  (ppm)
chloroalkane RCH2Cl
bromoalkane RCH2Br
iodoalkane RCH2I
ether RCH2OR’
alcohol RCH2OH
ketone CH3-C=O

25. Typical chemical shifts
Type of hydrogen Chemical shift,  (ppm)
aldehyde H-C=O
terminal alkene R2C=CH
alkene C=CH
aromatic ArH
alkyne RCCH
alcoholic OH ROH (variable)
amine RNH (variable)
carboxylic RCO2H

26. Peak areas
The area under an NMR peak is directly proportional to the number of protons giving rise to the peak.

27. A problem – C11H16

28. Spin - spin coupling – CH3CH2I
There are two principal absorptions. These are divided into three and four component peaks which are equally spaced.

29. Spin - spin coupling Let’s examine a simple molecule:
Consider the absorption by the CH2Br protons in the absence of the other proton of the -CBr2H :
a single peak

30. The CH2Br protons
The magnetic field experienced by these protons at any given moment is either slightly increased or reduced due to the spin of the CHBr2 proton.

31. The CH2Br protons
The magnetic field is increased if this proton is aligned with the applied field.
Therefore a lower external field is necessary to maintain resonance and the peak is situated at lower applied field:

32. The CH2Br protons
The magnetic field is reduced if this proton is aligned against this applied field.
The exterior field must be increased to maintain resonance and so we observe a peak at higher field:

33. Spin - spin coupling
The signal due to the CH2Br protons is divided into two peaks:-
A doublet of peaks of equal size.

34. Spin - spin coupling
Now let’s look at the absorption due to the -CHBr2 group.
It is affected by the spin of the neighboring protons. There are 4 possible spin combinations which are equally possible:
1 : 2 : 1
a triplet

35. Spin-spin coupling constantsJ J

36. CH3CH2CH3
Predict its 1H NMR spectrum.....
TMS

37. Spin - spin coupling
n equivalent neighbouring protons divide an NMR signal into n + 1 peaks.

38. Pascal’s triangle
The relative peak areas are given by Pascal’s triangle: 1

39. Coupling constants J = 2-6 Hz J = 0-7 Hz J = 2-13 Hz J = 5-14 Hz
cis - J = 2-15 Hz
trans - J = Hz

40. Proton exchange
Protons bonded to electronegative atoms undergo rapid exchange and do not show spin - spin coupling with neighboring protons. A time-averaged spectrum is observed, a singlet.
If such a proton is suspected, add a few drops of D2O. The rapid exchange of H - D causes the OH peak to disappear.

41. C8H9Br

42. Degree of unsaturation
Degree of unsaturation = (2NC - NX + NN – NH + 2)/2
NC = number of carbons
NX = number of halogens
NN = number of nitrogens
NH = number of hydrogens

43. C6H14O
SDBSWeb: 24/7/02

44. C6H14O

45. C8H8O2
SDBSWeb: 25/7/02

46. C8H8O2

47. C9H12O
SDBSWeb: 25/7/02

48. C9H12O

49. C8H11NO
SDBSWeb: 25/7/02

50. C8H11NO

51. Problems
Try problems on pages of Solomons and Fryhle 8th Edition or 9.28 – 9.33 on pages 421 – 423 of the 9th Edition.

52. Sketch a spectrum