1. Chapter 14 NMR Spectroscopy Organic Chemistry 6th Edition Dr. Halligan
CHM 236
Spring 2012
Chapter 14
NMR Spectroscopy
Organic Chemistry
6th Edition
Paula Yurkanis Bruice

2. Ch. 14 Overview Nuclear Magnetic Resonance (NMR) Spectroscopy Theory
Chemical shift
Splitting Patterns (N + 1 Rule)
Integration
Interpretation of 1H and 13C Spectra
13C DEPT and APT NMR
Two-Dimensional NMR Spectroscopy
Connection between NMR and MRI
X-Ray Crystallography

3. Nuclear Magnetic Resonance (NMR) Spectroscopy
Identify the carbon–hydrogen framework of an organic
compound
Certain nuclei, such as 1H, 13C, 15N, 19F, and 31P, have
non-zero value for their spin quantum number; this
property allows them to be studied by NMR

4. The spin state of a nucleus is affected by an applied
magnetic field:

5. The energy difference between the spin states increases with the strength of the applied magnetic field:

6. a-spin states
b-spin states
absorb DE
release DE
Signals detected by NMR

7. An NMR Spectrometer
In pulsed Fourier transform (FT) spectrometers, the
magnetic field is held constant, and a radio frequency (rf)
pulse of short duration excites all the protons
simultaneously

8. The electrons surrounding a nucleus decrease the effective magnetic field sensed by the nucleus:
Beffective = Bo – Blocal

9. Chemically equivalent protons: protons in the same chemical environment
Each set of chemically equivalent protons in a compound
gives rise to a signal in an 1H NMR spectrum of that
compound:

11. The Chemical Shift
The reference point of an NMR spectrum is defined by
the position of TMS (zero ppm):
The chemical shift is a measure of how far the signal is
from the reference signal
The common scale for chemical shifts = d
d =
distance downfield from TMS (Hz)
operating frequency of the spectrometer (MHz)

12. 1-bromo-2,2-dimethylpropane
1H NMR spectrum of
1-bromo-2,2-dimethylpropane
The greater the chemical shift, the higher the frequency
The chemical shift is independent of the operating
frequency of the spectrometer

14. Protons in electron-poor environments show signals at
high frequencies
Electron withdrawal causes NMR signals to appear at
higher frequency (at larger d values):

15. Characteristic Values of
Chemical Shifts

18. Diamagnetic Anisotropy
The unusual chemical shifts associated with hydrogens
bonded to carbons that form p bonds:
The p electrons are freer to move than the s electrons in
response to a magnetic field

19. The protons show signals at higher frequencies because
they sense a larger effective magnetic field:
benzene

20. The alkene and aldehyde protons also show signals at
higher frequencies:
alkene
aldehyde

21. The alkyne proton shows a signal at a lower frequency
than it would if the p electrons did not induce a magnetic
field:
alkyne

22. 1-bromo-2,2-dimethylpropane
1H NMR spectrum of
1-bromo-2,2-dimethylpropane
The area under each signal is proportional to the number
of protons giving rise to the signal:

23. Integration Line
The area under each signal is proportional to the number
of protons that give rise to that signal
The height of each integration step is proportional to the
area under a specific signal
The integration tells us the relative number of protons
that give rise to each signal, not the absolute number

25. Splitting of the Signals
An 1H NMR signal is split into N + 1 peaks, where N is
the number of equivalent protons bonded to adjacent
carbons
Coupled protons split each other’s signal
The number of peaks in a signal is called the multiplicity
of the signal
The splitting of signals, caused by spin–spin coupling,
occurs when different kinds of protons are close to one
another

26. It is not the number of protons giving rise to a signal that
determines the multiplicity of the signal
It is the number of protons bonded to the immediately
adjacent carbons that determines the multiplicity
a: a triplet
b: a quartet
c: a singlet

27. Equivalent protons do not split each other’s signal:

30. The ways in which the magnetic fields of three protons
can be aligned:

32. Long-range coupling occurs over systems, such as benzene
Splitting is observed if the protons are separated by no
more than three s bonds:
Long-range coupling occurs over systems, such as benzene

33. More Examples of 1H NMR Spectra
Triplet: two neighboring protons
Quintet: four neighboring protons

34. Triplets: two neighboring protons
Doublet: one neighboring proton
Sextet: five neighboring protons
Triplets: two neighboring protons
Septet: six neighboring protons

35. The three vinylic protons are at relatively high frequency
because of diamagnetic anisotropy

36. The signals for the Hc, Hd, and He protons overlap because the electronic effect of an ethyl substituent is similar to that of a hydrogen:

37. The signals for the Ha, Hb, and Hc protons do not overlap
because of the strong electron-withdrawing property of
the nitro group:

38. Coupling Constants
The coupling constant (J) is the distance between two
adjacent peaks of a split NMR signal in hertz:
Coupled protons have the same coupling constant

40. Summary
1. The number of chemical shifts  specify the number of proton environments in the compound
2. The chemical shift values specify the nature of the chemical environment: alkyl, alkene, etc.
3. The integration values specify the relative number of protons
4. The splitting specifies the number of neighboring protons
5. The coupling constants specify the orientation of the coupled protons

41. A Splitting Diagram for
a Doublet of Doublets

42. Complex Splitting
JAC = JAB
Triplet
JAC > JAB
Doublet of doublets

43. The trans coupling constant is greater than the cis

44. A Splitting Diagram for
a Quartet of Triplets

45. Why is the signal for Ha a quintet rather than a
triplet of triplet?

47. The Difference between a Quartet and a Doublet of Doublets
Methylene has three neighbors, appears as a quartet
Doublet
Doublet

48. When two different sets of protons split a signal, the
multiplicity of the signal is determined by using the N + 1
rule separately for each set of the hydrogens, as long as the coupling constants for the two sets are different
When the coupling constants are similar, the multiplicity
of a signal can be determined by treating both sets of
adjacent hydrogens as though they were equivalent

49. Replacing one of the enantiotopic hydrogens by a
deuterium or any other atom or group other than CH3 or
OH forms a chiral molecule:
prochiral
carbon
Ha is the pro-R-hydrogen, whereas Hb is the
pro-S-hydrogen; and they are chemically equivalent

50. Diastereotopic hydrogens have different chemical shifts:

51. Diastereotopic hydrogens are not chemically equivalent:

52. The three methyl protons are chemically equivalent
because of rotation about the C—C bond:
We see one signal for the methyl group in the 1H NMR
spectrum

53. 1H NMR spectra of cyclohexane-d11 at various temperatures:
axial
equatorial
equatorial
axial
The rate of
chair–chair
conversion is
temperature
dependent

54. Protons Bonded to Oxygen and Nitrogen
The greater the extent of the hydrogen bond, the greater the chemical shift
These protons can undergo proton exchange
They always appear as broad signals

55. pure ethanol
ethanol with acid

56. A 60-MHz 1H
NMR spectrum
A 300-MHz 1H
NMR spectrum

57. To observe well-defined splitting patterns, the difference
in the chemical shifts (in Hz) must be 10 times the
coupling constant values

58. 13C NMR Spectroscopy
The number of signals reflects the number of different
kinds of carbons in a compound.
The overall intensity of a 13C signal is about 6400 times
less than the intensity of an 1H signal.
The chemical shift ranges over 220 ppm.
The reference compound is TMS.

61. Proton-Decoupled 13C NMR of
2-Butanol

62. Proton-Coupled 13C NMR of 2-Butanol

63. The intensity of a signal is somewhat related to the
number of carbons giving rise to it
Carbons that are not attached to hydrogens give very
small signals

64. DEPT 13C NMR distinguishes CH3, CH2, and CH groups:

65. The COSY spectrum identifies protons that are coupled:
Cross peaks indicate pairs of protons that are coupled

66. COSY Spectrum of 1-Nitropropane

67. The HETCOR spectrum of 2-methyl-3-pentanone
indicates coupling between protons and the carbon to
which they are attached:

68. Unknown Identification Using Spectroscopy
Example 1: 13C-NMR of C5H9Br

69. Example 1: 1H-NMR of C5H9Br

70. Example 1: IR of C5H9Br
Answer:

71. Example 2: 13C-NMR of C6H10O4
24.1
33.4
174.4
Solvent:

72. Example 2: 1H-NMR of C6H10O4
11.97
1.50
2.21

73. Example 2: IR of C6H10O4
Answer: