1. Case Western Reserve University
Chapter 14
NMR Spectroscopy
Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall

2. Nuclear Magnetic Resonance (NMR) Spectroscopy
Identify the carbon–hydrogen framework of an organic
compound
Certain nuclei such as 1H, 13C, 19F, and 31P have
allowed spin states of +1/2 and –1/2; this property allows
them to be studied by NMR

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

4. The energy difference between the two spin states
depends on the strength of the magnetic field

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

6. An NMR Spectrometer

7. The electrons surrounding a nucleus affect the effective
magnetic field sensed by the nucleus

8. 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

13. The chemical shift is independent of the operating
frequency of the spectrometer

14. Electron withdrawal causes NMR signals to appear at
higher frequency (at larger d values)

15. Characteristic Values of
Chemical Shifts

17. 1-bromo-2,2-dimethylpropane
1H NMR spectrum of
1-bromo-2,2-dimethylpropane

18. 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 absolute number

19. Diamagnetic Anisotropy
The p electrons are less tightly held by the nuclei than are s electrons; they are more free to move in response to a magnetic field
Causes unusual chemical shifts for hydrogen bonded to
carbons that form p bonds

20. 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

21. 1H NMR Spectrum of
1,1-Dichloroethane

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

25. Splitting is observed if the protons are separated by more
than three s bonds
Long-range coupling occurs when the protons are
separated by more than three bonds and one of the
bonds is a double or a triple bond

27. More Examples of 1H NMR Spectra

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

30. The Difference between a Quartet and a Doublet of Doublets

31. The signals for the Hc, Hd, and He protons overlap

32. The signals for the Ha, Hb, and Hc protons do not overlap

33. 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

35. The trans coupling constant is greater than the cis

36. A Splitting Diagram for
a Doublet of Doublets

37. A Splitting Diagram for
a Quartet of Triplets

38. 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 when the
coupling constants for the two sets are different
When the coupling constants are similar, the N + 1 rule
can be applied to both sets simultaneously

39. The three methyl protons are chemically equivalent due
to rotation about the C–C bond
We see one signal for the methyl group in the 1H NMR
spectrum

40. 1H NMR spectra of cyclohexane-d11 at various temperatures
axial
equatorial
equatorial
axial
the rate of
chair–chair
conversion is
temperature
dependent

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

42. dry ethanol
ethanol with acid

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

44. 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

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

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

48. DEPT 13C NMR distinguish among CH3, CH2, and CH
groups

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

50. COSY Spectrum of 1-Nitropropane

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