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CHEM 12B
FOOTHILL COLLEGE
KLEIN: CHAPTER SIXTEEN OUTLINE
CHAPTER SIXTEEN: NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
1) Nuclear Spin and Magnetic Resonant Frequency
A) Spin Angular Momentum and Magnetic Moments
o Spin quantum number I = ½ nuclei
o Odd atomic number or odd mass number I≠0
o Spin ½ nuclei have 2 spin degenerate spin states  and 
o When nucleus is not in the presence of an external magnetic field, the spin states
have the same energy and so the spin property cannot be examined.
B) In presence of external magnetic field, spin states have different energies.
Absorption of radio frequency electromagnetic radiation causes increase in
population of higher energy spin state. This change in state can be recorded by
NMR spectroscopy.
C) Resonant Frequency: The energy difference between the spin states or the
frequency of em radiation that a given nucleus absorbs depends on its
gyromagnetic ratio  (property of nucleus which expresses its sensitivity to
magnetic field) and the strength of the external Magnetic Field
 0 = –  B0
where B0 is the strength of the external magnetic field
(1) Resonant frequency is proportional to field strength; Bigger magnet =
Higher frequency resonance (1H frequency in 1.41 T field ≈ 60 MHz; in
7.05 T field ≈ 300 MHz ≈ 10-4 kJ/mol)
(2) Larger E = Larger population difference (E=h≈ G0 = -RTln (N1/N0))
(3) Larger population difference = stronger signal
D) The resonant frequency also depends upon the local electronic environment of
any given nucleus (since electrons alter the magnetic environment) which is why
we may use resonant frequency to characterize the structure of an organic
compound.
(1) Calculating chemical shift:
where E is distance of a particular signal from TMS standard (in Hz)
and specfreq is the spectrometer frequency in MHz)
2) Interpreting the 1H NMR Spectrum
A) Chemical Shift (Section 16.5)
The resonant frequency is dependant on the extent to which nearby electrons
shield the nucleus from the external magnetic field: this is what makes NMR so
powerful as a probe of molecular structure.
i) Tables reveal resonant frequencies are characteristic of functional group
(1) Electronegative atoms deshield nearby nuclei shifting signal downfield (p
744)
(2) Anisotropic Sheilding: electrons in pi bonds create magnetic environments
that depend on the orientation to the external magnetic field (e.g.
Substituted benzene) (p 747)
B) Equivalent Hydrogens (# signals expected, Area/ Size of signal)
CHEM 12B
FOOTHILL COLLEGE
KLEIN: CHAPTER SIXTEEN OUTLINE
(1) Symmetry (Section 16.4)
(2) Diastereotopic protons (replacement test; p 737)
C) Spectroscopic Features converge to elucidate molecular structure:
i) Structure Elucidation from Molecular Formula: Degrees of Unsaturation (DU)
= Double Bond Equivalents (DBE) = Hydrogen Deficiency Index (HDI)
[(2n + 2) - # H - # X + # N)] where n = # carbons in formula (p721)
DBE =
2
ii) Number of signals: number of distinct molecular environments; reflection of
symmetry in molecule (Section 16.4)
iii) Signal position (Chemical Shift, or resonant frequency): reflection of
molecular environment
iv) Signal size (Integration/ Area): number of protons in that same environment
(Section 16.6)
v) Signal Shape (Multiplicity): number of neighboring protons (3 bonds or less
away) whose environment is different. (Section 16.7)
(1) Source of coupling and the coupling constant J
(2) Multiplicity rule #1 = n+1 where n = number of neighbors
(3) Multiplicity rule #2 = (n+1)(m+1) (p758)
where n= number of neighbors on left; m= number of neighbors on right
D) Combination Spectroscopy: Combining information from IR (and MS) with NMR
to elucidate molecular structure
E) Complications that we need to be aware of
i) What happens when two different nuclei have the nearly the same resonant
frequency?
(a) When they are not coupled
(b) When they are coupled
(c) When this is likely to happen (low magnetic fields)
ii) What happens when the nuclei are changing their magnetic environments
during the course of the NMR measurement?
(1) Acid-base exchange (p760)
(2) Slow conformational changes (p 742)
(3) Other equilibria
3) Interpreting the 13C NMR Spectrum: (Sections 16.11, 16.12)
A) Differences between 13C and 1H
B) Resonant Frequency (75 MHz in 7.05 T Field)
C) Range: 200 ppm corresponds to (200 ppm x 75 Hz/ppm) = 15,000 Hz (1H range
is ≈ (300 x 10)= 3000 Hz)
D) Coupling to other nuclei
i) 13C-13C (rare)
ii) 13C-1H (instrumentally decoupled)
E) Typical spectra are evaluated on:
i) Number of signals: symmetry
ii) Frequency of signals: functional group
iii) Size of signal gives only limited value
(small corresponds to C w/out attached protons)
CHEM 12B
FOOTHILL COLLEGE
KLEIN: CHAPTER SIXTEEN OUTLINE
4) LEARNING OUTCOMES:
A) Relate the spin quantum number I of a nucleus to the number of spin states.
B) Compare the absorption frequency of a nucleus in NMR spectroscopy to that of
other forms of spectroscopy (e.g. an electron in UV-Vis spectroscopy or IR
spectroscopy)
C) Compare the absorption frequency of the proton nucleus to carbon-13 and
phosphorus-31.
D) Predict the change in resonant frequency in the presence of a larger magnetic
field.
E) Calculate the resonant frequency in ppm given its frequency in Hz and vice-versa.
F) Determine the coupling constant J of a signal from close inspection of the NMR
spectrum and understand its relationship to the structure of the molecule.
G) Understand the limitations of a low-field spectrum based on non-first-order
coupling
H) Use the molecular formula of a molecule in combination with its NMR spectrum
to determine the structure of the molecule.
I) Assign the signals in an NMR spectrum to specific protons in the molecule
J) Predict the first-order 1H NMR spectrum of a simple molecule
CHEM 12B
FOOTHILL COLLEGE
KLEIN: CHAPTER SIXTEEN OUTLINE
5) SAMPLE EXAM PROBLEMS:
Determine the structure of an organic molecule with molecular formula C4H11N
consistent with the spectra below. Show all your work and label all signals used in your
analysis in order to receive full credit.
6)
7) Tell whether Ha and Ha’ are DIASTEREOTOPIC in each of the following compounds
(yes or no) then EXPLAIN what that means about their NMR signals:
8)
i. _____
_____
9) The coupling constant, J, between the protons of chloroethane is 7 Hz when the
spectrum is obtained at 250 MHz. What is the coupling constant between these
protons when the spectrum is acquired at 500 MHz?
10) Consider the IR and 1H NMR spectra of an unknown with the molecular formula
C5H12O.
Determine the structure of the unknown and ASSIGN the 1H NMR signals along with any
signals in the IR used to formulate your answer.
CHEM 12B
FOOTHILL COLLEGE
KLEIN: CHAPTER SIXTEEN OUTLINE
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