Structure Determination:
Nuclear
Magnetic Resonance Spectroscopy
Goals
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After this chapter you should be able to
n Predict
relative chemical shifts
n Analyze
13C NMR to determine structure
n Analyze
1H NMR to determine structure
n Use
the different forms of spectroscopy to solve structural conundrums.
Nuclear Spin:
The I Quantum Number
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This angular momentum is characterized by a nuclear
spin quantum number, I such that,
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I = 1/2n, where n is an integer
0,1,2,3...etc.
Which Nuclei Have Spin?
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Those nuclei for which I = 0 do not possess spin
angular momentum and do not exhibit magnetic resonance phenomena. The nuclei of
12C and 16O fall into this category.
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Nuclei for which I = 1/2
include 1H, 19F, 13C, 31P and 15N
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2H and 14N have I = 1.
Spin Causes Tiny Magnetic Fields Around Nuclei
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Nuclei with ˝ spin align with or against in external
magnetic field.
Without B
o With B
o
Energy Difference Between Different Spin States
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The energy difference between the spin states depends upon the magnitude of the
B
o Field.
Small Bo
Medium Bo Large Bo
Where DE=hn
Nuclear Magnetic Resonance
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When electromagnetic energy applied to the nuclei in
the magnetic field is equal to the energy difference in the spin states, the
nuclei in lower energy state jump to the higher state AND fall back to the
lower state releasing electromagnetic energy equal to the incident radiation.
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This phenomenon is called Nuclear magnetic resonance.
Shielding
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Nuclei experience different total magnetic field due to
the shielding effect of electrons in the local environment.
Beffective = Bo
- Blocal
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Blocal is affected by:
n Electron
withdrawing by electronegative elements
n Adjacent
nuclei having various spins
13C NMR
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12C has no NMR (even neutrons and
protons)
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13C NMR is used to detect the different
kinds of carbons in a compound.
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Only 1.1% of carbons are 13C and therefore
the signal strength is very low.
n Multiple
pass is required
n From
FT averaging, the signal emerges
Chemical Shift Correlations
13C NMR
Analyze This 13C NMR for C3H6O2
Analyze Other 13C NMR
1H NMR
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1H nuclei exhibit NMR.
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1H NMR is used to determine how equivalent
hydrogen atoms exist in a molecule.
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The number of each type of hydrogen can also be
determined.
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Structure can be deduced from high resolution NMR by
examining spin-spin coupling from adjacent hydrogens.
1H NMR Methyl Acetate
How Many Different Hydrogens?
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Ethyl Acetate
For Ethyl Acetate
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The
area under the NMR resonance is proportional to the number of hydrogens which
that resonance represents.
Quiz So Far
Spin-Spin Coupling
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An equivalent set of hydrogens will feel the magnetic
fields from hydrogen’s attached to an adjacent carbon.
Spin-Spin Splitting
Why Does the Chemical Shift Split
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CH2 hydrogens can orient in three ways
giving three different shifts to the CH3 hydrogens.
Why Does the Chemical Shift Split
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CH3 hydrogens can orient in four ways giving
three different shifts to the CH2 hydrogens.
Why Does the Chemical Shift Split
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Hydrogens more than one carbon away have a negligible
effect on the local Bo field.
Some Practice to Cement It
General Spectroscopy Problems
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When you go to the following site, click on “Problems”
in the upper left-hand corner then click on the problem you want to do.
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Remember to walk before you run…
