CHAPTER 13
Molecular Structure
by Nuclear Magnetic Resonance (NMR)
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Nuclear Magnetic Resonance Spectroscopy
exploits a property called Nuclear Spin
Any atomic nucleus that has an odd mass, an odd atomic number, or
both is a nucleus that behaves like it is spinning.
Just like for electrons, The 1H nucleus has two spin states which in a
magnetic field can have two different energies.
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Nuclei that possess Spin Angular Momentum have quantized states
whose energies are affected by an Applied Magnetic Field
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The stronger the Applied Magnetic Field the larger the energy
difference between the Spin States of the Nucleus
14,000 G (1.4 T)
60 MHz
70,500 G (7.0 T)
300 MHz
The actual energy difference is small. At 300 MHz, the energy
difference for a proton is about 3 x 10-5 kcal mol-1.
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When the protons are placed in a homogenous magnetic field and a
pulse of the correct energy is applied, the small excess of nuclei with the
lower energy spin state absorb energy and the change in spin state is
detected by the spectrometer
Image from www2.chemistry.msu.edu
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When electromagnetic radiation having the same energy
as the energy difference between the spin states strikes
the nucleus, the electromagnetic radiation is absorbed and
the slight excess of nuclei in the  state is reduced.
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NMR Schematic
Figure 14.1
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Proton nuclei in different electronic environments show different resonant
frequencies. The resonant frequencies are plotted as relative frequencies
called “chemical shifts” d.
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CHEMICAL SHIFT (d):
the shift in ppm of an NMR signal from the signal of TMS
dA = ( nA – nTMS) ÷ Operating frequency in MHz
Tetramethyl silane (TMS)
d0.9
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Why do different electronic environments lead to different signals?
e– density near the nucleus
decreases the E between its
spin states so its resonant
frequency decreases
Lower freq = upfield
Shift to the RIGHT = “UPFIELD” = smaller
E felt by nucleus = more shielded
Shift to the LEFT = “DOWNFIELD” = larger
E felt by nucleus= deshielded
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e- SHEILD NUCLEI FROM EXTERNAL MAGNETIC FIELD
Shielding: e– density near the nucleus decreases the E between its spin
states so its resonant frequency decreases
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Features of 1H NMR Spectrum:
1. Position of Signals on x-axis: Relative frequency of absorption:
CHEMICAL SHIFT
Info about electronic environment
2. Number of Signals
Magnetic EQUIVALENCE
Info about number of unique electronic environments (symmetry)
3. Size (Area) of signals:
INTEGRATION
Info about number of nuclei in each environment
4. Splitting Pattern of Signals
Info about number of neighboring nuclei
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Chemical Shift:
Characteristic of neighboring Functional groups
(e- shielding)
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The 9 protons on the three methyl groups are all in the same
electronic environment on average, but the methylene and
hydroxyl protons are unique.
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Features of NMR Spectrum:
Number of unique electronic environments
≤ number of chemically distinct proton sites on the molecule.
How many distinct environments are there for the protons on each of
the following molecules?
3 signals
5 signals
4 signals
For a compound with the molecular formula C4H10, how
many isomers are there?
How many chemically distinct environments are there
in each case?
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The number of distinct NMR signals may alone allow one to
determine the chemical structure given the molecular
formula.
Draw all isomers of C2H6O and tell how many unique
electronic environments each has
d3.3
d1.2
d3.6
d4.7
Draw all isomers of C4H10, and tell how many
unique electronic environments are there in
each case?
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Both isomers of C4H10 have two signals, but their
relative intensities and shapes differ
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Features of NMR Spectrum: Relative Size/Area of Signals
AREA under an NMR peak is proportional to the number of equivalent
nuclei contributing to the peak.
Electronic integration gives relative number of H’s only.
Use Molecular formula to detn absolute number of H’s responsible
for each signal:
Total #HA = (areaA/areaTot)• (Total #H’s in formula)
Total # H= 12
# H’s at d2.0 =[23/(23+67)]12 = 3
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Features of NMR Spectrum: SHAPE of signals
Multiplicity (#peaks to which each signal is split) depends on the
number of H’s at adjacent carbon(s)
Multiplicity = (#neighbors + 1)
If multiplicity =2; # neighbors = 1
multiplicity =3, # neighbors =2
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Features of an NMR spectrum: SHAPE
Spin Coupling: Neighboring nuclei “split” NMR signals
Usually n neighbors splits the signal into n+1 peaks
Multiplicity = n+1
Which Isomer of C5H10O gives rise to this spectrum?
Triplet => 2 neighbors
Quartet means the
nuclei responsible for
this signal have 3 H’s
“next door”
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