Nuclear Magnetic Resonance Spectroscopy

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Nuclear Magnetic Resonance Spectroscopy
Related to MRI in medicine (Magnetic Resonance
Imaging)
Most important spectroscopic technique in organic
chemistry
Reveals
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l ? 1-3 m (radio waves)
n 200, 300, 500, 600, MHz (radio frequency,
rf)
NMR involves only nuclei with odd mass and/or
odd atomic number. Table 13.1
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1H and 13C most important
Such nuclei have a spin
Spinning charged nucleus acts as a tiny magnet
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See Figure 13.1
Applied magnetic field Specific orientation
Random orientation (normal)
Parallel Lower E Antiparallel Higher E
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Spinning nucleus circles at some frequency, n
When precisely the right amount of energy is
applied, the nucleus can spin flip from lower
to higher energy state.
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Radio Frequency
(absorbed energy)
B0
B0
Nucleus is said to be in resonance
Absorbed energy registers as a signal (peak)
Size of DE depends on strength of B0
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See Figure 13.2
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Radio frequency and B0 determine DE.
Spectrum can be obtained two ways
1) Hold B0 constant and vary rf
or
2) Hold rf constant and vary B0
(Done in practice)
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See Figure 13.4
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Amount of B0 required to flip nucleus depends
on
1) Strength of applied rf
2) Type of nucleus (1H , 13C, etc.)
1H NMR (most common)
For any given radio frequency not all 1H flip at
exactly the same B0
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YH
Cloud of electron density around the 1H nucleus
shields it from the applied magnetic field, B0
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Nuclei in different environments are shielded
different amounts.
HH
ClH
Deshielded 1H
Shielded 1H
Each kind of 1H flips at a different B0
producing a different peak. 1H NMR spectrum shows
how many different 1H environments are present.
How?
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How many different 1H NMR signals are expected
for each compound below?
CH3CH2CH3

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How many different 1H NMR signals are expected?
In theory there will be signals in the 1H NMR
spectrum
but ...
accidental superimposition is common. May be
fewer than signals visible.
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An unidentified compound has the formula C3H6 and
shows only one 1H NMR signal. Propose a structure.
Degree of unsaturation
1H environments
1H environment
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Differences between B0 required for various 1H
types called
Expressed as
Compared to a standard defined as zero
Low electronegativity of Si makes methyl groups
strongly shielded.
Tetramethylsilane (TMS)
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Values of d are characteristic for various 1H
environments.
Tables available but normally not needed. Know
trends
Chemical shifts can indicate the
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See Figure 13.5
Y O,N,X
Allylic, Benzylic,
Table 13.2
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Table 13.3 (See also 1H NMR regions. Table 13.2)
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Signal intensity (area) is proportional to
See Figure 13.12
Integration shows ratios of number of 1H of each
type. 26 (integration) 13 (ratio) 39
(actual count)
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1H NMR signals split by neighboring 1H
1H can feel the spin state of adjacent 1H.
Signals are coupled
or
or
Spins can be the same or different. Equal
probability
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Without splitting would be
a
b
TMS d 0
But each 1H coupled to the other. Signals for a
and b appear as doublets
a
b
TMS d 0
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Distance between legs of split peak called
For coupled signals a and b, abbreviated Ja-b
Expressed in Hz
Ja-b Jb-a
Jb-a
Ja-b
a
b
d 0
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Consider Ha split by two Hb
Ha
Hb
Probability
1
2
1
Ha appears as a triplet with 121 intensity
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See Figure 13.14
BrCH2CH3
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Figure 13.13
In general a signal split by n equivalent
neighbors appears as a multiplet with
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See Table 13.4
Intensities of multiplet legs predicted by
Pascals triangle
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Summary of 1H NMR. What can be learned?
1) Number of different 1H environments from
2) Approximate type of environment for each from
3) Relative number of 1H of each type from
4) Number of neighboring 1H from
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Fig 13.15
Fig 13.16
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Figure 13.18
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Figure 13.19 Figure 13.20
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13C NMR Similar to 1H NMR in some respects
1) Number of peaks indicates the number of
different carbon environments
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(No Transcript)
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2) Chemical shifts can indicate type of
environments.
Downfield for 13C near electronegative groups.
Can have anisotropy from aromatic ring, CO, etc.
TMS d 0
Chemical shift range 0 - 220 ppm
See Figure 13.7
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13C NMR different from 1H NMR in some ways
1) Peak intensities do not, in general,
correspond to the number of nuclei.
Integration is possible but rarely done.
2) 13C 1 natural abundance. Two adjacent is
rare. Coupling between neighboring 13C not
observed
3) 13C less sensitive and less abundant than 1H
so spectra take longer to obtain.
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Fig 13.6. single acquisition HOCH2CH2CH2CH2CH3
200 aquisitions
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Figure 13.8
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See Figure 13.11
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See problem 13.56
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How can one know the number of H on each 13C?
Distortionless Enhancement by Polarization
Transfer
DEPT - Involves three experiments (Section 13.6)
Normal mode
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Figure 13.10