Chapter 13
Structure
Determination:
Nuclear
Magnetic
Resonance
Spectroscopy
Suggested Problems –
1-23,29,34,36-8,44-7,51
CHE2202, Chapter 13
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Nuclear Magnetic Resonance
Spectroscopy
• Nuclei are positively charged, have spin, and
interact with an external magnetic field, B0
• Magnetic rotation of nuclei is random in the
absence of a magnetic field
– In the presence of a strong magnet, nuclei adopt
specific orientations
CHE2202, Chapter 13
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Nuclear Magnetic Resonance
Spectroscopy
• When exposed to a certain frequency of
electromagnetic radiation, oriented nuclei
absorb energy which causes a spinflip from a
state of lower energy to higher energy
– Nuclear magnetic resonance - Nuclei are in
resonance with applied radiation
• Frequency that causes resonance depends
on:
– Strength of external magnetic field
– Identity of the nucleus
– Electronic environment of the nucleus
CHE2202, Chapter 13
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Nuclear Magnetic Resonance
Spectroscopy
• Larmor equation
– Relation between
• Resonance frequency of a nucleus
• Magnetic field and the magnetogyric ratio of the
nucleus
 γ 
ν =   B0
 2π 
CHE2202, Chapter 13
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Worked Example
• Calculate the amount of energy required to
spin-flip a proton in a spectrometer operating
at 300 MHz
– Analyze if the increase of spectrometer
frequency from 200 MHz to 300 MHz
increases or decreases the amount of energy
necessary for resonance
CHE2202, Chapter 13
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Worked Example
• Solution:
8
c 3.0 × 10 m / s
λ= =
; ν = 300 MHz = 3.0 ×108 Hz
ν
ν
8
c 3.0 × 10 m / s
λ= =
= 1.0 m
8
ν
3.0 × 10 Hz
1.20× 10-4 kJ / mol
E=
= 1.20 ×10-4 kJ / mol
1.0
– Increasing the spectrometer frequency from
200 MHz to 300 MHz increases the amount of
energy needed for resonance
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The Nature of NMR Absorptions
• Absorption frequencies differ across 1H and
13C molecules
• Shielding: Opposing magnetic field produced
by electrons surrounding nuclei to counteract
the effects of an external magnetic field
– Effect on the nucleus is lesser than the applied
magnetic field
– Beffective = Bapplied – Blocal
– Individual variances in the electronic environment
of nuclei leads to different shielding intensities
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NMR Spectrum of 1H and 13C
CHE2202, Chapter 13
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Working of an NMR Spectrometer
• Organic sample dissolved in a suitable solvent is
placed in a thin glass tube between the poles of
a magnet
• 1H and 13C nuclei respond to the magnetic field
by aligning themselves to one of the two
possible orientations followed by rf irradiation
• Varying the strength of the applied field causes
each nucleus to resonate at a slightly varied field
strength
• Absorption of rf energy is monitored by a
sensitive detector that displays signals as a peak
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Operation of a Basic NMR
Spectrometer
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NMR Spectrometer
• Time taken by IR spectroscopy is about 10–13 s
• Time taken by NMR spectroscopy is about 10–3 s
– Provides a blurring effect that is used in the
measurement of rates and activation energies of vary
fast processes
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Worked Example
• Explain why 2-chloropropene shows signals
for three kinds of protons in its 1H NMR
spectrum
• Solution:
– 2-Chloropropene has three kinds of protons
• Protons b and c differ because one is cis to the
chlorine and the other is trans
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The Chemical Shift
• The left segment of the chart is the
downfield
– Nuclei absorbing on the downfield have less
shielding as they require a lower field for
resistance
• The right segment is the upfield
– Nuclei absorbing on the upfield have more
shielding as they require a higher field
strength for resistance
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The NMR Chart
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The Chemical Shift
• Chemical shift is the position on the chart at
which a nucleus absorbs
• The delta (δ) scale is used in calibration of
the NMR chart
– 1 δ = 1 part-per-million of the spectrometer
operating frequency
Observed chemical shift (number of Hz away from TMS)
δ=
Spectrometer frequency in MHz
– The delta scale is used as the units of
measurement can be used to compare values
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across other instruments
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Worked Example
• The 1H NMR peak of CHCl3 was recorded on
a spectrometer operating at 200 MHz
providing the value of 1454 Hz
– Convert 1454 Hz into δ units
• Solution:
Observed chemical shift (number of Hz away from TMS)
δ=
Spectrometer frequency in MHz
1454Hz
δ=
= 7.27 δ for CHCl3
200MHz
CHE2202, Chapter 13
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Chemical Shifts in 1H NMR
Spectroscopy
• Chemical shifts are due to the varied
electromagnetic fields produced by electrons
surrounding nuclei
• Protons bonded to saturated, sp3-hybridized
carbons absorb at higher fields
• Protons bonded to sp2-hybridized carbons
absorb at lower fields
• Protons bonded to electronegative atoms
absorb at lower fields
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Regions of the 1H NMR Spectrum
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Representative Chemical Shifts
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Worked Example
• CH2Cl2 has a single 1H NMR peak
– Determine the location of absorption
• Solution:
• For CH2Cl2 , δ = 5.30
– The location of absorption are the protons
adjacent to the two halogens
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Integration of 1H NMR Absorptions:
Proton Counting
• In the figure, the peak caused by (CH3)3C–
protons is larger than the peak caused by –
OCH3 protons
• Integration of the area under the peak can be
used to quantify the different kinds of protons
CHE2202, Chapter 13
in a molecule
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Worked Example
• Mention the number of peaks in the 1H NMR
spectrum of 1,4-dimethyl-benzene (paraxylene or p-xylene)
– Mention the ratio of peak areas possible on
integration of the spectrum
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Worked Example
• Solution:
– There are two absorptions in the 1H NMR
spectrum of p-xylene
– The four ring protons absorb at 7.05 δ and the
six methyl-groups absorb at 2.23 δ
– The peak ratio of methyl protons:ring protons
is 3:2
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Worked Example
CHE2202, Chapter 13
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Worked Example
• How many distinct signals are present in the
13C NMR of p-xylene?
– Mention the ratio of peak areas possible on
integration of the spectrum
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Worked Example
CHE2202, Chapter 13
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Spin-Spin Splitting in 1H NMR
Spectra
• Multiplet: Absorption of a proton that splits
into multiple peaks
– The phenomenon is called spin-spin splitting
– Caused by coupling of neighboring spins
CHE2202, Chapter 13
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Spin-Spin Splitting in 1H NMR
Spectra
• Alignment of both –CH2Br proton spins with
the applied field will result in:
– Slightly larger total effective field and slight
reduction in the applied field to achieve resonance
• There is no effect if one of the –CH2Br proton
spins aligns with the applied field and the
other aligns against it
• Alignment of both –CH2Br proton spins
against the applied field results in:
– Smaller effective field and an increased applied
field to achieve resonance
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The Origin of Spin-Spin Splitting in
Bromoethane
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Spin-Spin Splitting in 1H NMR
Spectra
• n + 1 rule: Protons that exhibit n + 1 peaks in
the NMR spectrum possess
– n = number of equivalent neighboring protons
• Coupling constant is the distance between
peaks in a multiplet
CHE2202, Chapter 13
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Spin-Spin Splitting in 1H NMR
Spectra
• It is possible to identify multiplets in a
complex NMR that are related
– Multiplets that have the same coupling constant
can be related
– Multiplet-causing protons are situated adjacent to
each other in the molecule
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Rules of Spin-Spin Splitting
• Chemically equivalent protons do not
show spin-spin splitting
• The signal of a proton with n equivalent
neighboring protons is split into a multiplet
of n + 1 peaks with a coupling constant
• Two groups of protons coupled together
have the same coupling constant, J
CHE2202, Chapter 13
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Worked Example
• The integrated 1H NMR spectrum of a
compound of formula C4H10O is shown
below
– Propose a structure
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Worked Example
• Solution:
– The molecular formula (C4H10O) indicates that
the compound has no multiple bonds or rings
– The 1H NMR spectrum shows two signals,
corresponding to two types of hydrogens in
the ratio 1.50:1.00, or 3:2
– Since the unknown contains 10 hydrogens,
four protons are of one type and six are of the
other type
– The upfield signal at 1.22 δ is due to
saturated primary protons
CHE2202, Chapter 13
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Worked Example
– The downfield signal at 3.49 δ is due to
protons on carbon adjacent to an
electronegative atom - in this case, oxygen
– The signal at 1.23 δ is a triplet, indicating two
neighboring protons
– The signal at 3.49 δ is a quartet, indicating
three neighboring protons
– This splitting pattern is characteristic of an
ethyl group
– The compound is diethyl ether,
CH3CH2OCH2CH3
CHE2202, Chapter 13
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1H
NMR Spectroscopy and Proton
Equivalence
• Proton NMR is much more sensitive than 13C and
the active nucleus (1H) is nearly 100 % of the
natural abundance
• Proton NMR shows how many kinds of
nonequivalent hydrogens are in a compound
• Theoretical equivalence can be predicted by
comparing structures formed by replacing each H
with X and determining the number of different
compounds
• Equivalent H’s have the same signal while
nonequivalent H’s give different signals
CHE2202, Chapter 13
– There are degrees of nonequivalence
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1H
NMR Spectroscopy and Proton
Equivalence
• One use of 1H NMR is to ascertain the
number of electronically non-equivalent
hydrogens present in a molecule
• In relatively small molecules, a brief look at
the structure can help determine the kinds of
protons present and the number of possible
NMR absorptions
– Equivalence or nonequivalence of two protons
can be determined by comparison of
structures formed if each hydrogen were
replaced by an X group
CHE2202, Chapter 13
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1H
NMR Spectroscopy and Proton
Equivalence
• Possibilities
– If the protons are chemically unrelated and
non-equivalent, the products formed by
substitution would be different constitutional
isomers
CHE2202, Chapter 13
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1H
NMR Spectroscopy and Proton
Equivalence
– If the protons are chemically identical, the
same product would be formed despite the
substitution
CHE2202, Chapter 13
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1H
NMR Spectroscopy and Proton
Equivalence
– If the hydrogens are homotopic but not
identical, substitution will form a new chirality
center
• Hydrogens that lead to formation of enantiomers
upon substitution with X are called enantiotopic
CHE2202, Chapter 13
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1H
NMR Spectroscopy and Proton
Equivalence
– If the hydrogens are neither homotopic nor
enantiotopic, substitution of a hydrogen at C3
would form a second chirality center
CHE2202, Chapter 13
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Worked Example
• How many absorptions will (S)-malate, an
intermediate in carbohydrate metabolism
have in its 1H NMR spectrum? Explain
CHE2202, Chapter 13
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Worked Example
• Solution:
– Because (S)-malate already has a chirality
center(starred), the two protons next to it are
diastereotopic and absorb at different values
– The 1H NMR spectrum of (S)-malate has four
absorptions
CHE2202, Chapter 13
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More Complex Spin-Spin Splitting
Patterns
• Some hydrogens in a molecule possess
accidentally overlapping signals
– In the spectrum of toluene (methylbenzene),
the five aromatic ring protons produce a
complex, overlapping pattern though they are
not equivalent
CHE2202, Chapter 13
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More Complex Spin-Spin Splitting
Patterns
• Splitting of a signal by two or more
nonequivalent kinds of protons causes a
complication in 1H NMR spectroscopy
CHE2202, Chapter 13
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Tree Diagram for the C2 proton of
trans-cinnamaldehyde
CHE2202, Chapter 13
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Worked Example
• 3-Bromo-1-phenyl-1-propene shows a
complex NMR spectrum in which the vinylic
proton at C2 is coupled with both the C1
vinylic proton (J = 16 Hz) and the C3
methylene protons (J = 8 Hz)
– Draw a tree diagram for the C2 proton signal and
account for the fact that a five-line multiplet is
observed
CHE2202, Chapter 13
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Worked Example
• Solution:
– C2 proton couples with vinylic proton (J = 16)
Hz
• C2 proton’s signal is split into a doublet
– C2 proton also couples with the two C3
protons (J = 8 Hz)
• Each leg of the C2 proton doublet is split into a
CHE2202, Chapter 13
triplet to produce a total of six lines
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Worked Example
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Uses of 1H NMR Spectroscopy
• The technique is used to identify likely
products in the laboratory quickly and
easily
– NMR can help prove that hydroborationoxidation of alkenes occurs with nonMarkovnikov regiochemistry to yield the less
highly substituted alcohol
CHE2202, Chapter 13
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1H
NMR Spectra of
Cyclohexylmethanol
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Worked Example
• Mention how 1H NMR is used to determine
the regiochemistry of electrophilic addition
to alkenes
– Determine whether addition of HCl to 1methylcyclohexene yields 1-chloro-1methylcyclohexane or 1-chloro-2methylcyclohexane
CHE2202, Chapter 13
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Worked Example
• Solution:
– Referring to 1H NMR methyl group absorption
• The unsplit methyl group in the left appears as a
doublet in the product on the right
• Bonding of a proton to a carbon that is also
bonded to an electronegative atom causes a
downfield absorption in the 2.5–4.0 region
– 1H NMR spectrum of the product would confirm
the product to be 1-chloro-1-methylcyclohexane
CHE2202, Chapter 13
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13C
NMR Spectroscopy: Signal
Averaging and FT–NMR
• Carbon-13 is the only naturally occurring
carbon isotope that possesses a nuclear
spin, but its natural abundance is 1.1%
• Signal averaging and Fourier-transform NMR
(FT–NMR) help in detecting carbon 13
• Due to the excess random electronic
background noise present in 13C NMR, an
average is taken from hundreds or thousands
of individual NMR spectra
CHE2202, Chapter 13
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Carbon-13 NMR Spectra of 1Pentanol
CHE2202, Chapter 13
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13C
NMR Spectroscopy: Signal
Averaging and FT–NMR
• Spin-spin splitting is observed only in 1H
NMR
– The low natural abundance of 13C nucleus is
the reason that coupling with adjacent
carbons is highly unlikely
– Due to the broadband decoupling method
used to record 13C spectra, hydrogen coupling
is not seen
CHE2202, Chapter 13
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Characteristics of 13C NMR
Spectroscopy
•
13C
NMR provides a count of the different
carbon atoms in a molecule
• 13C resonances are 0 to 220 ppm downfield
from TMS
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Characteristics of 13C NMR
Spectroscopy
• General factors that determine chemical
shifts
– The electronegativity of nearby atoms
– The diamagnetic anisotropy of pi systems
– The absorption of sp3-hybridized carbons and
sp2 carbons
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Carbon-13 Spectra of 2-butanone
and para-bromoacetophenone
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Worked Example
• Classify the resonances in the 13C
spectrum of methyl propanoate,
CH3CH2CO2CH3
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Worked Example
• Solution:
– Methyl propanoate has four unique carbons
that individually absorb in specific regions of
the 13C spectrum
CHE2202, Chapter 13
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DEPT 13C NMR Spectroscopy
• DEPT-NMR (distortionless enhancement
by polarization transfer)
• Stages of a DEPT experiment
– Run a broadband-decoupled spectrum
– Run a DEPT-90
– Run a DEPT-135
• The DEPT experiment manipulates the
nuclear spins of carbon nuclei
CHE2202, Chapter 13
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DEPT-NMR Spectra for 6-methyl-5hepten-2-ol
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Uses of 13C NMR Spectroscopy
• Helps in determining molecular structures
– Provides a count of non-equivalent carbons
– Provides information on the electronic
environment of each carbon and the number of
attached protons
• Provides answers on molecule structure that
IR spectrometry or mass spectrometry cannot
provide
CHE2202, Chapter 13
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13C
NMR Spectrum of 1methylcyclohexane
CHE2202, Chapter 13
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Worked Example
• Propose a structure for an aromatic
hydrocarbon, C11H16, that has the
following 13C NMR spectral data:
– Broadband decoupled: 29.5, 31.8, 50.2,
125.5, 127.5, 130.3, 139.8 δ
– DEPT-90: 125.5, 127.5, 130.3 δ
– DEPT-135: positive peaks at 29.5, 125.5,
127.5, 130.3 δ; negative peak at 50.2 δ
CHE2202, Chapter 13
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Worked Example
• Solution:
– Calculate the degree of unsaturation of the
unknown compound
• C11H16 has 4 degrees of unsaturation
– Look for elements of symmetry
• 7 peaks appearing in the 13C NMR spectrum
indicate a plane of symmetry
– According to the DEPT-90 spectrum, 3 of the
kinds of carbons in the aromatic ring are CH
carbons
CHE2202, Chapter 13
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Worked Example
– The unknown structure is a monosubstituted
benzene ring with a substituent containing
CH2 and CH3 carbons
CHE2202, Chapter 13
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How Many Signals?
Each set of chemically equivalent protons give a signal in the
1H NMR spectrum.
CHE2202, Chapter 13
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How Many Signals?
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How Many Signals?
CHE2202, Chapter 13
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Relative Positions of the Signals
Protons in electron-poor environments
show signals at high frequencies.
Electron withdrawal causes NMR signals to appear at
a higher frequency (at a larger d value).
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Relative Positions of the Signals
The closer the electronegative the atom (or group),
the more it deshields the protons.
CHE2202, Chapter 13
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Where 1H NMR Signals Appear
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Where They Show a Signal
Methine protons appear at higher frequency than methylene protons,
which appear at a higher frequency than methyl protons.
CHE2202, Chapter 13
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60-MHz Versus 300-MHz
a 60-MHz
1H NMR spectrum
a 300-MHz
1H NMR spectrum
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Where 13C NMR Signals Appear
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A DEPT 13C NMR Spectrum
(Distinguishes CH3, CH2, and CH Groups)
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Two-Dimensional NMR Spectroscopy
(A COSY Spectrum)
Cross peaks indicate pairs of protons that are coupled.
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A COSY Spectrum of 1-Nitropropane
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A HETCOR Spectrum of
Ethyl Isopropyl Ketone
A HETCOR spectrum of indicates coupling between protons
and the carbon to which they are attached. CHE2202, Chapter 13
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Nuclear Magnetic Resonance (NMR)
Magnetic Resonance Imaging (MRI)
an MRI scanner
CHE2202, Chapter 13
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An MRI
The white region is a brain lesion.
The spectrum indicates an abscess.
CHE2202, Chapter 13
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