h-nmr 2012
TRANSCRIPT
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Nuclear Magnetic
Resonance Spectroscopy
Ref: Organic Chemistry, 5th EditionL. G. Wade, Jr.
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2
Introduction
• NMR is the most powerful tool available for
organic structure determination.
• It is used to study a wide variety of nuclei:
1H
13C
15N
19F
31P
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3
Nuclear Spin• A nucleus with an odd atomic number or
an odd mass number has a nuclear spin.• The spinning charged nucleus generates
a magnetic field.
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4
External Magnetic Field
When placed in an external field, spinningprotons act like bar magnets.
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5
Two Energy States
The magnetic fields of
the spinning nucleiwill align either withthe external field, oragainst the field.
A photon with the rightamount of energycan be absorbed
and cause thespinning proton toflip. =>
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6
ΔE and Magnet Strength• Energy difference is proportional to the
magnetic field strength.• ΔE = hν = γ h B0
2π
• Gyromagnetic ratio, γ, is a constant foreach nucleus (26,753 s-1gauss-1 for H).
• In a 14,092 gauss field, a 60 MHzphoton is required to flip a proton.
• Low energy, radio frequency. =>
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7
Magnetic Shielding• If all protons absorbed the same
amount of energy in a given magneticfield, not much information could be
obtained.
• But protons are surrounded by electrons
that shield them from the external field.
• Circulating electrons create an inducedmagnetic field that opposes the external
magnetic field. =>
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Chapter 13 8
Shielded ProtonsMagnetic field strength must be increased
for a shielded proton to flip at the samefrequency.
=>
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Chapter 13 9
Protons in a Molecule
Depending on their chemical
environment, protons in a molecule are
shielded by different amounts.
=>
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Chapter 13 10
NMR Signals
• The number of signals shows how many
different kinds of protons are present.
• The location of the signals shows how
shielded or deshielded the proton is.• The intensity of the signal shows the
number of protons of that type.
• Signal splitting shows the number of
protons on adjacent atoms. =>
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Chapter 13 11
The NMR Spectrometer
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Chapter 13 12
The NMR Graph
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Chapter 13 13
Tetramethylsilane
• TMS is added to the sample.
• Since silicon is less electronegativethan carbon, TMS protons are highlyshielded. Signal defined as zero.
• Organic protons absorb downfield (to
the left) of the TMS signal.
=>
Si
CH3
CH3
CH3
H3C
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Chapter 13 14
Chemical Shift
• Measured in parts per million.• Ratio of shift downfield from TMS (Hz)
to total spectrometer frequency (Hz).
• Same value for 60, 100, or 300 MHz
machine.
• Called the delta scale.=>
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Chapter 13 15
Delta Scale
=>
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Chapter 13 16
Location of Signals• More electronegative
atoms deshield more andgive larger shift values.
• Effect decreases with
distance.• Additional electronegative
atoms cause increase in
chemical shift.=>
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Chapter 13 17
Typical Values
=>
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Chapter 13 18
Aromatic Protons, δ7-δ8
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Chapter 13 19
Vinyl Protons, δ5-δ6
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Chapter 13 20
Acetylenic Protons, δ2.5
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Chapter 13 21
Aldehyde Proton, δ9-δ10
=>
Electronegative
oxygen atom
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Chapter 13 22
O-H and N-H Signals
• Chemical shift depends on concentration.• Hydrogen bonding in concentrated
solutions deshield the protons, so signal
is around δ3.5 for N-H and δ4.5 for O-H.
• Proton exchanges between the molecules
broaden the peak. =>
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Chapter 13 23
Carboxylic Acid
Proton, δ10+
=>
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Chapter 13 24
Number of SignalsEquivalent hydrogens have the same
chemical shift.
=>
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Chapter 13 25
Intensity of Signals• The area under each peak is
proportional to the number of protons.
• Shown by integral trace.
=>
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Chapter 13 26
How Many Hydrogens?When the molecular formula is known,
each integral rise can be assigned to aparticular number of hydrogens.
=>
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Chapter 13 27
Spin-Spin Splitting
• Nonequivalent protons on adjacent carbons
have magnetic fields that may align with oroppose the external field.
• This magnetic coupling causes the protonto absorb slightly downfield when the
external field is reinforced and slightly
upfield when the external field is opposed.• All possibilities exist, so signal is split. =>
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Chapter 13 28
1,1,2-TribromoethaneNonequivalent protons on adjacent carbons.
=>
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Chapter 13 29
Doublet: 1 Adjacent Proton
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Chapter 13 30
Triplet: 2 Adjacent Protons
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Chapter 13 31
The N + 1 RuleIf a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>
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Chapter 13 32
Range of Magnetic
Coupling
• Equivalent protons do not split each other.
• Protons bonded to the same carbon willsplit each other only if they are notequivalent.
• Protons on adjacent carbons normally willcouple.
• Protons separated by four or more bondswill not couple.=>
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Chapter 13 33
Splitting for Ethyl Groups
=>
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Chapter 13 34
Splitting forIsopropyl Groups
=>
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Chapter 13 35
Coupling Constants
• Distance between the peaks of multiplet• Measured in Hz
• Not dependent on strength of the external
field
• Multiplets with the same coupling
constants may come from adjacent groupsof protons that split each other.
=>
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Chapter 13 36
Values for
Coupling Constants
=>
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Chapter 13 37
Complex Splitting
• Signals may be split by adjacent
protons, different from each other, with
different coupling constants.• Example: Ha of styrene which is split by
an adjacent H trans to it (J = 17 Hz) and
an adjacent H cis to it (J = 11 Hz).
=>
C C
H
H
Ha
b
c
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Chapter 13 38
Splitting TreeC CH
H
Ha
b
c
=>
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Chapter 13 39
Spectrum for Styrene
=>
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Chapter 13 40
StereochemicalNonequivalence
• Usually, two protons on the same C areequivalent and do not split each other.
• If the replacement of each of the protons of
a -CH2 group with an imaginary “Z” givesstereoisomers, then the protons are non-
equivalent and will split each other.
=>
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Chapter 13 41
Some NonequivalentProtons
C CH
H
H
a
b
c
OH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b =>
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Chapter 13 42
Time Dependence• Molecules are tumbling relative to the
magnetic field, so NMR is an averaged
spectrum of all the orientations.
• Axial and equatorial protons on
cyclohexane interconvert so rapidly thatthey give a single signal.
• Proton transfers for OH and NH may occur
so quickly that the proton is not split byadjacent protons in the molecule.
=>
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Chapter 13 43
Hydroxyl
Proton
• Ultrapure samplesof ethanol show
splitting.
• Ethanol with a smallamount of acidic or
basic impurities will
not show splitting.
=>
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Chapter 13 44
N-H Proton
• Moderate rate of exchange.
• Peak may be broad.
=>
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Chapter 13 45
Identifying the O-H
or N-H Peak• Chemical shift will depend on
concentration and solvent.• To verify that a particular peak is due to
O-H or N-H, shake the sample with D2O
• Deuterium will exchange with the O-H
or N-H protons.
• On a second NMR spectrum the peakwill be absent, or much less intense.
=>
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Chapter 13 46
Carbon-13
•12
C has no magnetic spin.• 13C has a magnetic spin, but is only 1% of
the carbon in a sample.
• The gyromagnetic ratio of 13C is one-
fourth of that of 1H.
• Signals are weak, getting lost in noise.• Hundreds of spectra are taken, averaged.
=>
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Chapter 13 47
Fourier Transform NMR• Nuclei in a magnetic field are given a
radio-frequency pulse close to their
resonance frequency.
• The nuclei absorb energy and precess
(spin) like little tops.
• A complex signal is produced, then
decays as the nuclei lose energy.
• Free induction decay is converted tospectrum. =>
H d d C b
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Chapter 13 48
Hydrogen and Carbon
Chemical Shifts
=>
C bi d 13C
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Chapter 13 49
Combined 13C
and 1H Spectra
=>
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Chapter 13 50
Differences in13C Technique
• Resonance frequency is ~ one-fourth,15.1 MHz instead of 60 MHz.
• Peak areas are not proportional to
number of carbons.
• Carbon atoms with more hydrogens
absorb more strongly.=>
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Chapter 13 51
Spin-Spin Splitting
• It is unlikely that a
13
C would beadjacent to another 13C, so splitting bycarbon is negligible.
•13
C will magnetically couple withattached protons and adjacent protons.
• These complex splitting patterns are
difficult to interpret. =>
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Chapter 13 52
Proton Spin Decoupling
• To simplify the spectrum, protons arecontinuously irradiated with “noise,” sothey are rapidly flipping.
• The carbon nuclei see an average of allthe possible proton spin states.
• Thus, each different kind of carbon
gives a single, unsplit peak. =>
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Chapter 13 53
Off-Resonance Decoupling
• 13C nuclei are split only by the protonsattached directly to them.
• The N + 1 rule applies: a carbon with N
number of protons gives a signal with
N + 1 peaks.
=>
I t ti 13C NMR
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Chapter 13 54
Interpreting 13C NMR
• The number of different signals indicates
the number of different kinds of carbon.• The location (chemical shift) indicates the
type of functional group.
• The peak area indicates the numbers ofcarbons (if integrated).
• The splitting pattern of off-resonancedecoupled spectrum indicates the number
of protons attached to the carbon. =>
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Chapter 13 55
Two 13C NMR Spectra
=>
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Chapter 13 56
MRI• Magnetic resonance imaging, noninvasive
• “Nuclear” is omitted because of public’sfear that it would be radioactive.
• Only protons in one plane can be inresonance at one time.
• Computer puts together “slices” to get 3D.
• Tumors readily detected.=>
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Chapter 13 57
End of Chapter 13