Download - NMR spectroscopy
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ProtonNuclear Magnetic
Resonance SpectroscopyH1-NMR
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Chapter 13 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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Chapter 13 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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Chapter 13 4
External magnetic feild
N o M agne tic fe ild M agne tic fe ild
S p in -1 /2 (antiparallel to feild)
S p in +1 /2 (parallel to feild)
In the absence of magnetic field nuclei are randomly oriented.
But when magnetic field is applied some of them align parallel to magnetic field and some antiparallel.
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Chapter 13 5
External magnetic feildE
ne
rgy
increasing magnetic fe ild
w hen magnetic feild is 0
energy d iffernce betw en +1/2 and -1/2 sp in
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Chapter 13 6
External Magnetic FieldWhen placed in an external field, spinning protons
act like bar magnets.
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Chapter 13 7
Two Energy StatesThe magnetic fields of
the spinning nuclei will align either with the external field, or against the field.
A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. =>
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Chapter 13 8
Spin Flipping• The transition of a proton from the lower
energy state to the high energy state, by the absorption of radio wave frequency. This transition of a proton from spin state to is called spin flipping.
N o M agne tic fe ild M agne tic fe ild
EAdd radiation of energy= E
sta te
state
+1/2
-1/2
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Chapter 13 9
Precessional frequency
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Chapter 13 10
E and Magnet Strength• Energy difference is proportional to the
magnetic field strength.E = h = h B0
2
• Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for H).
• In a 14,092 gauss field, a 60 MHz photon is required to flip a proton.
• Low energy, radio frequency. =>
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Chapter 13 11
Magnetic Shielding
• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.
• But protons are surrounded by electrons that shield them from the external field.
• Circulating electrons create an induced magnetic field that opposes the external magnetic field. =>
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Chapter 13 12
Shielded Protons
Magnetic field strength must be increased for a shielded proton to flip at the same frequency.
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Chapter 13 13
Protons in a Molecule
Depending on their chemical environment, protons in a molecule are shielded by different amounts.
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Chapter 13 14
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 15
The NMR Spectrometer
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Chapter 13 16
The NMR Graph
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Chapter 13 17
Tetramethylsilane
• TMS is added to the sample.• Since silicon is less electronegative
than carbon, TMS protons are highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal.
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Si
CH3
CH3
CH3
H3C
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Chapter 13 18
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 19
Delta Scale
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Chapter 13 20
Location of Signals
• More electronegative atoms deshield more and give larger shift values.
• Effect decreases with distance.
• Additional electronegative atoms cause increase in chemical shift. =>
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Chapter 13 21
Typical Values
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Chapter 13 22
Aromatic Protons, 7-8
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Chapter 13 23
Vinyl Protons, 5-6
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Chapter 13 24
Acetylenic Protons, 2.5
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Chapter 13 25
Aldehyde Proton, 9-10
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Electronegativeoxygen atom
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Chapter 13 26
O-H and N-H Signals
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H.
• Proton exchanges between the molecules broaden the peak. =>
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Chapter 13 27
Carboxylic Acid Proton, 10+
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Chapter 13 28
Number of SignalsEquivalent hydrogens have the same
chemical shift.
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Chapter 13 29
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 30
How Many Hydrogens?
When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens.
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Chapter 13 31
Spin-Spin Splitting• Nonequivalent protons on adjacent carbons
have magnetic fields that may align with or oppose the external field.
• This magnetic coupling causes the proton to 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 32
1,1,2-TribromoethaneNonequivalent protons on adjacent carbons.
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Chapter 13 33
Doublet: 1 Adjacent Proton
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Chapter 13 34
Triplet: 2 Adjacent Protons
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Chapter 13 35
The N + 1 Rule
If a signal is split by N equivalent protons, it is split into N + 1 peaks.
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Chapter 13 36
Range of Magnetic Coupling
• Equivalent protons do not split each other.• Protons bonded to the same carbon will
split each other only if they are not equivalent.
• Protons on adjacent carbons normally will couple.
• Protons separated by four or more bonds will not couple. =>
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Chapter 13 37
Splitting for Ethyl Groups
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Chapter 13 38
Splitting for Isopropyl Groups
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Chapter 13 39
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 groups of protons that split each other. =>
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Chapter 13 40
Values for Coupling Constants
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Chapter 13 41
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 CH
H
Ha
b
c
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Chapter 13 42
Splitting TreeC C
H
H
Ha
b
c
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Chapter 13 43
Spectrum for Styrene
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Chapter 13 44
Stereochemical Nonequivalence
• Usually, two protons on the same C are equivalent and do not split each other.
• If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other. =>
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Chapter 13 45
Some Nonequivalent Protons
C CH
H
Ha
b
cOH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b =>
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Chapter 13 46
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 that they give a single signal.
• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. =>
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Chapter 13 47
Hydroxyl Proton
• Ultrapure samples of ethanol show splitting.
• Ethanol with a small amount of acidic or basic impurities will not show splitting.
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Chapter 13 48
N-H Proton
• Moderate rate of exchange.
• Peak may be broad.
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Chapter 13 49
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 peak will be absent, or much less intense. =>
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Chapter 13 50
Carbon-13
• 12C 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 51
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 to spectrum. =>
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Chapter 13 52
Hydrogen and Carbon Chemical Shifts
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Chapter 13 53
Combined 13C and 1H Spectra
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Chapter 13 54
Differences in 13C 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 55
Spin-Spin Splitting
• It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.
• 13C will magnetically couple with attached protons and adjacent protons.
• These complex splitting patterns are difficult to interpret. =>
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Chapter 13 56
Proton Spin Decoupling
• To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping.
• The carbon nuclei see an average of all the possible proton spin states.
• Thus, each different kind of carbon gives a single, unsplit peak. =>
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Chapter 13 57
Off-Resonance Decoupling
• 13C nuclei are split only by the protons attached directly to them.
• The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks. =>
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Chapter 13 58
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 of carbons (if integrated).
• The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =>
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Chapter 13 59
Two 13C NMR Spectra
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Chapter 13 60
MRI• Magnetic resonance imaging, noninvasive
• “Nuclear” is omitted because of public’s fear that it would be radioactive.
• Only protons in one plane can be in resonance at one time.
• Computer puts together “slices” to get 3D.
• Tumors readily detected. =>
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Chapter 13 61
End of Chapter 13