Chapter 13 Nuclear Magnetic Resonance Spectroscopy
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Transcript of Chapter 13 Nuclear Magnetic Resonance Spectroscopy
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Chapter 13Chapter 13
Nuclear Magnetic Nuclear Magnetic
Resonance SpectroscopyResonance Spectroscopy
Leroy WadeLeroy Wade
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Mass No.
At. No.
Nuclear Spin, I
Nuclei
OddOdd or
even1/2, 3/2, 5/2 1H, 13C, 19F
Even Even 0 12C, 16O
even odd 1, 2, 3 2H, 14N
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Origin of NMR Signals
Bo
-1/2 stateor
+1/2 stateor
E = hBo/2
Magnetic energy level for nuclei with I = 1/2
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1-Chlorobutane
CH3CH2CH2CH2Cl
Information contained in an
NMR spectrum includes:
1. Position (chemical shift) of the peaks – amount of shielding.
2. Intensities of signals – number of protons producing each signal.
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3. Splitting pattern – number of neighboring protons.
4. Number of signals – different types of proton present in the molecule.
- - protons that have different protons that have different chemical shifts are chemically chemical shifts are chemically nonequivalent – exist in different nonequivalent – exist in different molecular environment.molecular environment.
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Chemical shift (Chemical shift (, ppm), ppm)
CCCCHH22OCOCHH33NN
OCOCHH33
NCCNCCHH22OO
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are in identical environmentsare in identical environments - - have same have same chemical chemical
shiftshift
Replacement test: replacement by some Replacement test: replacement by some arbitrary "test group" generates same compoundarbitrary "test group" generates same compound
HH33CCHCCH22CCHH33
chemically equivalentchemically equivalent
Chemically equivalent protonsChemically equivalent protons
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Chemically equivalent protonsChemically equivalent protons
Replacing protons at C-1 and C-3 gives Replacing protons at C-1 and C-3 gives same compound (1-chloropropane)same compound (1-chloropropane)
C-1 and C-3 protons are chemically C-1 and C-3 protons are chemically equivalent and have the same chemical shiftequivalent and have the same chemical shift
HH33CCHCCH22CCHH33
chemically equivalentchemically equivalent
CCHH33CHCH22CCHH22ClClClClCCHH22CHCH22CCHH33
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2. CHEMICAL SHIFTS All protons present in a molecule don’t produce a single NMR signal. Protons in different chemical environments produce signal at different positions on the spectrum. For example, CH3CH2CH2CH2Cl produce a set of four signals, one for methyl and three for
methylene protons. The position of signal’s appearance on NMR spectrum is known as Chemical Shift.
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Measurement of Chemical Shift
Position of resonance signals are measured relative to (CH3)4Si, tetramethylsilane (TMS), used as a NMR reference substance or standard. All 12 protons of TMS produce a single sharp line on NMR spectrum.
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Resonance frequency of nuclei depends on the applied magnetic resonance frequency. In order to make the chemical shift values independent of the magnet strength, a ppm scale is introduced. This dimensionless chemical shift is represented by is defined as follows.
(ppm) = Spectrometer frequency (MHz)
Shift downfield from TMS (Hz)
1212
= 710 Hz
100 MHz= 7.1 ppm
= 426 Hz
60 MHz= 7.1 ppm
1313
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Characteristic Values of Chemical Shifts
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Vinyl and Aromatic Protons
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Areas of the Peaks
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2020
Not all NMR peaks are singlets. Not all NMR peaks are singlets.
When two different types of protons When two different types of protons
are close enough their magnetic are close enough their magnetic fields fields
interact with each other and signals interact with each other and signals
are splitted. are splitted.
Spin-Spin SplittingSpin-Spin Splitting
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Chemical shift (Chemical shift (, ppm), ppm)
ClCl22CCHHCCHH33
4 lines;4 lines;quartetquartet
2 lines;2 lines;doubletdoublet
CCHH33CCHH
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Chemical shift (Chemical shift (, ppm), ppm)
ClCl22CCHHCCHH33
4 lines;4 lines;quartetquartet
2 lines;2 lines;doubletdoublet
CCHH33CCHH
coupled protons are vicinal (three-bond coupling)
CH splits CH3 into a doublet
CH3 splits CH into a quartet
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Why do the methyl protons ofWhy do the methyl protons of1,1-dichloroethane appear as a doublet?1,1-dichloroethane appear as a doublet?
CC CC HHHH
ClCl
ClCl
HH
HHsignal for signal for methylmethyl protons is split into protons is split into a doubleta doublet
To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1
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Why do the methyl protons ofWhy do the methyl protons of1,1-dichloroethane appear as a doublet?1,1-dichloroethane appear as a doublet?
CC CC HHHH
ClCl
ClCl
HH
HHsignal for signal for methylmethyl protons is split into protons is split into a doubleta doublet
There are two orientations of the nuclear There are two orientations of the nuclear spin for the proton at C-1. One orientation spin for the proton at C-1. One orientation shields the protons at C-2; the other shields the protons at C-2; the other deshields the C-2 protons.deshields the C-2 protons.
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Why do the methyl protons ofWhy do the methyl protons of1,1-dichloroethane appear as a doublet?1,1-dichloroethane appear as a doublet?
CC CC HHHH
ClCl
ClCl
HH
HHsignal for signal for methylmethyl protons is split into protons is split into a doubleta doublet
The protons at C-2 "feel" the effect of both The protons at C-2 "feel" the effect of both the applied magnetic field and the local field the applied magnetic field and the local field resulting from the spin of the C-1 proton.resulting from the spin of the C-1 proton.
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CC CC HHHH
ClCl
ClCl
HH
HH"true" chemical"true" chemical
shift of methylshift of methyl
protons (no coupling)protons (no coupling)
this line correspondsthis line corresponds
to molecules in which to molecules in which
the nuclear spin of the nuclear spin of
the proton at C-1 the proton at C-1
reinforcesreinforces
the applied fieldthe applied field
this line correspondsthis line corresponds
to molecules in which to molecules in which
the nuclear spin of the nuclear spin of
the proton at C-1 the proton at C-1
opposesopposes
the applied fieldthe applied field
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Why does the methine proton ofWhy does the methine proton of1,1-dichloroethane appear as a quartet?1,1-dichloroethane appear as a quartet?
CC CC HHHH
ClCl
ClCl
HH
HHsignal for signal for methinemethine proton is split into proton is split into a quarteta quartet
TheThe protonproton at C-1 "feels" the effect of the at C-1 "feels" the effect of the applied magnetic field and the local fields applied magnetic field and the local fields resulting from the spin states of the three resulting from the spin states of the three methyl protons. The possible combinations methyl protons. The possible combinations are shown on the next slide.are shown on the next slide.
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CC CC HHHH
ClCl
ClCl
HH
HH There are eight combinations of nuclear spins for the three methyl protons.
These 8 combinations split the signal into a 1:3:3:1 quartet.
Why does the methine proton ofWhy does the methine proton of1,1-dichloroethane appear as a quartet?1,1-dichloroethane appear as a quartet?
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For simple cases, the multiplicity of a signalFor simple cases, the multiplicity of a signalfor a particular proton is equal to the number for a particular proton is equal to the number of equivalent vicinal protons + 1.of equivalent vicinal protons + 1.
The N+1 rule
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The range of Magnetic couplingThe range of Magnetic coupling
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Splitting Patterns of Common MultipletsSplitting Patterns of Common Multiplets
Number of equivalentNumber of equivalent AppearanceAppearance Intensities of linesIntensities of linesprotons to which H protons to which H of multipletof multiplet in multipletin multipletis coupledis coupled
11 DoubletDoublet 1:11:1
22 TripletTriplet 1:2:11:2:1
33 QuartetQuartet 1:3:3:11:3:3:1
44 PentetPentet 1:4:6:4:11:4:6:4:1
55 SextetSextet 1:5:10:10:5:11:5:10:10:5:1
66 SeptetSeptet 1:6:15:20:15:6:11:6:15:20:15:6:1
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Splitting Patterns:
The Ethyl Group
CHCH33CHCH22X is characterized by a triplet-X is characterized by a triplet-quartet pattern (quartet at lower field than quartet pattern (quartet at lower field than the triplet)the triplet)
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Chemical shift (Chemical shift (, ppm), ppm)
BrCBrCHH22CCHH33
4 lines;4 lines;quartetquartet
3 lines;3 lines;triplettriplet
CCHH33
CCHH22
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Splitting Patterns:
The Isopropyl Group
(CH(CH33))22CHX is characterized by a doublet-CHX is characterized by a doublet-
septet pattern (septet at lower field than the septet pattern (septet at lower field than the doublet)doublet)
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Chemical shift (Chemical shift (, ppm), ppm)
7 lines;7 lines;septetseptet
2 lines;2 lines;doubletdoublet
CCHH33
CCHH
Br
H
C CH3
CH3
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Chemical shift (Chemical shift (, ppm), ppm)
OCOCHH33
skewed doubletsskewed doublets
HH HH
HHHH
ClCl OCOCHH33
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Couplinmg Constants
The distance between the peaks of a multiplet (in Hz) is called the Coupling Constant. Coupling constants are represented by J, and the coupling constant between Ha and Hb is represented by Jab.
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Exercise:
Fig 13-30
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Complex Splitting
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m-Nitrostyrenem-Nitrostyrene
Consider the proton shown in red.
It is unequally coupled to the protons shown in blue and yellow.
Jcis = 12 Hz; Jtrans = 16 Hz
HH
HHOO22NN
HH
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m-Nitrostyrenem-Nitrostyrene
16 Hz16 Hz
12 Hz12 Hz 12 Hz12 Hz
The signal for the proton shown in red appears as a doublet of doublets.
HH
HHOO22NN
HH
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HH
HHOO22NN
HH
doublet of doubletsdoublet of doublets
doubletdoublet doubletdoublet
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Time Dependence of NMR Spectroscopy
Most conformational changes occur faster than NMR can detect them.
An NMR spectrum is the weighted average An NMR spectrum is the weighted average of the conformations.of the conformations.
For example: Cyclohexane gives a single For example: Cyclohexane gives a single peak for its H atoms in NMR. Half of the peak for its H atoms in NMR. Half of the time a single proton is axial and half of the time a single proton is axial and half of the time it is equatorial. The observed chemical time it is equatorial. The observed chemical shift is half way between the axial chemical shift is half way between the axial chemical shift and the equatorial chemical shift.shift and the equatorial chemical shift.
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1H NMR Spectra of O-H, N-H proton containing molecules
The chemical shift for O—H and N-H is variable ( 0.5-5 ppm) and depends on temperature and concentration.
Splitting of the O—H proton is sometimes observed, but often is not. It usually appears as a broad peak.
Adding D2O converts O—H to O—D. The O—H peak disappears.
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13.1213.121313C NMR SpectroscopyC NMR Spectroscopy
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11H and H and 1313C NMR compared:C NMR compared:
both give us information about the number of both give us information about the number of chemically nonequivalent nuclei chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent (nonequivalent hydrogens or nonequivalent carbons)carbons)
both give us information about the both give us information about the environment of the nuclei (hybridization state, environment of the nuclei (hybridization state, attached atoms, etc.)attached atoms, etc.)
it is convenient to use FT-NMR techniques for it is convenient to use FT-NMR techniques for 11H; it is standard practice for H; it is standard practice for 1313C NMRC NMR
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11H and H and 1313C NMR compared:C NMR compared:
1313C requires FT-NMR because the signal for a C requires FT-NMR because the signal for a carbon atom is 10carbon atom is 10-4-4 times weaker than the times weaker than the signal for a hydrogen atomsignal for a hydrogen atom
a signal for a a signal for a 1313C nucleus is only about 1% as C nucleus is only about 1% as intense as that for intense as that for 11H because of the magnetic H because of the magnetic properties of the nuclei, andproperties of the nuclei, and
at the "natural abundance" level only 1.1% of at the "natural abundance" level only 1.1% of all the C atoms in a sample are all the C atoms in a sample are 1313C (most are C (most are 1212C)C)
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11H and H and 1313C NMR compared:C NMR compared:
1313C signals are spread over a much wider C signals are spread over a much wider range than range than 11H signals making it easier to H signals making it easier to identify and count individual nucleiidentify and count individual nuclei
Figure 1 shows the Figure 1 shows the 11H NMR spectrum of 1-H NMR spectrum of 1-chloropentane; Figure 2 shows the chloropentane; Figure 2 shows the 1313C C spectrum. It is much easier to identify the spectrum. It is much easier to identify the compound as 1-chloropentane by its compound as 1-chloropentane by its 1313C C spectrum than by its spectrum than by its 11H spectrum.H spectrum.
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Chemical shift (Chemical shift (, ppm), ppm)
ClClCCHH22
Figure 1Figure 1
CCHH33ClClCCHH22CHCH22CHCH22CHCH22CCHH33
11HH
5353Chemical shift (Chemical shift (, ppm), ppm)
Figure 2Figure 2
ClClCHCH22CHCH22CHCH22CHCH22CHCH33
020406080100120140160180200
1313CC
CDClCDCl33
a separate, distinct peak appears for each of the 5 carbons
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1313C Chemical ShiftsC Chemical Shifts
are measured in ppm (are measured in ppm ())
from the carbons of TMSfrom the carbons of TMS
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1313C Chemical shifts are most affected by:C Chemical shifts are most affected by:
• electronegativity of groups attached to carbonelectronegativity of groups attached to carbon • hybridization state of carbonhybridization state of carbon
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Electronegativity EffectsElectronegativity Effects
Electronegativity has an even greater effect Electronegativity has an even greater effect on on 1313C chemical shifts than it does on C chemical shifts than it does on 11H H chemical shifts.chemical shifts.
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Types of CarbonsTypes of Carbons
(CH(CH33))33CCHH
CCHH44
CCHH33CCHH33
CHCH33CCHH22CHCH33
(CH(CH33))44CC
primaryprimary
secondarysecondary
tertiarytertiary
quaternaryquaternary
ClassificationClassification Chemical shift, Chemical shift, 11HH 1313CC
0.20.2
0.90.9
1.31.3
1.71.7
-2-2
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1616
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Replacing H by C (more electronegative) deshieldsReplacing H by C (more electronegative) deshieldsC to which it is attached.C to which it is attached.
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Electronegativity effects on CHElectronegativity effects on CH33
CCHH33FF
CCHH44
CCHH33NHNH22
CCHH33OHOH
Chemical shift, Chemical shift, 11HH
0.20.2
2.52.5
3.43.4
4.34.3
1313CC
-2-2
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7575
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Electronegativity effects and chain lengthElectronegativity effects and chain length
ChemicalChemicalshift, shift,
ClCl CHCH22 CHCH22 CHCH22 CHCH22 CHCH33
4545 3333 2929 2222 1414
Deshielding effect of Deshielding effect of ClCl decreases as decreases as number of bonds between number of bonds between ClCl and C increases. and C increases.
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1313C Chemical shifts are most affected by:C Chemical shifts are most affected by:
• electronegativity of groups attached to carbonelectronegativity of groups attached to carbon • hybridization state of carbonhybridization state of carbon
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Hybridization effectsHybridization effects
spsp33 hybridized hybridized carbon is more carbon is more shielded than shielded than spsp22
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138138
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3636 126-142126-142spsp hybridized hybridized carbon is carbon is more more shielded shielded than than spsp22, , but less but less shielded shielded than than spsp33
CHCH33HH CC CC CHCH22 CHCH22
6868 8484 2222 2020 1313
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Carbonyl carbons are especially deshieldedCarbonyl carbons are especially deshielded OO
CHCH22 CC OO CHCH22 CHCH33
127-134127-1344141 14146161171171
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Table: Table: 1313C Chemical ShiftC Chemical Shift
Type of carbonType of carbon Chemical shift (Chemical shift (),),ppmppm
Type of carbonType of carbon Chemical shift (Chemical shift (),),ppmppm
RRCCHH33 0-350-35
CCRR22RR22CC
65-9065-90CCRRRRCC
RR22CCHH22 15-4015-40
RR33CCHH 25-5025-50
RR44CC 30-4030-40
100-150100-150 110-175110-175
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Type of carbonType of carbon Chemical shift (Chemical shift (),),ppmppm
Type of carbonType of carbon Chemical shift (Chemical shift (),),ppmppm
RRCCHH22BrBr 20-4020-40
RRCCHH22ClCl 25-5025-50
35-5035-50RRCCHH22NHNH22
50-6550-65RRCCHH22OHOH
RRCCHH22OROR 50-6550-65
RRCCOROR
OO
160-185160-185
RRCCRR
OO
190-220190-220
RRCC NN 110-125110-125
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1313C NMR and Peak IntensitiesC NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals. Pulse-FT NMR distorts intensities of signals. Therefore, peak heights and areas can be Therefore, peak heights and areas can be deceptive.deceptive.
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CHCH33
OHOH
Figure 3Figure 3
Chemical shift (Chemical shift (, ppm), ppm)
020406080100120140160180200
7 carbons give 7 signals, but intensities are not equal