Dr. Wolf's CHM 201 & 202 14-1 Chapter 14 Organometallic Compounds.
Dr. Wolf's CHM 201 & 202 13- 1 13.3 Introduction to 1 H NMR Spectroscopy.
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Transcript of Dr. Wolf's CHM 201 & 202 13- 1 13.3 Introduction to 1 H NMR Spectroscopy.
Dr. Wolf's CHM 201 & 202 13- 1
13.313.3Introduction to Introduction to
11H NMR SpectroscopyH NMR Spectroscopy
Dr. Wolf's CHM 201 & 202 13- 2
11H and H and 1313CC
both have spin = ±1/2both have spin = ±1/2
11H is 99% at natural abundanceH is 99% at natural abundance
1313C is 1.1% at natural abundanceC is 1.1% at natural abundance
The nuclei that are most useful toThe nuclei that are most useful to
organic chemists are:organic chemists are:
Dr. Wolf's CHM 201 & 202 13- 3
Nuclear SpinNuclear Spin
A spinning charge, such as the nucleus of A spinning charge, such as the nucleus of 11H H or or 1313C, generates a C, generates a magnetic fieldmagnetic field. The . The magnetic fieldmagnetic field generated by a nucleus of spin generated by a nucleus of spin +1/2 is opposite in direction from that +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.generated by a nucleus of spin –1/2.
+ +
Dr. Wolf's CHM 201 & 202 13- 4
++
+
+
+
The distribution of The distribution of nuclear spins is nuclear spins is random in the random in the absence of an absence of an external magnetic external magnetic field.field.
Dr. Wolf's CHM 201 & 202 13- 5
++
+
+
+An external magnetic An external magnetic field causes nuclear field causes nuclear magnetic moments to magnetic moments to align parallel and align parallel and antiparallel to applied antiparallel to applied field.field.
HH00
Dr. Wolf's CHM 201 & 202 13- 6
++
+
+
+
There is a slight There is a slight excess of nuclear excess of nuclear magnetic moments magnetic moments aligned parallel to aligned parallel to the applied field.the applied field.
HH00
Dr. Wolf's CHM 201 & 202 13- 7
no difference in absence of magnetic fieldno difference in absence of magnetic fieldproportional to strength of external magnetic field proportional to strength of external magnetic field
Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States
+
+
EE E E ''
increasing field strengthincreasing field strength
Dr. Wolf's CHM 201 & 202 13- 8
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbedThe frequency of absorbedelectromagnetic radiationelectromagnetic radiationis proportional tois proportional to
the energy difference betweenthe energy difference betweentwo nuclear spin statestwo nuclear spin stateswhich is proportional towhich is proportional to
the applied magnetic fieldthe applied magnetic field
UnitsUnits
HzHz
kJ/molkJ/mol(kcal/mol)(kcal/mol)
tesla (T)tesla (T)
Dr. Wolf's CHM 201 & 202 13- 9
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation is different for different elements, radiation is different for different elements, and for different isotopes of the same element.and for different isotopes of the same element.
For a field strength of 4.7 T:For a field strength of 4.7 T:11H absorbs radiation having a frequencyH absorbs radiation having a frequencyof 200 MHz (200 x 10of 200 MHz (200 x 1066 s s-1-1))1313C absorbs radiation having a frequencyC absorbs radiation having a frequencyof 50.4 MHz (50.4 x 10of 50.4 MHz (50.4 x 1066 s s-1-1))
Dr. Wolf's CHM 201 & 202 13- 10
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation for a particular nucleus (such as radiation for a particular nucleus (such as 11H)H)depends on its molecular environment. depends on its molecular environment.
This is why NMR is such a useful toolThis is why NMR is such a useful toolfor structure determination.for structure determination.
Dr. Wolf's CHM 201 & 202 13- 11
13.413.4Nuclear ShieldingNuclear Shielding
andand11H Chemical ShiftsH Chemical Shifts
What do we mean by "shielding?"What do we mean by "shielding?"
What do we mean by "chemical shift?"What do we mean by "chemical shift?"
Dr. Wolf's CHM 201 & 202 13- 12
ShieldingShielding
An external magnetic field An external magnetic field affects the motion of the affects the motion of the electrons in a molecule, electrons in a molecule, inducing a magnetic field inducing a magnetic field within the molecule.within the molecule.
The direction of the The direction of the induced magnetic field is induced magnetic field is opposite to that of the opposite to that of the applied field.applied field.
CC HH
HH 00
Dr. Wolf's CHM 201 & 202 13- 13
ShieldingShielding
The induced field shields The induced field shields the nuclei (in this case, C the nuclei (in this case, C and H) from the applied and H) from the applied field.field.
A stronger external field is A stronger external field is needed in order for energy needed in order for energy difference between spin difference between spin states to match energy of states to match energy of rf radiation.rf radiation.
CC HH
HH 00
Dr. Wolf's CHM 201 & 202 13- 14
Chemical ShiftChemical Shift
Chemical shift is a Chemical shift is a measure of the degree to measure of the degree to which a nucleus in a which a nucleus in a molecule is shielded.molecule is shielded.
Protons in different Protons in different environments are shielded environments are shielded to greater or lesser to greater or lesser degrees; they have degrees; they have different chemical shifts.different chemical shifts.
CC HH
HH 00
Dr. Wolf's CHM 201 & 202 13- 15
Chemical ShiftChemical Shift
Chemical shifts (Chemical shifts () are ) are measured relative to the measured relative to the protons in protons in tetramethylsilane (TMS) tetramethylsilane (TMS) as a standard.as a standard.
SiSi CHCH33
CHCH33
CHCH33
HH33CC
==position of signal - position of TMS peakposition of signal - position of TMS peak
spectrometer frequencyspectrometer frequencyx 10x 1066
Dr. Wolf's CHM 201 & 202 13- 16
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
measured relative to TMSmeasured relative to TMS
UpfieldUpfieldIncreased shieldingIncreased shielding
DownfieldDownfieldDecreased shieldingDecreased shielding
(CH(CH33))44Si (TMS)Si (TMS)
Dr. Wolf's CHM 201 & 202 13- 17
Chemical ShiftChemical Shift
Example: The signal for the proton in chloroform Example: The signal for the proton in chloroform (HCCl(HCCl33) appears 1456 Hz downfield from TMS at a ) appears 1456 Hz downfield from TMS at a
spectrometer frequency of 200 MHz.spectrometer frequency of 200 MHz.
==position of signal - position of TMS peakposition of signal - position of TMS peak
spectrometer frequencyspectrometer frequencyx 10x 1066
==1456 Hz - 0 Hz1456 Hz - 0 Hz
200 x 10200 x 1066 Hx Hxx 10x 1066
= 7.28= 7.28
Dr. Wolf's CHM 201 & 202 13- 18
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
7.28 ppm7.28 ppm
HH CC
ClCl
ClCl
ClCl
Dr. Wolf's CHM 201 & 202 13- 19
13.513.5Effects of Molecular StructureEffects of Molecular Structure
onon11H Chemical ShiftsH Chemical Shifts
protons in different environments experience protons in different environments experience different degrees of shielding and have different degrees of shielding and have
different chemical shiftsdifferent chemical shifts
Dr. Wolf's CHM 201 & 202 13- 20
Electronegative substituents decreaseElectronegative substituents decrease
the shielding of methyl groupsthe shielding of methyl groups
least shielded H most shielded H CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si
4.3 3.2 2.2 0.9 0.0
Dr. Wolf's CHM 201 & 202 13- 21
Electronegative substituents decrease shieldingElectronegative substituents decrease shielding
HH33C—CC—CHH22—C—CHH33
OO22N—CN—CHH22—C—CHH22—C—CHH33
0.90.9 0.90.9 1.31.3
1.01.0 4.34.3 2.02.0
Dr. Wolf's CHM 201 & 202 13- 22
Effect is cumulativeEffect is cumulative
CHCHClCl33 7.3 7.3
CHCH22ClCl22 5.3 5.3
CHCH33ClCl 3.1 3.1
Dr. Wolf's CHM 201 & 202 13- 23
Methyl, Methylene, and MethineMethyl, Methylene, and Methine
CCHH33 more shielded than CH more shielded than CH2 2 ; ; CCHH22
more shielded than Cmore shielded than CHH
HH33CC CC
CCHH33
CHCH33
HH
0.90.9
1.61.6
0.80.8
HH33CC CC
CCHH33
CHCH33
CCHH22
0.90.9
CHCH33
1.21.2
Dr. Wolf's CHM 201 & 202 13- 24
Protons attached to spProtons attached to sp22 hybridized carbon hybridized carbonare less shielded than those attachedare less shielded than those attached
to spto sp33 hybridized carbon hybridized carbon HH HH
HHHH
HH
HH
CC CC
HHHH
HH HH
CHCH33CHCH33
7.37.3 5.35.3 0.90.9
Dr. Wolf's CHM 201 & 202 13- 25
But protons attached to sp hybridized carbonBut protons attached to sp hybridized carbonare more shielded than those attachedare more shielded than those attached
to spto sp22 hybridized carbon hybridized carbon
CC CC
HHHH
HH HH
5.35.3
2.42.4CHCH22OCHOCH33CC CCHH
Dr. Wolf's CHM 201 & 202 13- 26
Protons attached to benzylic and allylicProtons attached to benzylic and allyliccarbons are somewhat less shielded than usualcarbons are somewhat less shielded than usual
1.51.5 0.80.8
HH33CC CHCH33
1.21.2
HH33CC CHCH22
2.62.6
HH33C—CHC—CH22—CH—CH33
0.90.9 0.90.9 1.31.3
Dr. Wolf's CHM 201 & 202 13- 27
Proton attached to C=O of aldehydeProton attached to C=O of aldehydeis most deshielded C—His most deshielded C—H
2.42.4
9.79.7
1.11.1
CC CC
OO
HH
HH
CHCH33
HH33CC
Dr. Wolf's CHM 201 & 202 13- 28
Type of protonType of proton Chemical shift (Chemical shift (),),ppmppm
Type of protonType of proton Chemical shift (Chemical shift (),),ppmppm
CCHH RR 0.9-1.80.9-1.8
1.5-2.61.5-2.6CCHH CCCC
2.0-2.52.0-2.5CCHH CC
OO
2.1-2.32.1-2.3CCHH NNCC
CCHH ArAr 2.3-2.82.3-2.8
2.52.5CCHH CCCC
Dr. Wolf's CHM 201 & 202 13- 29
Type of protonType of proton Chemical shift (Chemical shift (),),ppmppm
Type of protonType of proton Chemical shift (Chemical shift (),),ppmppm
CCHH BrBr 2.7-4.12.7-4.1
9-109-10CC
OO
HH
2.2-2.92.2-2.9CCHH NRNR
3.1-4.13.1-4.1CCHH ClCl
6.5-8.56.5-8.5HH ArAr
CC CC
HH
4.5-6.54.5-6.5
3.3-3.73.3-3.7CCHH OO
Dr. Wolf's CHM 201 & 202 13- 30
Type of protonType of proton Chemical shift (Chemical shift (),),ppmppm
1-31-3HH NRNR
0.5-50.5-5HH OROR
6-86-8HH OArOAr
10-1310-13CC
OO
HHOO
Dr. Wolf's CHM 201 & 202 13- 31
13.613.6
Interpreting Proton NMR Interpreting Proton NMR
SpectraSpectra
Dr. Wolf's CHM 201 & 202 13- 32
1. number of signals1. number of signals
2. their intensity (as measured by area 2. their intensity (as measured by area under peak)under peak)
3. splitting pattern (multiplicity)3. splitting pattern (multiplicity)
Information contained in an NMRInformation contained in an NMRspectrum includes:spectrum includes:
Dr. Wolf's CHM 201 & 202 13- 33
Number of SignalsNumber of Signals
protons that have different chemical shifts protons that have different chemical shifts are chemically nonequivalentare chemically nonequivalent
exist in different molecular environmentexist in different molecular environment
Dr. Wolf's CHM 201 & 202 13- 34
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
CCCCHH22OCOCHH33NN
OCOCHH33
NCCNCCHH22OO
Dr. Wolf's CHM 201 & 202 13- 35
are in identical environmentsare in identical environments
have same chemical shifthave same chemical shift
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
Dr. Wolf's CHM 201 & 202 13- 36
HH33CCHCCH22CCHH33
chemically equivalentchemically equivalent
CCHH33CHCH22CCHH22ClClClClCCHH22CHCH22CCHH33
Chemically equivalent protonsChemically equivalent protons
Replacing protons at C-1 and C-3 gives same Replacing protons at C-1 and C-3 gives same compound (1-chloropropane)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
Dr. Wolf's CHM 201 & 202 13- 37
replacement by some arbitrary test group replacement by some arbitrary test group generates diastereomersgenerates diastereomers
diastereotopic protons can have differentdiastereotopic protons can have differentchemical shiftschemical shifts
Diastereotopic protonsDiastereotopic protons
CC CC
BrBr
HH33CC
HH
HH
5.3 ppm5.3 ppm
5.5 ppm5.5 ppm
Dr. Wolf's CHM 201 & 202 13- 38
not all peaks are singletsnot all peaks are singlets
signals can be split by coupling of signals can be split by coupling of nuclear spinsnuclear spins
13.713.7Spin-Spin SplittingSpin-Spin Splitting
ininNMR SpectroscopyNMR Spectroscopy
Dr. Wolf's CHM 201 & 202 13- 39
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
ClCl22CCHHCCHH33Figure 13.12 (page 536)Figure 13.12 (page 536)
4 lines;4 lines;quartetquartet
2 lines;2 lines;doubletdoublet
CCHH33CCHH
Dr. Wolf's CHM 201 & 202 13- 40
Two-bond and three-bond couplingTwo-bond and three-bond coupling
CC CC
HH
HH
CC CC HHHH
protons separated byprotons separated bytwo bondstwo bonds
(geminal relationship) (geminal relationship)
protons separated byprotons separated bythree bondsthree bonds
(vicinal relationship)(vicinal relationship)
Dr. Wolf's CHM 201 & 202 13- 41
in order to observe splitting, protons cannot in order to observe splitting, protons cannot
have same chemical shifthave same chemical shift
coupling constant (coupling constant (22J or J or 33J) is independent J) is independent
of field strengthof field strength
Two-bond and three-bond couplingTwo-bond and three-bond coupling
CC CC
HH
HH
CC CC HHHH
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
ClCl22CCHHCCHH33Figure 13.12 (page 536)Figure 13.12 (page 536)
4 lines;4 lines;
quartetquartet
2 lines;2 lines;
doubletdoublet
CCHH33CCHH
coupled protons are vicinal (three-bond coupling)coupled protons are vicinal (three-bond coupling)
CCHH splits C splits CHH33 into a doublet, C into a doublet, CHH33 splits C splits CHH into a quartet into a quartet
Dr. Wolf's CHM 201 & 202 13- 43
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 To explain the splitting of the protonsprotons at C-2, at C-2, we first focus on the two possible spin we first focus on the two possible spin orientations of the orientations of the protonproton at C-1 at C-1
Dr. Wolf's CHM 201 & 202 13- 44
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 spin There are two orientations of the nuclear spin for the proton at C-1. One orientation shields for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C-the protons at C-2; the other deshields the C-2 protons.2 protons.
Dr. Wolf's CHM 201 & 202 13- 45
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 The protons at C-2 "feel" the effect of both the applied magnetic field and the local field 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.
Dr. Wolf's CHM 201 & 202 13- 46
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
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
Dr. Wolf's CHM 201 & 202 13- 47
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
The The 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.
Dr. Wolf's CHM 201 & 202 13- 48
CC CC HHHH
ClCl
ClCl
HH
HH There are eight combinations of There are eight combinations of nuclear spins for the three methyl nuclear spins for the three methyl protons.protons.
These 8 combinations split the These 8 combinations split the signal into a 1:3:3:1 quartet.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?
Dr. Wolf's CHM 201 & 202 13- 49
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 splitting rule for The splitting rule for 11H NMRH NMR
Dr. Wolf's CHM 201 & 202 13- 50
13.813.8Splitting Patterns:Splitting Patterns:The Ethyl GroupThe Ethyl Group
CHCH33CHCH22X is characterized by a triplet-quartet X is characterized by a triplet-quartet
pattern (quartet at lower field than the triplet)pattern (quartet at lower field than the triplet)
Dr. Wolf's CHM 201 & 202 13- 51
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
BrCBrCHH22CCHH33
4 lines;4 lines;quartetquartet
3 lines;3 lines;triplettriplet
CCHH33
CCHH22
Dr. Wolf's CHM 201 & 202 13- 52
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
Table 13.2 (page 540)Table 13.2 (page 540)
Dr. Wolf's CHM 201 & 202 13- 53
13.913.9Splitting Patterns:Splitting Patterns:
The Isopropyl GroupThe Isopropyl Group
(CH(CH33))22CHX is characterized by a doublet-septet CHX is characterized by a doublet-septet
pattern (septet at lower field than the doublet)pattern (septet at lower field than the doublet)
Dr. Wolf's CHM 201 & 202 13- 54
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
BrCBrCHH(C(CHH33))22
7 lines;7 lines;septetseptet
2 lines;2 lines;doubletdoublet
CCHH33
CCHH
Dr. Wolf's CHM 201 & 202 13- 56
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
Dr. Wolf's CHM 201 & 202 13- 57
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)
Dr. Wolf's CHM 201 & 202 13- 58
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 13.23 (a) shows the Figure 13.23 (a) shows the 11H NMR spectrum H NMR spectrum of 1-chloropentane; Figure 13.23 (b) shows of 1-chloropentane; Figure 13.23 (b) shows the the 1313C spectrum. It is much easier to identify C spectrum. It is much easier to identify the compound as 1-chloropentane by its the compound as 1-chloropentane by its 1313C C spectrum than by its spectrum than by its 11H spectrum.H spectrum.
Dr. Wolf's CHM 201 & 202 13- 59
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift (, ppm), ppm)
ClClCCHH22 CCHH33ClClCCHH22CHCH22CHCH22CHCH22CCHH33
11HH
Dr. Wolf's CHM 201 & 202 13- 60Chemical shift (Chemical shift (, ppm), ppm)
ClClCHCH22CHCH22CHCH22CHCH22CHCH33
020406080100120140160180200
1313CC
CDClCDCl33
a separate, distinct a separate, distinct peak appears for peak appears for each of the 5 carbonseach of the 5 carbons
Dr. Wolf's CHM 201 & 202 13- 61
13.1513.151313C Chemical ShiftsC Chemical Shifts
are measured in ppm (are measured in ppm ())
from the carbons of TMSfrom the carbons of TMS
Dr. Wolf's CHM 201 & 202 13- 62
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
Dr. Wolf's CHM 201 & 202 13- 63
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.
Dr. Wolf's CHM 201 & 202 13- 64
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
88
1616
2525
2828
Replacing H by C (more electronegative) deshieldsReplacing H by C (more electronegative) deshieldsC to which it is attached.C to which it is attached.
Dr. Wolf's CHM 201 & 202 13- 65
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
2727
5050
7575
Dr. Wolf's CHM 201 & 202 13- 66
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.
Dr. Wolf's CHM 201 & 202 13- 67
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
Dr. Wolf's CHM 201 & 202 13- 68
Hybridization effectsHybridization effects
spsp33 hybridized hybridized carbon is more carbon is more shielded than shielded than spsp22
114114
138138
3636
3636 126-142126-142
spsp hybridized hybridized carbon is carbon is more more shielded than shielded than spsp22, but less , but less shielded than shielded than spsp33
CHCH33HH CC CC CHCH22 CHCH22
6868 8484 2222 2020 1313
Dr. Wolf's CHM 201 & 202 13- 69
Carbonyl carbons are especially deshieldedCarbonyl carbons are especially deshielded OO
CHCH22 CC OO CHCH22 CHCH33
127-134127-1344141 14146161171171
Dr. Wolf's CHM 201 & 202 13- 70
Table 13.3 (p 549)Table 13.3 (p 549)
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
Dr. Wolf's CHM 201 & 202 13- 71
Table 13.3 (p 549)Table 13.3 (p 549)
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
Dr. Wolf's CHM 201 & 202 13- 72
13.1613.161313C 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.
Dr. Wolf's CHM 201 & 202 13- 73
CHCH33
OHOH
Figure 13.24 (page 551)Figure 13.24 (page 551)
Chemical shift (Chemical shift (, ppm), ppm)
020406080100120140160180200
7 carbons give 7 7 carbons give 7 signals, but signals, but intensities are not intensities are not equalequal