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


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


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 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


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


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


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



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


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


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


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


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


CC CC HHHH

ClCl

ClCl

HH


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


CC CC HHHH

ClCl

ClCl

HH


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


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


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


BrCBrCHH(C(CHH33))22

7 lines;7 lines;septetseptet

2 lines;2 lines;doubletdoublet

CCHH33

CCHH

Dr. Wolf's CHM 201 & 202 13- 55

13.1413.141313C NMR SpectroscopyC NMR Spectroscopy

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


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


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


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


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)



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)


020406080100120140160180200

7 carbons give 7 7 carbons give 7 signals, but signals, but intensities are not intensities are not equalequal

Dr. Wolf's CHM 201 & 202 13- 74

End of Chapter 13End of Chapter 13

dr. wolf's chm 201 & 202 13- 1 13.3 introduction to 1 h nmr spectroscopy

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