1 Chapter 13 Nuclear Magnetic Resonance Spectroscopy Leroy Wade.

Chapter 13Chapter 13

Nuclear Magnetic Nuclear Magnetic

Resonance SpectroscopyResonance Spectroscopy

Leroy WadeLeroy Wade

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

Origin of NMR Signals

-1/2 stateor

+1/2 stateor

E = hBo/2

Magnetic energy level for nuclei with I = 1/2

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.

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

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

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

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.

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.

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)

= 710 Hz

100 MHz= 7.1 ppm

= 426 Hz

60 MHz= 7.1 ppm

Characteristic Values of Chemical Shifts

Vinyl and Aromatic Protons

Areas of the Peaks

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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ClCl22CCHHCCHH33

4 lines;4 lines;quartetquartet

2 lines;2 lines;doubletdoublet

CCHH33CCHH

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ClCl22CCHHCCHH33

CCHH33CCHH

coupled protons are vicinal (three-bond coupling)

CH splits CH3 into a doublet

CH3 splits CH into a quartet

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

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

CC CC HHHH

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.

CC CC HHHH

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.

CC CC HHHH

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

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

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.

CC CC HHHH

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?

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

The range of Magnetic couplingThe range of Magnetic coupling

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

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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BrCBrCHH22CCHH33

3 lines;3 lines;triplettriplet

CCHH33

CCHH22

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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7 lines;7 lines;septetseptet

CCHH33

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OCOCHH33

skewed doubletsskewed doublets

ClCl OCOCHH33

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.

Exercise:

Fig 13-30

Complex Splitting

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

HHOO22NN

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.

HHOO22NN

doublet of doubletsdoublet of doublets

doubletdoublet doubletdoublet

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.

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.

13.1213.121313C NMR SpectroscopyC NMR Spectroscopy

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

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)

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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ClClCCHH22

Figure 1Figure 1

CCHH33ClClCCHH22CHCH22CHCH22CHCH22CCHH33

5353Chemical shift (Chemical shift (, ppm), ppm)

Figure 2Figure 2

ClClCHCH22CHCH22CHCH22CHCH22CHCH33

020406080100120140160180200

1313CC

CDClCDCl33

a separate, distinct peak appears for each of the 5 carbons

1313C Chemical ShiftsC Chemical Shifts

are measured in ppm (are measured in ppm ())

from the carbons of TMSfrom the carbons of TMS

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

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.

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

Replacing H by C (more electronegative) deshieldsReplacing H by C (more electronegative) deshieldsC to which it is attached.C to which it is attached.

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

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.

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

Hybridization effectsHybridization effects

spsp33 hybridized hybridized carbon is more carbon is more shielded than shielded than spsp22

114114

138138

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

Carbonyl carbons are especially deshieldedCarbonyl carbons are especially deshielded OO

CHCH22 CC OO CHCH22 CHCH33

127-134127-1344141 14146161171171

Table: Table: 1313C Chemical ShiftC Chemical Shift

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

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

160-185160-185

RRCCRR

190-220190-220

RRCC NN 110-125110-125

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.

CHCH33

Figure 3Figure 3

020406080100120140160180200

7 carbons give 7 signals, but intensities are not equal

1 Chapter 13 Nuclear Magnetic Resonance Spectroscopy Leroy Wade.

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