13.3 Introduction to 1H NMR Spectroscopy

1
13.3Introduction to 1H NMR Spectroscopy
2
The nuclei that are most useful toorganic
chemists are

• 1H and 13C
• both have spin 1/2
• 1H is 99 at natural abundance
• 13C is 1.1 at natural abundance

3
Nuclear Spin

• A spinning charge, such as the nucleus of 1H or
  13C, generates a magnetic field. The magnetic
  field generated by a nucleus of spin 1/2 is
  opposite in direction from that generated by a
  nucleus of spin 1/2.

4
The distribution of nuclear spins is random in
the absence of an external magnetic field.

5
An external magnetic field causes nuclear
magnetic moments to align parallel and
antiparallel to applied field.

H0
6
There is a slight excess of nuclear magnetic
moments aligned parallel to the applied field.

H0
7
Energy Differences Between Nuclear Spin States
increasing field strength

• no difference in absence of magnetic field
• proportional to strength of external magnetic
  field

8
Some important relationships in NMR

• The frequency of absorbedelectromagnetic
  radiationis proportional to
• the energy difference betweentwo nuclear spin
  stateswhich is proportional to
• the applied magnetic field

9
Some important relationships in NMR

• The frequency of absorbed electromagneticradiatio
  n is different for different elements, and for
  different isotopes of the same element.
• For a field strength of 4.7 T 1H absorbs
  radiation having a frequency of 200 MHz (200 x
  106 s-1) 13C absorbs radiation having a
  frequency of 50.4 MHz (50.4 x 106 s-1)

10
Some important relationships in NMR

• The frequency of absorbed electromagneticradiati
  on for a particular nucleus (such as 1H)depends
  on its molecular environment. This is why NMR
  is such a useful toolfor structure determination.

11
13.4Nuclear Shieldingand1H Chemical Shifts

• What do we mean by "shielding?"
• What do we mean by "chemical shift?"

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Shielding

• An external magnetic field affects the motion of
  the electrons in a molecule, inducing a magnetic
  field within the molecule.
• The direction of the induced magnetic field is
  opposite to that of the applied field.

H 0
13
Shielding

• The induced field shields the nuclei (in this
  case, C and H) from the applied field.
• A stronger external field is needed in order for
  energy difference between spin states to match
  energy of rf radiation.

H 0
14
Chemical Shift

• Chemical shift is a measure of the degree to
  which a nucleus in a molecule is shielded.
• Protons in different environments are shielded
  to greater or lesser degrees they have
  different chemical shifts.

H 0
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Chemical Shift

• Chemical shifts (d) are measured relative to the
  protons in tetramethylsilane (TMS) as a standard.

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UpfieldIncreased shielding
DownfieldDecreased shielding
(CH3)4Si (TMS)
Chemical shift (?, ppm)measured relative to TMS
17
Chemical Shift

• Example The signal for the proton in
  chloroform (HCCl3) appears 1456 Hz downfield from
  TMS at a spectrometer frequency of 200 MHz.

d 7.28
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? 7.28 ppm
Chemical shift (?, ppm)
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13.5Effects of Molecular Structureon1H
Chemical Shifts

• protons in different environments experience
  different degrees of shielding and have different
  chemical shifts

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Electronegative substituents decreasethe
shielding of methyl groups
21
Electronegative substituents decrease shielding
H3CCH2CH3
d 1.0
d 4.3
d 2.0
O2NCH2CH2CH3
22
Effect is cumulative

• CHCl3 ? 7.3
• CH2Cl2 ? 5.3
• CH3Cl ? 3.1

23
Methyl, Methylene, and Methine
CH3 more shielded than CH2 CH2 more shielded
than CH
24
Protons attached to sp2 hybridized carbonare
less shielded than those attachedto sp3
hybridized carbon
CH3CH3
? 7.3
? 5.3
? 0.9
25
But protons attached to sp hybridized carbonare
more shielded than those attachedto sp2
hybridized carbon
26
Protons attached to benzylic and allyliccarbons
are somewhat less shielded than usual
H3CCH2CH3
27
Proton attached to CO of aldehydeis most
deshielded CH
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Type of proton
Chemical shift (?),ppm
Type of proton
Chemical shift (?),ppm
2.1-2.3
0.9-1.8
2.5
1.5-2.6
2.3-2.8
2.0-2.5
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Type of proton
Chemical shift (?),ppm
Type of proton
Chemical shift (?),ppm
2.2-2.9
4.5-6.5
3.1-4.1
6.5-8.5
2.7-4.1
9-10
3.3-3.7
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Type of proton
Chemical shift (?),ppm
1-3
0.5-5
6-8
10-13
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13.6Interpreting Proton NMR Spectra
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Information contained in an NMRspectrum includes

• 1. number of signals
• 2. their intensity (as measured by area under
  peak)
• 3. splitting pattern (multiplicity)

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Number of Signals

• protons that have different chemical shifts are
  chemically nonequivalent
• exist in different molecular environment

34
OCH3
NCCH2O
Chemical shift (?, ppm)
35
Chemically equivalent protons

• are in identical environments
• have same chemical shift
• replacement test replacement by some arbitrary
  "test group" generates same compound

H3CCH2CH3
chemically equivalent
36
Chemically equivalent protons

• Replacing protons at C-1 and C-3 gives same
  compound (1-chloropropane)
• C-1 and C-3 protons are chemically equivalent
  and have the same chemical shift

CH3CH2CH2Cl
ClCH2CH2CH3
H3CCH2CH3
chemically equivalent
37
Diastereotopic protons

• replacement by some arbitrary test group
  generates diastereomers
• diastereotopic protons can have
  differentchemical shifts

? 5.3 ppm
? 5.5 ppm
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13.7Spin-Spin SplittinginNMR Spectroscopy

• not all peaks are singlets
• signals can be split by coupling of nuclear spins

39
Figure 13.12 (page 536)
Cl2CHCH3
4 lines quartet
2 lines doublet
CH3
CH
Chemical shift (?, ppm)
40
Two-bond and three-bond coupling
H
H
H
H
protons separated bytwo bonds(geminal
relationship)
protons separated bythree bonds(vicinal
relationship)
41
Two-bond and three-bond coupling
H
H
H
H

• in order to observe splitting, protons cannot
  have same chemical shift
• coupling constant (2J or 3J) is independent of
  field strength

42
Figure 13.12 (page 536)
Cl2CHCH3
4 lines quartet
2 lines doublet
CH3
CH

• coupled protons are vicinal (three-bond coupling)
• CH splits CH3 into a doublet, CH3 splits CH
  into a quartet

Chemical shift (?, ppm)
43
Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
signal for methyl protons is split into a 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 of1,1-dichloroethane
appear as a doublet?
signal for methyl protons is split into a doublet

• There are two orientations of the nuclear spin
  for the proton at C-1. One orientation shields
  the protons at C-2 the other deshields the C-2
  protons.

45
Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
signal for methyl protons is split into a doublet

• The protons at C-2 "feel" the effect of both the
  applied magnetic field and the local field
  resulting from the spin of the C-1 proton.

46
Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
"true" chemicalshift of methylprotons (no
coupling)
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Why does the methine proton of1,1-dichloroethane
appear as a quartet?
signal for methine proton is split into a quartet

• The proton at C-1 "feels" the effect of the
  applied magnetic field and the local fields
  resulting from the spin states of the three
  methyl protons. The possible combinations are
  shown on the next slide.

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Why does the methine proton of1,1-dichloroethane
appear as a quartet?

• There are eight combinations of nuclear spins
  for the three methyl protons.
• These 8 combinations split the signal into a
  1331 quartet.

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The splitting rule for 1H NMR

• For simple cases, the multiplicity of a
  signalfor a particular proton is equal to the
  number of equivalent vicinal protons 1.

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13.8Splitting PatternsThe Ethyl Group

• CH3CH2X is characterized by a triplet-quartet
  pattern (quartet at lower field than the triplet)

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BrCH2CH3
4 lines quartet
3 lines triplet
CH3
CH2
Chemical shift (?, ppm)
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Table 13.2 (page 540)

• Splitting Patterns of Common Multiplets

Number of equivalent Appearance Intensities of
linesprotons to which H of multiplet in
multipletis coupled 1 Doublet 11 2 Triplet
121 3 Quartet 1331 4 Pentet 14641 5
Sextet 15101051 6 Septet 1615201561

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13.9Splitting PatternsThe Isopropyl Group

• (CH3)2CHX is characterized by a doublet-septet
  pattern (septet at lower field than the doublet)

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BrCH(CH3)2
2 lines doublet
7 lines septet
CH3
CH
Chemical shift (?, ppm)
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13.1413C NMR Spectroscopy
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1H and 13C NMR compared

• both give us information about the number of
  chemically nonequivalent nuclei (nonequivalent
  hydrogens or nonequivalent carbons)
• both give us information about the environment
  of the nuclei (hybridization state, attached
  atoms, etc.)
• it is convenient to use FT-NMR techniques for
  1H it is standard practice for 13C NMR

57
1H and 13C NMR compared

• 13C requires FT-NMR because the signal for a
  carbon atom is 10-4 times weaker than the signal
  for a hydrogen atom
• a signal for a 13C nucleus is only about 1 as
  intense as that for 1H because of the magnetic
  properties of the nuclei, and
• at the "natural abundance" level only 1.1 of
  all the C atoms in a sample are 13C (most are 12C)

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1H and 13C NMR compared

• 13C signals are spread over a much wider range
  than 1H signals making it easier to identify and
  count individual nuclei
• Figure 13.23 (a) shows the 1H NMR spectrum of
  1-chloropentane Figure 13.23 (b) shows the 13C
  spectrum. It is much easier to identify the
  compound as 1-chloropentane by its 13C spectrum
  than by its 1H spectrum.

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1H
CH3
ClCH2
ClCH2CH2CH2CH2CH3
Chemical shift (?, ppm)
60
13C
ClCH2CH2CH2CH2CH3

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

CDCl3
Chemical shift (?, ppm)
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13.1513C Chemical Shifts

• are measured in ppm (?)from the carbons of TMS

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13C Chemical shifts are most affected by

• electronegativity of groups attached to carbon
• hybridization state of carbon

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Electronegativity Effects

• Electronegativity has an even greater effect on
  13C chemical shifts than it does on 1H chemical
  shifts.

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Types of Carbons
0.2
0.9
1.3
1.7
Replacing H by C (more electronegative)
deshieldsC to which it is attached.
65
Electronegativity effects on CH3
Chemical shift, d
CH4
CH3NH2
CH3OH
CH3F
66
Electronegativity effects and chain length
Chemical shift, d
45
33
29
22
14
Deshielding effect of Cl decreases as number of
bonds between Cl and C increases.
67
13C Chemical shifts are most affected by

• electronegativity of groups attached to carbon
• hybridization state of carbon

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Hybridization effects

• sp3 hybridized carbon is more shielded than sp2

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Carbonyl carbons are especially deshielded
O
CH2
C
41
14
61
171
127-134
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Table 13.3 (p 549)
Type of carbon
Chemical shift (?),ppm
Type of carbon
Chemical shift (?),ppm
RCH3
0-35
65-90
R2CH2
15-40
100-150
R3CH
25-50
110-175
R4C
30-40
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Table 13.3 (p 549)
Type of carbon
Chemical shift (?),ppm
Type of carbon
Chemical shift (?),ppm
RCH2Br
20-40
110-125
RCH2Cl
25-50
RCOR
160-185
35-50
RCH2NH2
50-65
RCH2OH
RCR
190-220
RCH2OR
50-65
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13.1613C NMR and Peak Intensities

• Pulse-FT NMR distorts intensities of signals.
  Therefore, peak heights and areas can be
  deceptive.

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Figure 13.24 (page 551)

• 7 carbons give 7 signals, but intensities are
  not equal

Chemical shift (?, ppm)
74
End of Chapter 13