13.7 SpinSpin Splitting in NMR Spectroscopy

1
13.7Spin-Spin SplittinginNMR Spectroscopy
2
Figure 13.12 (page 536)
Cl2CHCH3
4 lines quartet
2 lines doublet
CH3
CH
Chemical shift (?, ppm)
3
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)
4
Two-bond and three-bond coupling
H
H
H
H
protons separated bytwo bonds(geminal
relationship)
protons separated bythree bonds(vicinal
relationship)
5
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

6
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

7
Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
signal for methyl protons is split into a doublet

• . One orientation shields the protons at C-2
  the other deshields the C-2 protons.

8
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.

9
Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
"true" chemicalshift of methylprotons (no
coupling)
10
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.

11
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.

12
The splitting rule for 1H NMR

• For simple cases, the multiplicity of a
  signalfor a particular proton is n 1

13
13.8Splitting PatternsThe Ethyl Group

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

14
Figure 13.15 (page 539)
BrCH2CH3
4 lines quartet
3 lines triplet
CH3
CH2
Chemical shift (?, ppm)
15
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

16
13.9Splitting PatternsThe Isopropyl Group

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

17
Figure 13.17 (page 540)
BrCH(CH3)2
2 lines doublet
7 lines septet
CH3
CH
Chemical shift (?, ppm)
18
13.10Splitting PatternsPairs of Doublets

• Splitting patterns are not always symmetrical,
  but lean in one direction or the other.

19
Pairs of Doublets
H
H

• Consider coupling between two vicinal protons.
• If the protons have different chemical shifts,
  each will split the signal of the other into a
  doublet.

20
Pairs of Doublets
H
H

• Let ?? be the difference in chemical shift in Hz
  between the two hydrogens.
• Let J be the coupling constant between them in
  Hz.

21
AX
??

• When ?? is much larger than J the signal for
  each proton is a doublet, the doublet is
  symmetrical, and the spin system is called AX.

22
AM
??

• As ??/J decreases the signal for each proton
  remains a doublet, but becomes skewed. The outer
  lines decrease while the inner lines increase,
  causing the doublets to "lean" toward each other.

23
AB
??

• When ?? and J are similar, the spin system is
  called AB. Skewing is quite pronounced. It is
  easy to mistake an AB system of two doublets for
  a quartet.

24
A2

• When ?? 0, the two protons have the same
  chemical shift and don't split each other. A
  single line is observed. The two doublets have
  collapsed to a singlet.

25
Figure 13.19 (page 542)
skewed doublets
OCH3
Chemical shift (?, ppm)
26
13.11Complex Splitting Patterns

• Multiplets of multiplets

27
m-Nitrostyrene

• Consider the proton shown in red.
• It is unequally coupled to the protons shown in
  blue and white.
• Jcis 12 Hz Jtrans 16 Hz

28
m-Nitrostyrene

• The signal for the proton shown in red appears
  as a doublet of doublets.

12 Hz
12 Hz
29
Figure 13.20 (page 543)
doublet
doublet
doublet of doublets
30
13.121H NMR Spectra of Alcohols

• What about H bonded to O?

31
OH

• The chemical shift for OH is variable (? 0.5-5
  ppm) and depends on temperature and
  concentration.
• Splitting of the OH proton is sometimes
  observed, but often is not. It usually appears
  as a broad peak.
• Adding D2O converts OH to OD. The OH peak
  disappears.

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13.13NMR and Conformations
33
NMR is "slow"

• Most conformational changes occur faster than
  NMR can detect them.
• An NMR spectrum is the weighted average of the
  conformations.
• For example Cyclohexane gives a single peak
  for its H atoms in NMR. Half of the time a
  single proton is axial and half of the time it is
  equatorial. The observed chemical shift is half
  way between the axial chemical shift and the
  equatorial chemical shift.