Interpreting Proton NMR Spectra

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Interpreting Proton NMR Spectra
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Information contained in an NMRspectrum includes

• 1. number of signals (protons)
• 2. chemical shift (type of proton)
• 3. their intensity or the area under peak
  (number of protons for each signal)
• 4. splitting pattern (multiplicity)

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Number of Signals

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

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

• are in identical environments
• have same chemical shift
• protons are equivalent by symmetry

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Diastereotopic protons

• some protons are not equivalent
• if the protons were replace they would generate
  diastereomers
• diastereotopic protons can have
  differentchemical shifts, but not always

? 2.4 ppm
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Diastereotopic protons
2R
2R, 3S
2R, 3R
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Enantiotopic protons

• are in mirror-image environments
• if the protons were replace they would generates
  enantiomers
• enantiotopic protons have the samechemical shift

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Enantiotopicprotons
R
S
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Spin-Spin SplittinginNMR Spectroscopy

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

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Cl2CHCH3
Chemical shift (?, ppm)
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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

• 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. Proton at C-1
  can be spin up or spin down

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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

• For the 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.

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(No Transcript)
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Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
Therefore the methyl protons are split into a
doublet by the neighboring proton

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

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Why do the methyl protons of1,1-dichloroethane
appear as a doublet?
"true" chemicalshift of methylprotons (no
coupling)
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Cl2CHCH3
Chemical shift (?, ppm)
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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. There are eight possible
  combinations shown on the next slide.

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

• These 8 combinations split the signal into a
  quartet with a 1331 ratio.

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Fig. 13.14
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• CH splits CH3 into a doublet
• CH3 splits CH into a quartet

Cl2CHCH3
4 lines quartet
2 lines doublet
CH3
CH
Signals will be split by n1, where n is the
number of neighboring protons
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Two-bond and three-bond coupling
H
H
H
H
protons separated bytwo bonds(geminal
relationship)
protons separated bythree bonds(vicinal
relationship)
neighboring protons can be two or three bonds away
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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

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Two-bond and three-bond coupling
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Splitting 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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• 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
Quintet (Pentet) 14641 5 Sextet 151010
51 6 Septet 1615201561
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Splitting 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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Splitting PatternsPairs of Doublets

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

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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.

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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.

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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.

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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.

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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.

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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.

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skewed doublets
OCH3
Chemical shift (?, ppm)
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Complex Splitting Patterns

• Multiplets of multiplets

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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

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m-Nitrostyrene

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

12 Hz
12 Hz
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doublet
doublet
doublet of doublets
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Fig. 13.23a
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1H NMR Spectra of Alcohols

• What about H bonded to O?

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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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OH NH
O
R
H
C
O

• The chemical shift for OH in carboxylic acid
  varies between ? 10-14 ppm
• Protons on other heteroatoms like nitrogen can
  also exchange NH proton and usually appears as a
  broad peak between ? 0.5-3.0 for aliphatic or ?
  3.0-5.0 for aromatic.
• Amides, pyrroles and indoles appear between
  5.0-8.5 ppm.

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Fig. 13.21
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NMR and Conformations
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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.

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Index of Hydrogen DeficiencyDegree of
Unsaturation

• relates molecular formulas to multiple bonds and
  rings

For a molecular formula, CcHhNnOoXx, the degree
of unsaturation can be calculated by
Degree ½ (2c 2 - h - x n)
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Rings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum
ofrings plus multiple bonds. Using catalytic
hydrogenation, the number ofmultiple bonds can
be determined.
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C5H8
C8H18
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C10H14
C4H8