NMR Nuclear Magnetic Resonance

1
NMR Nuclear Magnetic Resonance

• Proton NMR
• Symmetry

Index
NMR-basics
H-NMR
2
Homotopic protons A2 spin System
Chemical shift equivalence Isochronous nuclei
These nuclei are interchangeable by symmetry or
rapid exchange
Protons are equivalent in chiral and achiral
environment
3
Homotopic protons examples
CH2Cl2
CH2CF2
A2X2
A2
A4
4
Enantiotopic Protons
Plane of symmetry
A2X3
A2X
d 6.15 J 53.6 Hz
Enantiotopic protons are Equivalent in Achiral
environment (like CDCl3) Non-equivalent in
Chiral environment (optically active solvent)
5
A2X2 spin system??
AAXX
H1 H2 gt same shift chemically equivalent
(homotopic)
3JH1-F4 gt cis
3JH2-F4 gt trans
The two protons are coupled to the same nuclei
with different coupling!
Magnetic Non-Equivalence
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Magnetic Equivalence
The 2 H geminal to Fluorine are enantiotopic
The 2 H geminal to Chlorine are enantiotopic
The 2 H(1,3) and 2 H(2,4) are chemically
equivalent
Is this an A2M2X2 spin system?
These protons are chemically equivalent but are
magnetically non-equivalent because they have
different couplings with neighbors
AAMMXX spin system
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No symmetry Asymmetric center

ABX
A B
Protons A and B have different shifts they are
Diastereotopic Accidental overlap can occur
producing deceptively simple spin system
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H1-NMR
OH
CH3
CH
9
Dissymmetric center
Plane of symmetry
Enantiotopic groups
?
Ha1 Ha2
Hb1 Hb2
Diastereotopic protons
A2B2X
10
X
AB
AB
AB
X
11
2 dissymmetric centers in symmetrical molecule
Me
Symmetrical C2 axis Enantiotopic protons
Symmetrical s plane Diastereotopic protons
CH
CH2
HA
HB
Mixture of 2 isomers
12
Equivalence, non-equivalence and symmetry
d H1 ? d H2
d Me1 ? d Me2
d H1 ? d H2 AB
d H3 ? d H4 AB
d H3 d H4 A2
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Example of dissymmetric spin system
d A 3.40
d B 3.55
2JAB 9.4 Hz
3JA-Me 3JB-Me 7.0 Hz
ABX3
dq
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Chemical Shift Non-Equivalence over a distance
Diastereotopic protons

AB
AB
2 doublets
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Magnetic Equivalence
Magnetic equivalence
JA1-B1 ? JA2-B1
Enantiotopic protons
A1 and A2 are Magnetically different
Diastereototopic protons
AABB
16
AABB
17
AABB
2 sets of homotopic protons magnetically
non-equivalent
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AABB para
19
AABB Ortho
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Spin System Pople Notation

• Each Chemical Shift is designated by a letter
• Dn -gt Difference in Shift in Hz
• J -gt Coupling in Hz
• If the ratio Dn/J is Small (lt8), Letters used to
  designate the shift are closeAB, ABC This
  represent case of second order spectra These
  spectra must be simulated with the help of
  quantum mechanic equations. Such programs are
  available on Nuts or Mestrec or Spectrometer
  software. This case is also called strongly
  coupled
• If the ratio Dn/J is large (gt8), Letters used to
  designate the shift are farAM, AX This case
  give rise to first order type spectra Is is also
  refer to as weakly coupled case

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Pople Notation
A2X (if the shift difference of CH2 and CH is
large compare to coupling).
A2B (if the shift difference of CH2 and CH is
small compare to coupling).
3 Spins
AMX -gt if the 3 spins have large chemical shift
difference
ABX -gt if 2 spins are close and 1 is far away
ABC -gt if 3 spins are close
When nuclei have identical shift but different
magnetic coupling, prime symbol is used. For
example
AABB or AAXX
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AABBC
JAX Jcis 10 Hz JAM Jtrans 17 Hz JMX
Jgem 2 Hz
A dd
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Virtual Coupling
Virtual coupling
First order
Same shift
CH2-OH
CH3 broad
CH2b
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Virtual Coupling
Me broad doublet
A2B2CX3
Because of the close shifts of ABC protons we
observe virtual coupling
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Virtual Coupling Symmetrical chains
1 2 3
4
1) CO2Me CH2 CH2 CO2Me
A2 A2 A4
Singlet
1 2 3
4 5
2) CO2Me CH2 CH2 CH2 CO2Me
A2 X2 A2 A4X2
Triplet, Quintet
1 2 3
4 5
3) CO2Me CH2 CH2 CH2 CH2 CO2Me
A2 X2 X2 A2
A2 A2 X2 X2 Complex spectra
Same shift, different J with A/A Virtual coupling
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Virtual Coupling
3) CO2Me CH2 CH2 CH2 CH2 CO2Me
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Symmetrical Molecules with 2 chiral centers
1r, 3r erythro
1r, 3s Meso
H1 H2
H1 H2
diastereotopic protons ABX2
Enantiotopic protons Magnetically
non-equivalent AAXX
Due to fast rotation, J is average A2X2
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Chiral Centers in Symmetrical Molecules
Erythro axis of symmetry
Meso plane of symmetry
H1 ? H1 diastereotopic
H1 ? H1 diastereotopic
H3 ? H3 diastereotopic
H3 ? H3 diastereotopic
Group1 Group3
Group1 Group3
H2 ? H2 diastereotopic
H2 H2 enantiotopic
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Chiral Centers in polymers
Isotactic polymer AB
Syndiotactic polymer A B ? A2
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Calculating Shifts for simple aliphatic compounds
d 0.23 SSi(d)
CH3Cl d(calc)2.76 d(exp.)3.1
CH2Cl2 d(calc)5.29 d(exp.)5.3
CHCl3 d(calc)7.82 d(exp.)7.27
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Calculating Shifts for aliphatic compounds
d 0.933 SSi(d)
1 2 3
e.g. CH3-CO-CO-CH3
Subst. Effect value
- C2-C3 0.244
O (at C2) 1.021
O (at C3) 0.004
-CR3 (at C3) -0.038
SSi(d) 1.231
d 0.933 1.231 2.164
Experimental 2.23
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Calculating Shifts for olefinic compounds
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Calculating Shifts for olefinic compounds
H1
d 5.23 Phgem OEttrans
1.35 (-1.28)
d 5.32
H2
d 5.23 Phtrans OEtgem
(-0.10) 1.18
d 6.33
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Calculating Shifts for olefinic compounds
(deciding which isomer)
Experimental 8.22 ppm
Which one ??
Double bond is further conjugated
35
Calculating Shifts for aromatic compounds
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Aromatic substitution pattern ortho
AA XX
Typical spectra for ortho (symmetrical)
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Aromatic substituent pattern para
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Aromatic substituent pattern
t J8.1
t J1.8
dt J7.7, 1.5
ddd J8.1, 2.2, 1.1
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Aromatic substituent pattern
td J7.4, 1.1
dd J8.1, 0.7
td J8.1, 1.5
dd J7.7, 1.5
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C5H9NO4
t
NO2
d
CH3
O
CH
C
CH2
CH3
O
q
q
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C5H8O
CH3
CH2
CH
CH
t, J7.4
CHO
Trans J
CHO, d J8.1 Hz
ddt, J15.8, 8.1, 1.5
CH2
dt, J15.8, 6.9
42
C4H6O2
I C H/2 1 2
H
CH3
O
s
C
C
C
H
H
O
dd 6.6, 1.5
dd 14.0, 1.5
Jtrans 14.0 Jcis 6.6
2Jgem 1.5
dd 14.0, 6.6
CH
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6
3
400 MHz
5
6
4
6, d (9.2)
3, d(2.4)
5
3
5, dd 9.2, 2.4
6, dt 7.9, 1.0
3, ddd 5.0, 1.5, 0.9
5, ddd 7.9, 7.4, 1.6
4, ddd 7.4, 5.0 , 1.0
80 MHz
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Proton and Heteronuclear NMR
Index
NMR-basics
H-NMR
NMR-Symmetry
Heteronuclear-NMR