Chemical Shift
– magnetic induction of the pi electrons in an
aromatic ring (Fig. 13.11)
Anisotropy of Aromatic compounds: in plane and above
H
H
H
H
H
CH3
H
dring  7.27-6.95 ppm
H
dCH2  -0.51 ppm
H
H
CH3
H
H
H
H
H
H
H
dring  8.14-8.64 ppm
dMe  -4.25 ppm
H
H
H
H
H
H
H
dOUTSIDE  9.28 ppm
dINSIDE  -2.99 ppm
H
H
H
H
H
H
H
Anisotropy: Aromatic
Deshielded
Electronic effects
+
H2C
H2C
O
O
H3C
H3C
O
H
H
7.10 ppm
O
H
COOEt
6.28 ppm
H
H
COOEt
6.83 ppm
COOEt
EtOOC
H
O
O
O
6.10 ppm
7.71 ppm
5.93 ppm
H
7.07 ppm
H
H
H
H
H
6.28 ppm
6.38 ppm
Electronic effects: conjugation with carbonyl
O
7
3
5
2
6
8
7
O
1
4
6
5
4
3
Electronic effects: conjugation with carbonyl
O
1
2
deshielded
3
4
6
5
7.75
6.20
8
7
6
5
4
3
2
Electronic effects: conjugation with heteroatom
H
O
H
O
+
C
H
H
shielded
S
6.06 ppm
H
S
H
H
5.81 ppm
5.48 ppm
H
O
6.22 ppm
H 5.78
H
ppm
H
4.82 ppm H
O
Electronic effects: no conjugation with heteroatom
O
5
4
2
6.5
6.0
3
1
5.5
5.0
4.5
4.0
3.5
Electronic effects: conjugation with heteroatom
O
5
4
2
3
1
shielded
2.65
6.35
6.30
7.0
4.95
6.5
6.0
5.5
2.60
4.90
5.0
4.5
4.0
3.5
3.0
2.5
2.55
Electronic effects: conjugation with carbonyl
m
o
O
CH3
p
8.0
deshielded
7.5
8
7
6
deshielded
5
4
3
2
Electronic effects: conjugation with heteroatom
H3C
O
Shielded
shielded
o
m
7.3
7.5
p
7.2
7.0
7.1
6.5
7.0
6.0
6.9
5.5
6.8
5.0
4.5
4.0
3.5
Electronic effects: conjugation with heteroatom
H3C
NH
m
Shielded
o
p
shielded
7.0
7.5
7.0
6.5
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
Aromatic: inductive effect and resonance effect
F
7.5
7.4
7.3
7.2
7.1
7.0
6.9
Cl
7.4
7.3
7.2
7.1
Br
7.6
7.5
7.4
7.3
7.2
7.1
Calculating Shifts
for aromatic
compounds
NMR

5/14/2013
Common Aromatic Patterns
15
H
H
Ortho
6-10 Hz
H
para
1-4
Hz
H
H
H
meta
0-2 Hz
H
8-10 Hz
H
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
P-Chloroaniline (C6H6ClN)
Hb&Hb’
Ha
Ha’
Hb
Hb’
The Molecular Ion peak
from Mass Spectrometry
would have indicated the
presence of the single
Chlorine atom and Nitrogen.
Ha&Ha’






Para Di-Substituted Benzene ring
Ha & Ha’ have same Chemical Shift
Hb & Hb’ have same Chemical Shift
Ha is split into doublet by Hb
Hb is split into doublet by Ha
Two sets of peaks produced by relative
electronegativity of Amino & Cl groups
2 Amino Protons
5/14/2013
17
Hydrogen bond
meta bromo nitro benzene
Br
H
H
B
A
+
H
O
Calculated shifts
dHA=8.44 dHB=7.82
dHC=7.31
dHD=8.19
N
C
H
O
-
D
HA
HB
HD
8.5
8.0
HC
7.5
Aromatic substitution pattern: ortho
AA’ XX’
Typical spectra for ortho (symmetrical)
O
CH3
O
CH3
8.00
7.90
7.80
7.70
7.60
7.50
7.40
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
2,4-Dinitroanisole (C7H6N2O5)
3H


c
b
a
5/14/2013
b
a
c


The Methoxy group is
moderately activating,
while the Nitro groups are
strongly deactivating
(electron withdrawing)
Net effect is to Decrease
the electron density about
the ring protons
The a & b protons are
Ortho to the strongly
deactivating Nitro groups,
thus, they have reduced
electron density and their
Chemical Shift is down
field relative to the “c”
proton
All protons interact to
produce Spin-Spin
Coupling.
21
AB-Spectra
AMX
C6 H4 O5 N2
I = 6 - 4/2 + 2/2 +1
I= 6
Phenyl = 4 I
NO2 = 1 I
AMX
AFMX
d
J
C5 H4 N Br
I = 5 – 4/2 – 1/2 +1/2 +1
I = 4 (aromatic ring)
Aromatic substituent pattern
O
O
CH3
CH3
NO 2
NO 2
t
J=1.8
dt
J=7.7, 1.5
ddd
J=8.1, 2.2, 1.1
8.5
8.0
t
J=8.1
Aromatic substituent pattern
O
O
CH3
CH3
NO 2
NO 2
td
J=7.4, 1.1
dd
J=8.1, 0.7
8.1
8.0
7.9
7.8
7.7
~td
J=8.1, 1.5
7.6
dd
J=7.7, 1.5
7.5
7.4