Coupling to nuclei with spin > ½
2
JHD = 2.2 Hz
81% abundance
1
H NMR spectrum of deuterated acetone18% abundance
(400 MHz)
Note: Some spin >1/2 nuclei have large quadrupole moments (eg 14N,
35Cl, 10B). Couplings to these nuclei are rarely observed except under
highly symmetrical environment.
Exercise
1. Explain the appearance of 1H NMR spectrum of the olefinic
proton of dimethyl fumarate (400 MHz)
167.9 Hz
15.8 Hz
H
MeO2C
CO2Me
H
More examples on equivalence
HA'
HA
OH
Cl
HB
HB'
HA
NO2
HX'
HX
Cl
HB
HA
HA'
HA
HA
H
Cl
HB'
Br
H
HX
HA'
OH
H
H3C
O
H
HA
H
C
O
CH3
CH3
HB
average magnetically equivalence
Hax
Heq
HA
HA'
HA
HA'
H
CH3
but not
C
Cl
Br
Heterotopicity and NMR
• Do not confuse with chemically & magnetically nonequivalent nuclei.
• Two nuclei, even if they are isochronous, that have different coupling constant with respect to the
third nucleus are magnetically nonequivalent (anisogamous).
H
H
F
F
F
F
H
H
H
H
F
F
F
H
C
H
F
Br
H
F
isogamous H
F
H
C
F
F
H
H
H
Br
CO2H
Br
Br
H
H
H
H
H
H
H
Cl
anisogamous H
• In a spin system A2B2, substitution of one of the B-nuclei by a different nucleus Z leads to a system
A2BZ in which the remaining 2 A nuclei are no longer interconvertible by a symmetry operation then
the A nuclei are magnetically non-equivalent.
Heterotopicity and NMR
• The anisochrony of diastereotopic nuclei will be observed only if the diastereotopic nature of the
nuclei is maintained on the NMR timescale (rotation barrier > 5 kcal/mol or 21 kJ/mol).
1,1-difluorocyclohexane
rate of ring inversion ~ 105/sec
cf 884 Hz difference of δF
Heterotopicity and NMR
• The three protons in methyl groups are, in fact, heterotopic. They give only one NMR signal
because the rapid rotation of the CH3-X bond make these protons interchange under usual
experimental conditions.
b
b
b
X1
X2
X3
X1
X2
X3
a
c
a
c
a
c
X1
X2
X3
• The two Xs in CX2YR are enantiotopic when R is achiral, therefore only one signal will be
observed. On the other hand if R is chiral, they will be diastereotopic which cannot be interchanged
by rotation, hence giving rise to two signals.
b
b
b
X1
X2
Y
X1
X2
Y
a
a
a
a
a
a
Y
X2
X1
b
b
b
X1
X2
Y
X1
X2
Y
a
c
a
c
a
c
Y
X2
X1
Exercise
Analyze the coupling constant for the following multiplets