CH447 CLASS 9
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 3
Synopsis. Proton equivalence. Spin-spin coupling; origins and proton-proton coupling in 1H NMR
spectra. Simple spin-spin splitting patterns.
Equivalent and Non-Equivalent Protons
There are two types of equivalence to consider in NMR spectroscopy.
1. Chemical equivalence. Protons that are chemically equivalent are
described as those that, if substituted by a different group lead to the
same product. Alternatively, chemically equivalent nuclei can be
interchanged by a symmetry operation on the molecule.
2. Magnetic equivalence.
Protons that are magnetically equivalent are
those chemically equivalent protons that have the same coupling constant
to any nucleus (such as a proton on an adjacent carbon atom). Coupling
constants arise from spin-spin splitting, which is observed only for
magnetically non-equivalent nuclei.
In many cases chemical or stereochemical equivalence of hydrogen atoms
can be taken as magnetic equivalence of protons.
Note that a high degree of molecular symmetry results in fewer nonequivalent hydrogens, which gives simpler 1H NMR spectra (i.e. with fewer
lines).
Note also that rapid rotational conformational isomerism (including “ring flips”)
render magnetically non-equivalent nuclei equivalent on average, giving a
simpler 1H NMR spectrum. This is a normal situation with non-diastereotopic
protons (see below) in open-chain or flexible ring systems.
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Examples of Chemical and Magnetic Equivalence and Non-equivalence of Protons
Spin systems (where appropriate) are given by Pople notation; A, B, C, X
H
H
H
F
H
C
Br
A2X2
A2X2
H
C
H
F
H
H
Cl
Chemically equivalent Chemically equivalent
Magnetically equivalent Magnetically equivalent
Rotational axis and
Rotational axis and
plane of symmetry
plane of symmetry
Homotopic protons
Homotopic protons
Chemically equivalent
Magnetically equivalent
Plane of symmetry
Enantiotropic protons
Cl
HA
A2BX2
HA
H3 C
H
AX2
Chemically equivalent
Magnetically non-equivalent*
Rotational axis and
plane of symmetry
Homotopic protons
F
HA
ABX
HB
AA'XX'
C
Chemically equivalent Chemically equivalent
Magnetically equivalent Magnetically non-equivalent
Plane of symmetry
Rotational axis and
plane of symmetry
Enantiotropic protons
Homotopic protons
Ph
Cl
H
H
H
HB
F
C
H
H
HX
H
HA
Br
Chemically non-equivalent
Magnetically non-equivalent
Diastereotopic protons
HB
Cl
AA'BB'
HB'
Cl
H A'
Chemically equivalent
Magnetically non-equivalent
Plane of symmetry
Homotopic protons
* At room temperature, rapid rotation makes makes the methyl protons magnetically
equivalent,
on average, thus it is an A3X2 spin system
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NMR Spectroscopy and Proton Equivalence
For 1H NMR, the area under a signal (peak) is directly proportional to the
number of magnetically equivalent protons giving rise to that signal,
although in practice, magnetically similar protons may be integrated
together.
An integration report (measurement of area) is an important part of an
experimental NMR spectrum. This is illustrated below.
Examples (gross magnetic environments are given by small letters; numbers of
proton types are in red):
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Equivalence of Protons: Dynamic Situations and the NMR Timescale
Conformational Isomerism and Free Rotation
Conformational isomerism occurs so rapidly at normal temperatures with
respect to the NMR timescale that only an average signal is obtained in a
normal experiment for a proton involved in conformational equilibrium. Processes
of this kind can be slowed down, by lowering the temperature. At a particular
temperature, the conformational isomerism becomes so slow that a signal for
each separate proton environment can be seen in the 1H NMR spectrum. This is
illustrated for undecadeuteriocyclohexane:
H(ax)
D
D10
H(eq)
D10
D
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Exchange Processes
Chemical exchange processes of the type below are common for protons
bonded to oxygen and nitrogen (in particular) and are catalyzed by even small
amounts of acid.
R
O
H
R
O
H
+
+
R'
H2O
O
H
R
R
H + R'
O
O
H
+
O
H
H2O
Protons on oxygen or nitrogen readily, and fairly slowly (with respect to the NMR
time-scale) exchange with the environment. Hence 1H NMR signals for OH and
NH2 (etc) are characteristically broad and occur over an unusually wide chemical
shift range. Furthermore, shaking the sample with D2O causes the broad OH or
NH2 signals to disappear, as protons are exchanged for (non-resonating)
deuterium.
Diastereotopic Protons
Diastereotopic protons are those that if replaced, individually, by a different
group, give different diastereoisomeric products.
Typical examples include the two protons of a methylene group that is attached
to a center of chirality, and most terminal alkene protons. Diastereoisomeric
protons are non-equivalent in whatever the conformation the molecule is able to
exist: they cannot be made equivalent by rotation about single bonds that occurs
during conformational isomerism, as illustrated below.
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Cl
H
Cl
CH3
HA
HA
H
HB
CH3
HB
HB
H
Cl
Cl
Cl
CH3
Cl
HA
The diastereotopic protons A and B couple with each other and (separately) with
the methine proton on the other carbon atom.
Spin-Spin Coupling: Splitting of 1H NMR Signals
The local magnetic field experienced by a proton of interest (H A) is influenced by
the spin orientations of a neighboring non-equivalent proton (Hx), especially if the
number of chemical bonds separating the two protons is only 2 or 3.
If the magnetic environments of the coupling spectra are very different, the
protons are labeled A and X to denote this. The coupling in this case is known as
first-order coupling and is relatively easy to interpret and to measure coupling
constants (J) – see class 10.
If two coupling protons are magnetically similar, they are labeled A and B, the
spin-spin splitting is known as second-order and can be considerably distorted
from the familiar first-order pattern, especially in multiple proton spin systems,
such as A2B2C2, etc.
For the time being, we will consider only first-order situations. Two common
situations are described overleaf, but see other examples in textbooks.
Firstly, consider the proton of interest coupling with only one neighboring proton:
HA
C
HX
Geminal or
terminal
alkene
protons
HA
HX
C
C
Vicinal, non-terminal
alkene or aromatic
protons
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The signal for HA will appear as a DOUBLET, since this proton experiences two
magnetic fields according to the two spin orientations the second proton (HX), in
the applied magnetic field, as illustrated below.
( HX spin -1/2 )
HA
( HX spin 1/2 )
uncoupled
only TWO
EQUAL
combinations
for HX
coupled
Bo
A similar, but more complex picture emerges if there are two neighboring protons
HX coupling with the proton of interest HA, as in
HA
HX
C
C
HX
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HX total spin
-1
0 (two ways)
HA
uncoupled
+1
HA coupled
Bo
The distance between individual peaks of a multiplet is measured in Hz and is
usually equal to the coupling constant, symbol J. This is illustrated more fully
below. See class 10 for coupling constant notation.
HA
HX
C
C
HA
C
O
HX
31
P
19
F
Some of the more common spin-spin couplings seen in 1H NMR spectra are
summarized in the table.
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Coupling proton(s) Name of multiplet
Relative intensities Example (proton
of interest in bold)
>CH-
Doublet (d)
1:1
-CH2-
Triplet (t)
1:2:1
-CH3
Quartet (q)
1:3:3:1
Br2CH—CH2Br
1,1,2tribromoethane
Br2CH—CH2Br
1,1,2tribromoethane
CH3—CH2Br
bromoethane
Thus first-order coupling patterns in 1H NMR spectroscopy conform to the
“n + 1 rule” (more generally the 2nI + 1 rule, where I is the nuclear spin).
Another quite common situation is the isopropyl group, as in 2-bromopropane,
(CH3)2CHBr. The methine proton resonates as a septuplet (7 lines).
More Complex Spin –Spin Splitting Patterns
“Roofing” and Multiplet Distortion
Completely symmetrical spin-spin patterns, such as those above, are only seen
when the chemical shift difference between the coupled protons is much bigger
than the coupling constant. As the chemical shift difference decreases, the spinspin pattern becomes distorted, at first in a mild form of distortion, known as
“roofing”, as shown below.
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In situations where the chemical shift difference is only slightly greater than the
coupling constant, the distortion can be severe, as shown below for coupled
protons >CHA—CHB<. These are the simplest examples of second-order spectra.
Splitting by Two or More Non-equivalent Protons (ABC, ABX and AMX
Systems)
More complex spectra are observed when a signal is split by two or more types
of non-equivalent protons, as is the case with trans-cinnamaldehyde (3-phenyl-2propenal, an AMX system), below. The signal for the C 2 (vinylic) proton is split by
both the aldehyde proton and the (non-equivalent) C3 (vinylic) proton. Note also
the overlap of phenyl and vinylic signals.
C(1)HXO
C(3)HM
C(2)HA
The “tree diagram” below explains the splitting pattern for the vinylic C2 proton.
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Proton on C2
O
HM
3
1
2 C
C
C
Hx
J(2-3) = 12 Hz
HA
J(1-2) = 6 Hz
Another example of an AMX (or ABX) spin system is acrylonitrile, but where the
coupling between A and B happens to be very small and the H A, HB and HX
coupling constants are different, according to cis or trans stereochemical
orientation (see Class 10).
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Some Special Aspects of NMR Spin-Spin Coupling
Coupling Between Equivalent Protons
Coupling is only observed between NON-EQUIVALENT protons. For example,
no coupling is seen between protons A in the 1H NMR spectra of the following:
A
CH2
Cl
A
CH2
CH3
Cl
HA
C
C
CH3
HA
Also, coupling does not normally occur between protons that make up the same
group, like CH3 and CH2, since they are usually equivalent – see below, however,
for some exceptions.
Coupling Between Geminal Protons
This occurs in methylene groups and terminal alkene CH2 only if the two
protons are magnetically non-equivalent.
Examples
O
CH3
HA
CH3
C
Cl
H
HA
C
C
Br
HB
HA
C
HB
HB
H
H
O
Br
H
All the geminal protons above are examples of diastereotopic protons
See class 10 for examples of 2JHH (geminal coupling constants).
Long Range Coupling
These occur through FOUR or more bonds and, with few exceptions (see Class
10), are either very small or unobservable in normal cases.
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For example, no coupling is normally observed between protons A and B in the
following compounds.
A
CH3
A
CH3
A
CH3
C
B
CH2 Br
Br
A
CH3
B
BrCH2
B
CH2 Br
C
CH3
A
A
CH2R
HB
A
CH3
HB
HB
HB
CH3
A
See class 10 for examples of long range coupling constants
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