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.
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.
An integration report (measurement of area) is an integral part of an experimental
NMR spectrum. This is illustrated below.
Equivalent and Non-Equivalent Protons
Equivalent protons are described as those that, if substituted by a different group
lead to the same product. Chemical equivalence and stereochemical equivalence
of hydrogen atoms can be taken as magnetic equivalence of protons. Note that a
high degree of molecular symmetry results in fewer non-equivalent hydrogens,
which gives simpler 1H NMR spectra (i.e. with fewer lines).
EXAMPLES (magnetic environments, are given by small letters; numbers of
equivalent protons are in red).
O
H
a
O
O
C
C
C
C
Ha
Ha
Ha
succinic anhydride
1
a
CH3
a
CH3
C
Cl
a
CH3
a
CH3
2-chloropropane
Br
Hb
a
CH3
C
2-bromopropane
CH3 a
Hb
Hb
a
CH3
1,1-dimethylcyclopropane
Hc
Ha
Hb
C
3
Hb
1-chloropropane
C
Hc
2
Hb
C
Cl
Hc
vinyl chloride
3
Hc
a
CH3
Hc
c
CH2
b
CH2
3
Hb
C
Hb
a
CH3
2
Hb
methylpropene
Cl
b
CH
a
CH3
a
CH3
a
CH3
Hd
Hb
Hb
Hb
trans-1,2-dimethylcyclopropane
3
cis-1,2-dimethylcyclopropane
2
4
Equivalence of Protons: a Closer Look
Dynamic Situations: 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
H(eq)
D10
D10
D
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 (nonresonating) 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.
Cl
H
Cl
CH3
HA
HA
H
HB
Cl
CH3
HB
HB
H
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 (HA) is influenced by
the spin orientations of a neighboring non-equivalent proton (HB), especially if the
number of chemical bonds separating the two protons is only 2 or 3. Two
common situations are described overleaf, but see other examples in textbooks.
HA
C
HA
HB
C
C
HB
geminal or
terminal
alkene
vicinal or alkene
or aromatic
The signal for HA will appear as a DOUBLET, since this proton experiences two
magnetic fields according to the two spin orientations of proton B (H B), in the
applied magnetic field, as illustrated below.
( HB spin -1/2 )
HA
( HB spin 1/2 )
uncoupled
only TWO
EQUAL
combinations
for HB
coupled
Bo
A similar, but more complex picture emerges if there are two neighboring protons
HB coupling with the proton of interest HA, as in
HA
HB
C
C
HB
HB 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.
HA
HB
C
C
HA
C
O
P
F
HB
Some of the more common spin-spin couplings seen in 1H NMR are summarized
in the table.
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
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.
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<.
Overlapping Signals
This is common in proton NMR spectra and is typified by aromatic resonances.
For example, the aromatic region in the spectrum of toluene (methylbenzene)
shows a complex overlapping pattern, even though the protons are not all
equivalent:
Splitting by Two or More Non-equivalent Protons (ABX 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), below. The signal for the C2 (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.
The “tree diagram” below explains the splitting pattern for the vinylic C2 proton.
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:
Cl
A
CH2
A
CH2
CH3
Cl
HA
C
CH3
C
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 not equivalent.
Examples
O
CH3
HA
CH3
H
HA
C
C
Br
HB
HA
C
C
Cl
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, are either
very small or unobservable in normal cases.
For example, no coupling is 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