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CHEM 3306
Nuclear Magnetic Resonance (NMR) Spectroscopy
Physics
Nuclear Spin: It is experimentally observed that some nuclei act as small magnets.
Nuclei have a charge, and a spinning charged particle generates a magnetic field, so it is
said that these nuclei have "spin". Only nuclei with an odd number of protons (atomic
number) or an odd number of neutrons (isotope) have spin.
Magnetic moment
Direction of spin
Observation of nuclear spin: Most macroscopic manifestations of magnetism are due to
electron spin. Nuclear spin is usually observed by first placing a nucleus in an external
magnetic field (H
o
). The nuclear magnetic moment is quantized and can either line up
parallel to H
o
or antiparallel to it. Remember that the north pole of a magnet is attracted
to the south pole. Thus the antiparallel alignment is more stable.
H
o
parallel
(high energy)
antiparallel
(low energy)
As with any situation with more than one energy level, there will be a Bolzmann
distribution, with a majority of nuclei in the low energy state, but some nuclei in the
higher energy state, In fact, as the energy differences are quite small (radio frequency) the
equilibrium is close to 1:1.
As with any quantized phenomenon, absorption of a
photon can occur from the low energy state to the high-
energy state. It happens that in this case, the photon is in
the radio frequency range. The frequency depends on a
number of factors, including the strength of H
o
and the
identity of the nucleus. At higher magnetic field, the
energy difference between parallel and antiparallel is
larger and the population difference is similarly larger, so higher field instruments are
inherently more sensitive.
antiparallel
parallel
H
o
antiparallel
parallel
hν
parallel
antiparallel
H
o
(increasing magnetic field)
Energy
hν absorbed
hν not absorbed
frequency
absorption
13
C NMR Spectroscopy
Perhaps the easiest place to start is with
13
C NMR. The natural abundance of the magnetic
13
C isotope is only about 1%, while 99% is the non-magnetic
12
C. This means that for a
13
C nucleus in a molecule, there is a 99% chance that a neighboring carbon is
12
C and
non-magnetic. Most
13
C NMR spectra are “proton-decoupled”, meaning that we don’t see
the
1
H nuclei, even though they are also magnetic. More on coupling later.
Chemical Shift: The nuclei are (almost always) in an atom or a molecule and are thus
surrounded by electrons. The magnetic spin of the electrons "shield" the nuclei so they do
not see quite as strong a magnetic field as is applied outside the molecule. This means
that it takes a lower energy photon to cause the absorption.
parallel
antiparallel
E
n
e
r
g
y
hν
Shielding: weaker
field and lower frequency
Deshielding: stronger field
and higher frequency
Changes in the electron density change the "effective field" the nucleus sees and change
the frequency of the absorption. These changes are quite small, only 1 to 200 parts per
million (ppm) for
13
C, but they are observable. Indeed, you know of many factors, which
influence the electron density or partial charge on an atom. These are manifest in the
NMR spectra and are referred to as "upfield" or "downfield" shifts relative to
t etra m ethyl s ilane [(CH 3
)
4
Si; TMS].
There are a few trends to remember. More substitution generally shifts the chemical shift
downfield. The reasons for this are subtle – meaning I can’t give you a simple
explanation. Just remember the trend.
H
3
C
C
H
2
H
2
C
C H
3
H
3
C
C
H
2
H
2
C
CH
3
methyl
0 - 30 ppm
methylene
15 - 55 ppm
CH
3
C
CH
3
H
H
3
C
CH
3
C
CH
3
CH
3
H
3
C
methine
25 - 55 ppm
quaternary
30 - 40 ppm
DEPT- 135 : This experiment gives positive peaks for methyl and
methine carbons, negative peaks for methylene carbons and quartenary
carbons disappear.
O
O
O
O
OH
O
Integration: The more hydrogens, the stronger the absorption. Within a given
compound, the integrated signal intensity for a given hydrogen will be proportional to the
number of equivalent hydrogens. Thus, the relative intensities of different hydrogens give
the relative numbers of hydrogens in the molecule. So a methyl group should integrate to
three hydrogens and a methine to one hydrogen. (Note: in
13
C NMR spectroscopy,
intensity does not necessarily reflect the number of carbons.)
Spin-Spin Coupling: There is an even more subtle, and very important effect that is
observed in
1
H NMR spectroscopy. If there are two
1
H nuclei in a molecule, they both
are magnets and influence each other! Consider a sample containing H a
b
and the
effect that the two spins of H a
can have on H
b
.
some molecules other molecules resulting spectrum
H
o
H
a
H
b
stronger net field
for H
b
H
o
H
a
H
b
weaker net field
for H
b
frequency
absorption
H
a
H
b
downfield
Coupling Constants. Note that the separation between the two peaks from H a
are the
same as those from H b
. This is the coupling constant, J (in units of Hz). The magnitude
of J is always the same in the two nuclei that are coupled to each other and it depends
only on the relationship between the two nuclei. Thus the magnitude of J provides
significant information about that relationship. If there are more hydrogens, each
contributes to the splitting, in a predictable way.
Ethyl group splitting. Consider a molecule such as bromoethane (CH 3
CH
2
Br). First, the
methylene hydrogens (CH 2
) will be shifted further downfield than the methyl hydrogens
(CH 3
) because they are closer to the electron withdrawing Br (chemical shift). Second,
each of the methylene hydrogens has the same
chemical shift and do not couple with each
other. Third, each of the methylene hydrogens
is next to three methyl hydrogens and is
coupled to them.
CH
3
CH
2
Br
+ + + 1
+ + - + - + - + + 3
+ - - - + - - - + 3
- - - 1
This gives four peaks, in a 1:3:3:1 ratio.
frequency
absorption
downfield
CH
2
Br
CH
3
Based on these two examples, it is apparent that the splitting of a hydrogen (or several
equivalent hydrogens) will reflect the number of neighboring hydrogens.
Neighbors splitting Intensity
0 singlet 1
1 doublet 1 1
2 triplet 1 2 1
3 quartet 1 3 3 1
4 quintet 1 4 6 4 1
Pascal's triangle
Second order effects: Note that intensity pattern is not perfect, but that the peaks tend to
lean towards each other. Indeed, as the chemical shifts of two peaks approach each other,
they eventually coalesce into a single peak.
Thus, the actual NMR spectrum of bromoethane will look like this:
CH
2
Br
J
CH
3
absorption
J
3H
2 neighbors
triplet
1.55 ppm
integration
coupling
chemical shift
J
J
CH
3
CH
2
Br
2H
3 neighbors
quartet
3.15 ppm
J
In summary, the things to look for in the spectrum are (1) the integration, which tells you
how many equivalent hydrogens there are (2) the couplings, which tell you how many
neighboring hydrogens there are and (3) the chemical shift (frequency) which tells you
something about what electron withdrawing groups are near the hydrogens.
Equivalent hydrogens and stereochemistry: It is important to be able to look at a
structure and determine whether some hydrogens will have the same chemical shift
and/or are magnetically equivalent, as this determines the coupling patterns that are
observed in the NMR spectrum. The first example shown below is of enantiotopic
hydrogens. They have the same chemical shift and do not couple to each other. The
spectrum of the indicated hydrogens would simply be a triplet, from coupling to the
adjacent hydrogens. The second example shows diastereotopic hydrogens. The actual
NMR spectrum for this compound is shown on the following page.
Homotopic protons will have the same chemical shift. They may or may not be
magnetically equivalent. In a methyl group for example, they are magnetically equivalent
and are considered as a unit.
Diasterotopic hydrogens can accidentally have the same chemical shift, but usually they
will have different chemical shifts. Thus they can be, and often are, magnetically
nonequivalent. Think : What makes compounds diastereomers? The same factors make
hydrogens diastereomeric.
Enantiotopic hydrogens have the same chemical shift in achiral environments.
Enantiomers
Enantiotopic
hydrogens. Must have
the same chemical
shifts and cannot
couple to each other
HC
HC
C
H
CH
C
H
C C
H
C
H C
OCH
3
O
H
H
HC
HC
C
H
CH
C
H
C C
H
C
H C
OCH
3
O
H
D
HC
HC
C
H
CH
C
H
C C
D
C
H C
OCH
3
O
H
H
Diastereomers
Diastereotopic
hydrogens. Can have
different chemical
shifts and couple to
each other
HC
HC
C
H
CH
C
H
C C
H
C
H
2
N C
OCH
3
O
H
H
stereocenter
AKA: stereogenic
(chiral) center
HC
HC
C
H
CH
C
H
C C
H
C
H
2
N C
OCH
3
O
H
D
HC
HC
C
H
CH
C
H
C C
D
C
H
2
N C
OCH
3
O
H
H
H
3
C
H
2
C
C
H
2
Br
H
3
C
H
2
C
C
H
2
H
2
C
C
H
2
H
C
O
H
H
H
