CHEM 3411 Introduction to NMR Exam Practice
Introduction to NMRDr. Niki Shoup
CHEM 3411—Summer 2021
What is NMR?
• Nuclear magnetic resonance (NMR) is one of the most powerful
techniques for structural identification in organic chemistry
• NMR measures the interaction of electromagnetic radiation
(specifically radio waves) with the nucleus of an atom.
• This measurement takes place in an applied magnetic field
• There are several different types of NMR spectroscopy but the most
common for organic chemists are carbon and hydrogen.
What is NMR?
• Protons and neutrons in a nucleus
behave as if they are spinning
• If an odd number of protons or
neutrons are present, then the
atom will have a net nuclear spin
• This spinning charge will generate a
magnetic moment, which also
generates a tiny magnetic field!
• The magnetic moment is
perpendicular to the axis of nuclear
spin
What is NMR?
• Every atom has its own magnetic moment.
• By exposing these disordered atoms to an external magnetic field, we
can align the individual magnetic moments either with or against the
field.
What is NMR?
What is NMR?
• When an atom with an α spin state is exposed to radio waves with the
correct energy, it’ll absorb photons an perform a spin flip to β spin
state.
• The stronger the magnetic field the greater the energy gap.
What is NMR?
• The applied magnetic field causes
the electrons to circulate
• This will induce an opposing
magnetic field (diamagnetism)
• Nuclei that experience less of the
magnetic field are said to be
shielded, which means they
require less energy to spin flip
• Deshielded nuclei are surrounded
by less electron density.
What is NMR?
• The amount of radio wave energy necessary for the spin flip to occur
depends on the electronic environment of the atom.
• When α is flipped to β, energy is absorbed, and the atoms are said to
be in resonance.
• This is not the same resonance as electron flow, this is referring to energy
movement of magnetic dipoles.
• The amount of energy required for each atomic spin flip tells up
about the electron environment
• Which can tell us about the structure.
Acquiring an NMR Spectrum
• To generate an NMR spectrum, you need a strong magnetic field and
radio wave energy
• The strength of the magnetic field affects the energy gap
Acquiring an NMR Spectrum
• The strong magnetic field is generated when a high current is passed
through a superconducting material at extremely low temperatures
(~4 K)
• The greater the current the greater the magnetic field
• We use a pulsed Fourier-transform spectrometer: we briefly pulse
radio energy to excite the sample
• The atoms in the sample excite and then relax, which emits energy
• This happens in a pulsed fashion
• We record the energy as a free induction decay (FID)
Acquiring an NMR Spectrum
• The FID contains all of the information we need!
• The computer performs a Fourier-transform separates the signals so an
individual signal can be observed for each atom that was excited.
• Because we pulsed the sample, we need the FT in order to stitch together several
FID’s
• Because most samples are ran as a liquid, we dissolve the solids/liquids in a
deuterated solvent.
• The deuterated solvent will help prevent the solvent from drowning out the signal
• We then place the sample in the magnetic field and the tube is spun at a
high rate to average the magnetic field variations and tube imperfections.
Acquiring an NMR Spectrum
• If we can we use deuterated chloroform
(CDCl3)
• For proton we will see a small residual signal
at 7.3 ppm
• For carbon we see a residual signal at 77
ppm
• We can also run the sample at room
temperature
Carbon-13 NMR Spectroscopy
• Carbon NMR would be the most ideal atom to analyze for organic
chemists because that is the atom we spend the most time trying to
manipulate
• But the relative abundance of carbon-13 (the NMR active isotope) is
only 1%
• For NMR this means we need to have a sensitive enough receiver coil
or a very concentrated sample.
• Most modern instruments can get a 13C NMR
• We will consider the number of signals and the chemical shift of the
signals
Chemical Shift
• The NMR is initially taken in hertz (Hz) but these numbers are big and
can be hard to deal with, so we need to switch the unit to ppm
• In order to determine the “location” of the signal we have to
determine which compound well call “zero”
• The compound we determined is tetramethylsilane (TMS)
• Each of the protons and carbons of TMS are considered equally
shielded so it is an ideal compound for a zero point.
• The NMR software will take this information and convert the signal
from Hz to ppm
Chemical Shift
• Some definitions:
• Upfield—right side of spectrum, low field strength, shielded
• Downfield—left side of spectrum, high field strength, deshielded
• The chemical shifts for 13C NMR are between 0-200 ppm
• Each signal represents a carbon in a unique electronic environment.
• Remember that symmetry can cause carbon atoms to be in the same
electronic environment.
13C
NMR Chemical Shifts
• We can get general ranges for where signals will show up based upon
the deshielding of the carbon
Example Spectrum
Example Spectrum
• This spectrum
shows what
happens when we
run the spectrum at
different Rf values.
• This is a specific
example for
nonequivalent
carbons and why
we want to use a
higher Rf when
possible.
1H
NMR (AKA Proton NMR)
• Carbon NMR is incredibly useful for determining the type and number
of carbons in a compound, but it cannot be used to give a full
structure
• For that we will use proton NMR!
• Proton NMR can tell us a lot of information that we can use to
elucidate the structure of a compound
• To do this we will use:
•
•
•
•
Number of signals
Signal location (chemical shift)
Signal intensity (number of protons, we can get this from the integration)
Signal shape (splitting patter)
1H
NMR
• The number of signals can tell us the number of different kinds of
proton environments are present in a compound
• Identical protons are considered chemically equivalent. These
protons can be defined as homotopic on enatiotopic
• Homotopic protons: if the molecule has an axis of rotational
symmetry that allows one proton to be rotated onto the other
without changing the molecule (flip like a pancake!)
1H
NMR
• To test for homotopic protons, we can do the replacement test.
• This is when we replace a proton with another atom and see if we have the
same compound.
1H
NMR
• Enatiotopic protons are when you have a plane of reflection that
makes one proton the mirror image of the other (fold like a
quesadilla)
• We can use the replacement test to prove it is a pro-chiral site and
therefore the protons are enatiotopic
1H
NMR
• When you have diastereotopic protons (the replacement tests
produce diastereomers) those protons are not equivalent and will
produce their own signals
• With all of this information we can determine how many signals a
compound should produce
1H
NMR
• Just like with carbon NMR, proton NMR will base its chemical shift on
TMS being the “zero” on the spectrum
• Most proton signals are between 0-10 ppm
• We use the same upfield and downfield terms to describe a proton
NMR as a carbon NMR
Higher Energy
Lower Energy
1H
NMR
• Alkane protons give a chemical shift between 1-2 ppm
• Protons can be sifted downfield when nearby electronegative atoms
cause deshielding through inductive effects
1H
NMR
• Let’s look at a table of
approximate proton NMR
shifts
• The same trends are also
seen with carbon NMR
• Note how downfield the
alkenes and aromatic
compounds are
Magnetic Anisotropy
• Electrons in a pi system are subjected
to an external magnetic field, they
circulate and produce a magnetic field
causing diamagnetic anisotropy
• This means that different regions of
localized space have different
magnetic strength
• Because of this compounds in
aromatic systems and alkenes will be
more downfield.
1H
NMR
• The integration of the signal can tell us how many protons are
generating the signal
• The NMR analysis software will do this for you
1H
NMR
• The values that the computer gives is often annoying, so we have to
find a signal we feel confidant that we know the number of protons
and normalize the rest of the spectrum to it.
2.96
2.00
3.12
2.10
1H
NMR
• Remember that the integrations are relative quantities rather than an absolute
count of protons
• Just think about the fact that a computer comes up with some sort of relative number…
• Symmetry will also affect the integration
• 3-pentanone will have a relative integration of 2:3 but an absolute integration
(proton count) of 4:6
1H
NMR
• So far we have discussed the
number of signals, the location of
the signals, and the relative
integration of the signals
• But what about the shape!
• The multiplicity (splitting pattern,
or shape of the signal) is the
number of peaks in a given signal
1H
NMR
• The splitting pattern is the result of the affects that protons have on
each other
• This has to do with the individual magnetic fields and how they interact with
each other
• So lets consider two neighboring protons Ha and Hb
• Because proton align with or against the external magnetic field,
there are two different magnetic environments for Ha
1H
NMR
• What the multiplicity means is that we can determine how many
neighbors a proton environment has.
• Let’s look at another example where Ha is has two equivalent Hb
neighbors
1H
NMR
• Another example!
• What happens when Ha has 3 equivalent neighbors?
1H
NMR
• That means splitting
occurs using the n+1
rule. That means
that the proton that
you are observing is
split by its neighbor
into n+1 signals.
• This will follow
pascals triangle
1H
NMR
• The rules for splitting
• Equivalent protons cannot split each other
• Splitting is within a 2-3 bond distance
• Remember Ha will split Hb and Hb will split Ha!!!!!
1H
NMR
• The degree to which the protons will couple to each other is call the
coupling constant or the J-value
• The J-value is the distance between the peaks in a splitting pattern
and it is measured in Hz.
• The J-value is a function of the dihedral angle.
• Protons that are splitting each other will have the same coupling
constant
• Jab = Jba
1H
NMR
• Higher field strength instruments (the ones with a stronger magnet
and therefore a higher RF value) will give better resolution.
• Let’s look at the difference between a 60 MHz instrument (left) verses
a 300 MHz instrument (right)
1H
NMR
• Here are some common splitting patters:
• An isolated ethyl group will have a quartet and
a triplet with the same coupling constant
• A t-butyl group will have a singlet with a
relative integration of 9
• An isolated isopropyl group will give a doublet
and a septet!
1H
NMR
• The n+1 rule only applies to equivalent protons. When we have nonequivalent protons splitting the same proton, we will see complex
splitting
• What this means is that Hb will be split by protons Ha and Hc
differently
• If Jab is greater than Jbc then the signal will appear as a quartet of
triplets
1H
NMR
• If Jbc is grater that Jab the signal will be a triplet of quartets
• If Jab and Jbc have similar values, then we get signal overlap. We call
this a multiplit.
1H
NMR
• Some protons do not participate in splitting.
• Example include alcohol protons, amine protons, thiol, protons, and amide
protons.
• These protons can freely exchange with each other and the NMR
solvent
• This means that they are too transient and are not “around” long
enough for the NMR to read that protons magnetic influence on the
neighboring protons.
1H
NMR
• Let’s look at ethanol for an example
• In most alcohol samples there is always transient water
1H
NMR
• NMR is a powerful way for us to determine the full structure of a
compound
• Unlike IR (which gives us only functional group information) or MS
(which gives us the molecular weight and maybe the empirical
formula), NMR can give us the structure of a compound
1H
NMR
• Here is a process you can use to analyze an NMR spectrum
1. Calculate the degree of unsaturation or hydrogen deficiency
index (HDI).
2. Consider the number of NMR signals and integration (gives clues
about the symmetry in the molecule)
3. Analyze each signal, and draw fragments consistent with each
signal
4. Assemble the fragments into a complete structure
5. Verify the proposed structure is consistent with all of the spectral
data
Examples
Example
Example—Putting it Together
Example—Putting it Together
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Question 2
Question 3
Question 4
Question 5
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Question 7
Question 8
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Question 1
2 pts
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Using IR how can you determine if this reaction has moved forward?
OMHO
OH
,
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the loss of sp3 C H stretches and the gain of an OH stretch
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the loss of sp2 C H stretches and the gain of a C-O stretch
the loss of sp2 C H stretches and the gain of an OH stretch
the loss of sp3 C-H stretches and the gain of a C-o stretch
Question 2
2 pts
Consider the following reaction would you expect there to be a rearrangement product?
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Question 2
2 pts
Consider the following reaction, would you expect there to be a rearrangement product?
3 M HCI
OH
yes a methylsift
100%
O no, this reaction will go SN2
Collapse
O yes a hydride shift
no, you are already forming the most stable carbocation
Question 3
2 pts
Why do we use NBS for radical brominations for allylic compounds?
O it will give bromination at the allylic position
to prevent double additions
It is an easier compound to use
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Question 4
2 pts
What is the R/S configurations of the following molecule?
BE
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Question 5
2 pts
What is the rate determining step of the following reaction?
OH
MRSO
both nucleophilic attack of water and the final proton transfer step to form a neutrompound
the final proton transfer step to form a neutral compound
the nucleophilic attack of water
the proton transfer step and formation of the carbocation
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2 pts
Protons H, and H, in the following compound are
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Question 7
2 pts
Which of the indicated protons in the following compound would appear most upfield in the 'H
NMR spectrum?
III
OH
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2 pts
Which of the indicated protons in the following compound would appear most upheld in the ‘H
NMR spectrum?
III
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These protons all have the same chemical shift
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Question 8
2 pts
How many signals would you expect to find in the SC NMR spectrum of the following
compounds?
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Question 8
How many signals would you expect to find in the 13C NMR spectrum of the following
compounds?
2 pts
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Question 9
2 pts
Which of the following bonds has the weakest absorption?
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Which of the following bonds has the weakest absorption?
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Question 10
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Which of the following is true about the molecular weight and the Mom/2
value for the following compound?
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NH2
even molecular weight, mi-116
even molecular weight, mi-132
odd molecular weight mir-121
even molecular weight, m/z 96
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Question 10
2 pts
Which of the following is true about the molecular weight and the M’–m/z
value for the following compound?
NH
even molecular weight, m/2 – 116
O even molecular weight.mz – 132
O odd molecular welcht, m/2 – 121
O even molecular weight, m/96
odd molecular weight, m/115
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