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NUCLEAR MAGNETIC RESONANCE
SPECTROSCOPY
Objective: Use molecular modeling to predict the number
of equivalent 1H nuclei in a compound, and verify the prediction
using 1H- NMR.
AN INTRODUCTION TO 1H-NMR SPECTROSCOPY:
Atoms, interact with light, that is, electromagnetic radiation,
that is, energy. When atoms absorb energy in the UV and visible regions
of the electromagnetic spectrum, electrons jump from low energy to
higher energy orbitals. This is the basis of UV-visible spectroscopy,
atomic absorption spectroscopy, and fluorescence spectroscopy.
Infrared radiation (or heat radiation) is of lower energy; it is not energetic
enough to affect electrons; instead, it is absorbed by molecules and sets
atoms to vibrating and twisting along their bonds. This is the basis of IR
spectroscopy.
NMR spectroscopy uses low energy radio frequency
radiation to alter the spin of protons. The frequency of energy absorbed
gives information about the environment surrounding the nuclei, and
helps determine the structure of complex molecules.
BACKGROUND:
THE NUCLEUS AS MAGNET:
A spinning Hydrogen atom nucleus, a proton, 1H, acts like a small spinning magnet.
Magnets move in response to other magnets nearby (an external magnetic field). For a
proton there are two orientations - with the external magnetic field [the  state] and
against the external magnetic field [the  state] – These two states have different energy
levels.
Two energy levels exist for the spin of a proton in a hydrogen atom . [FIG 1]
β state
ΔE = hν = hγHo
2π
 state
Applied magnetic field
= 0.
Applied magnetic field
increases.
spins align with () or against (β)
the applied magnetic field.
The proton may be in a low energy level (aligned with the external magnetic field) or a
higher energy level (against the external magnetic field) when an external energy source
is applied to “flip the spin” of the proton. The energy difference between the two states,
ΔE, is the energy of electromagnetic radiation required to flip the spin
The energy required to flip the spin of a typical proton is low, about 4 x 10 -5 kJ/mole.
This is in the radio frequency range. When a proton experiences the right combination
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of radio frequency energy and external magnetic field, and flips its spin, it is said to be
“in resonance”.
THE NMR INSTRUMENT:
The simplest NMR instrument consists of a sample chamber, a magnet, a source
of radio frequency radiation and a detector
Amplifier
Detector
[FIG 5].
Transmitter
Sample tube
Magnet
Magnet.
The
of the
NMR
Audio
Amplifier
Sweep generator
Recorder
Super-cooled superconducting magnet
spinning sample tube
Radio frequency Sweep Generator
Radio frequency transmitter and coil
Receiver coil
Radio frequency amplifier
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When protons absorb energy, the NMR generates a signal peak, which is
proportional to the number of protons being flipped:
Peak area is proportional
to the number of protons
generating the signal.
Hb
O
Ha
C Ha
Ha
FIG 62
Hb
Ha
Ho ------------
Downfield
Spectrum of methanol.
The Hb- bonded to O is
less shielded than the –
Ha’s bonded to C. The Hb
signal peak area is 1/3 the
signal peak area of the
Ha’s. All three Ha’s are
equivalent.
Upfield
A proton is a proton is a proton. If all protons were always equal then all
would have exactly the same ΔE, and only one peak would be observed for any
compound containing hydrogen atoms. This would make NMR an amusing, if limited,
parlor trick. However, while all protons are created equal, protons reside in different
neighborhoods. The environment of a proton affects the amount of energy required to
flip its spin.
Electrons surround any proton in an atom. Electrons, being negative charges in
motion, create a small but measurable magnetic field. This field opposes the applied
external magnetic field experienced by the nucleus. Electrons are said to shield the
proton from the applied magnetic field. In a molecule, different protons have different
electronic environments; they are more shielded or less shielded (deshielded) by their
attendant electrons and thus absorb different amounts of energy. Protons in the same
electronic environment absorb the same amount of energy and are equivalent. (see fig
2).
The energy absorbed by a given proton or set of protons in a compound is
measured relative to a reference compound. The standard used most often is
tetramethylsilane (TMS).
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FIG. 3
CH3a
C – CH3a
Hb-O
CH3a
a
protons
b proton
δ value (ppm)
TMS
Protons. The
NMR is
calibrated
such that
TMS has a δ
value of 0.
3
2
1
0
Upfield (More Shielded)
Downfield (Less Shielded)
The difference in energy absorption by a proton is called a chemical shift (δ); its
units are parts per million (ppm). TMS is used to calibrate the NMR and provide a 0
reference point. Its δ value is set at 0.0 ppm.
The intensity of the NMR signal is proportional to the number of protons
contributing to the signal. The peak area for each signal is proportional to the
number of equivalent protons generating that signal. The integrator on the NMR
instrument traces out an integral-sign shaped step for each peak. The height of the
step (the integral) is proportional to the number of protons. Typically, one calculates
the simplest whole number ratio of protons from the integration data. The pink lines on
Figures 2 and 3 are integration lines. Their height is proportional to peak area. That is,
in Figure 2, the integration line for the “a” proton –(CH3) peak is 3X higher than the line
for the “b” proton [-OH] peak. In Figure 3, the integration line for the “a” proton –(CH3)3
peak is 9X higher than the line for the “b” proton [-OH] peak.
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To summarize:

The number of peaks tells you the number of different equivalent
protons in a compound;
The chemical shift (δ) tells you about the electronic environment
of the proton; UPFIELD…the 1H is near an electronegative atom, or
is attached to a structure with a cloud of electrons about it, like a
benzene ring. The electrons form the electronegative atom or the
benzene ring SHIELD the 1H form the applied magnetic field of the
NMR. DOWNFIELD: The opposite.


The integration values tell you the relative number of equivalent protons
which contribute to each peak‘s signal
EXPERIMENTAL:
A. You and your partner will be assigned one of the following compounds:
1. Methyl acetate (C3H6O2)
2. Acetone (C3H6O)
3. Cyclohexane (C6H12)
4. Toluene (C7H8]
5. Methyl Benzoate (C8H8O2)
6. P-dioxane (C4H8O2)
7. Acetonitrile (C2H3N)
8.
Benzaldehyde (C7H6O)
9.
Anisole (C7H8O)
10. Benzyl Alcohol (C7H8O)
A 1.
BEFORE YOU COME TO LAB: Draw at least two plausible Lewis
structures for your assigned molecule, bearing in mind your molecular
geometry and VSEPR exercises
../../../CHEM_103/CHEM_103/CHEM_103_Labs/IR_Lab/VSEPRstudent.doc.
That is, remember the octet rule. Keep in mind that carbon has 4 bonds. Oxygen
has 2 bonds. Hydrogen has 1 bond. Note on each structure:
a.
Lone pairs of electrons;
b.
Which bonds may freely rotate;
c.
Which bonds are locked into position;
d.
Bond angles.
e. Hybridization of each atom
1. IN LAB: Build models for 2 of your favorite predicted structures.
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2. Predict, from your models, how many equivalent sets of protons (that is, how
many protons are there in how many identical environments) each of your
predicted structure has.
3. On the basis of the relative electronegativity of carbon, oxygen and nitrogen,
which protons in your predicted structures will be most downfield?
4. Obtain an NMR spectrum of your compound & analyze it. That is:
a. How many different sets of equivalent protons are there?
b. What is the simplest whole number ratio of the integrations of each
peak?
6. Which of your two structures is consistent with the NMR data?
Explain.
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FOR STAFF USE:
SET UP: Work individually
IN LAB:
1.
Place out model kits (top shelf glassroom).
2.
Assemble molecules on whitecards.
3.
Whiteboard: write down structures (students will sign up for NMR in this
manner).
INSTRUMENT ROOM PREP:
4.
Reagents: Can be found in GCHE Stockroom.
5.
Place each reagent in a labeled beaker.
6.
Prepare NMR tubes in advance and place in corresponding beaker.
(TMS/CDCL3). A VERY SMALL AMOUNT OF REAGENT IS
SUFFICIENT!!!!!!
7.
Acetone bottle.
8.
Disposable pipets (available if needed, include bulbs).
9.
REAGENTS:
Methyl acetate (C3H6O2)
Acetone (C3H6O)
Cyclohexane (C6H12)
Toluene (C7H8]
Methyl Benzoate (C8H8O2)
P-dioxane (C4H8O2)
Acetonitrile (C2H3N)
Benzaldehyde (C7H6O)
Benzyl Alcohol (C7H8O
8. Place on cart and bring in to Instrument room.