spectroscopy NMR is the most powerful tool
available for organic structure determination.
It is used to study a wide variety of nuclei with
spin quantum numbers (l) of , including 1H,
13C, 15N, 19F, and 31P.
Our focus in this lab will be primarily on 1H, or
or
proton NMR.
The spinning charged nucleus generates a
magnetic field.
When the sample is placed in an external
magnetic
magnetic field, the nuclei respond to that field.
the
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The magnetic fields of the
spinning nuclei will align
either with the external
field,
field, or against the field.
A photon with the right
amount of energy (radio
waves) can be absorbed by
the molecule and cause the
spinning proton to flip to
the high energy state.
When the nucleus relaxes
back to its original state,
energy is given off, and we
measure that energy as our
NMR spectrum.
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The number of signals shows how many
different kinds of protons are present.
The location of the signals shows how
shielded or deshielded the proton is.
The intensity of the signal shows the number
of protons of that type.
Signal splitting shows the number of protons
on adjacent carbon atoms.
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Protons that are
chemically equivalent
give one signal in the
NMR.
Look for symmetry if
if
you can group protons
with a subscript, they
are
are equivalent.
Example to the right:
2-chloropropane has 2
kinds of protons the
the
protons on carbons 1
and 3 and the proton on
carbon 2.
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The easiest way to decide if hydrogens are
equivalent is to replace each one with another
atom, like chlorine.
Each time you draw a new isomer, you have
found a different type of hydrogen.
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Equivalent hydrogens show one signal in the
NMR spectrum.
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If all protons absorbed the same amount of
energy in a given magnetic field, not much
information could be obtained.
But protons are surrounded by electrons
that shield them from the external field, just
lik
like a jacket shields you from a chilly day.
Circulating electrons create an induced
magnetic field that opposes the external
magnetic field.
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Depending on their chemical environment,
protons in a molecule are shielded by different
amounts.
Protons nearer to electronegative atoms, like O,
are less shielded, which means they absorb
energy at a lower field.
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TMS standard
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This is the location of the signals in the
spectrum.
Measured in parts per million (ppm).
Tetramethylsilane (TMS) [(CH3)4Si] sometimes
added as internal standard, and the TMS signal
is
is defined as 0 ppm.
The delta () scale is the ratio of shift downfield
from TMS to total spectrometer frequency.
Using the scale allows the same scale to be
used regardless of frequency of spectrometer.
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Electronegative atoms
deshield protons and give
larger
larger shift values.
Effect decreases with
distance.
distance.
Additional electronegative
atoms cause increased
deshielding, resulting
further shifting downfield.
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The figure below gives a representation of the
common chemical shifts for various types of
protons.
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Aromatic protons are typically found around 7-8ppm.
The electron system of the benzene ring deshields
protons on the ring.
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The shift chemical of an O-H or N-H proton
depends on both concentration and the
solvent used.
Hydrogen bonding in concentrated solutions
deshields the proton, so signal is around 3.5
for N-H and 4.5 for O-H.
Proton exchanges between the molecules
th
broaden the peak.
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The area under each peak is proportional to the
number of protons giving that signal.
Area is usually shown by an integral trace.
The height of the integral represents the area of
the
the peak.
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When the molecular formula is known, each
integral rise can be assigned to a particular
number of hydrogens.
Total hydrogens = 12
Total area = 23 + 67 = 90
90 12 = 7.5 area units per H
67 7.5 = 8.93, so 9 H
23 7.5 = 3.07, so 3 H
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We can get information about the number of hydrogens
on adjacent carbons from the signal splitting.
Non-equivalent protons on adjacent carbons have
magnetic fields, too, and they may align with or oppose
too and
the external field.
This is called spin-spin coupling, and it causes the
original signal to be split.
The proton signal shifts slightly downfield when the
external field is reinforced and slightly upfield when the
external field is opposed.
This is probably confusing in words, but easy to
understand
understand with examples.
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First note that there are two different types of
protons, and proton Ha is further downfield
because it is on a C with two bromine atoms.
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If a set of equivalent protons is adjacent to carbons
with N protons, it is split into N + 1 peaks.
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Equivalent protons do not split each other.
Protons bonded to the same carbon will split
each other only if they are not equivalent.
th
Protons on adjacent carbons normally will
couple
couple and cause splitting.
Protons separated by four or more bonds will
not generally show splitting.
NOTE: In general, we dont see splitting for
hydrogen
hydrogen bonded to atoms other than carbon.
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It is also common to see more complex splitting
depending on the arrangement of the molecule.
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Learn to recognize this pattern of quartet-triplet.
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Usually, two protons on the same C are
equivalent and do not split each other.
If the replacement of each of the protons of a
group with an imaginary Z gives
stereoisomers
stereoisomers, then the protons are nonequivalent and will often split each other.
This is common in molecules with chiral centers.
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Chemical shift will depend on concentration
and solvent.
To verify that a particular peak is due to
O-H or N-H, shake the sample with D2O.
Deuterium will exchange with the O-H or
N-H protons.
On a second NMR spectrum the peak will
be absent, or much less intense.
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12C
has no magnetic spin.
13C has a magnetic spin, but is only 1% of the
carbon in a sample.
The gyromagnetic ratio of 13C is one-fourth of
that of 1H.
Signals are weak, getting lost in noise.
Hundreds of spectra are taken, averaged.
Not used much in our lab.
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NMR is a powerful tool for elucidating
structure of molecules
NMR alone may not reveal functional group
When used in conjunction with IR, a unique
structure for the molecule can usually be
determined
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