Proton nmr spectroscopy present
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Transcript of Proton nmr spectroscopy present
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• Nuclear magnetic resonance spectroscopy is a powerful analytical techniqueused to characterize organic molecules by identifying carbon-hydrogenframeworks within molecules.
• Two common types of NMR spectroscopy are used to characterize organicstructure: 1H NMR is used to determine the type and number of H atoms in amolecule; 13C NMR is used to determine the type of carbon atoms in themolecule.
• The source of energy in NMR is radio waves which have long wavelengths, andthus low energy and frequency.
• When low-energy radio waves interact with a molecule, they can change thenuclear spins of some elements, including 1H and 13C.
Introduction to NMR Spectroscopy
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Menu
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All atoms, except those that have an even atomic number and an even mass
number, have a property called spin.
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These are some atoms that possess spin.
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Nuclei with spin are active in nuclear magnetic resonance (n.m.r.) spectroscopy.
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The nucleus of a H atom is a proton. Hydrogen atoms are present in most organic
compounds, so proton n.m.r is a useful way to study them.
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Nuclei with spin behave as if they were tiny bar magnets. They can respond to an
applied magnetic field.
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They can align with the magnetic field.
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Nuclei aligned with a magnetic field are in a relatively low energy state.
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Nuclei with spin can also align against the magnetic field.
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Nuclei aligned against the magnetic field are in a higher energy state than nuclei
aligned with the field.
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Energy is needed to move a nucleus to the higher energy state. The amount of
energy needed depends upon the chemical environment of the atom.
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Let’s look at an n.m.r. spectrum for ethanol, CH CH OH.3 2
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Notice that zero is on the right on the horizontal axis.
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The horizontal axis represents the chemical shift. This is given the symbol
δ (delta) and it is measured in parts per million (ppm).
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This is a simplified low-resolution spectrum for ethanol.
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The three hydrogen atoms in the CH group produce this peak.3
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The two hydrogen atoms in the CH group produce this peak.2
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The hydrogen atom in the OH group produces this peak.
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A proton n.m.r. spectrum can give us a lot of useful information about a molecule.
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It can tell us how many different chemical environments there are in the
molecule. Hydrogen atoms in different environments are non-equivalent.
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It can’t tell us how many hydrogen atoms the molecule contains, but it can tell us
the ratio of the number of hydrogen atoms in each chemical environment.
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It can give us information about the nature of the different chemical environments.
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It can also give us information about adjacent non-equivalent hydrogen atoms in
different chemical environments.
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This is the displayed formula for ethanol. How many different chemical
environments does it have?
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There are three equivalent hydrogen atoms in this chemical environment.
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There are two equivalent hydrogen atoms in this chemical environment.
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There is just one hydrogen atom in this chemical environment.
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What peaks would we expect in a low-resolution n.m.r. spectrum of ethanol?
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The CH group contains three hydrogen atoms, which form a large peak3
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The CH group contains two hydrogen atoms, which form a smaller peak.2
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The OH group contains one hydrogen atom, which forms a small peak.
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The OH group contains one hydrogen atom, which forms a small peak.
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The position of each peak on the n.m.r. spectrum gives us information about the
corresponding chemical environment.
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The hydrogen atom in the OH group is attached to an oxygen atom, which is very
electronegative. A hydrogen atom like this is deshielded.
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The peak it produces is shifted downfield in the spectrum.
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The hydrogen atoms in the CH group are far from the oxygen atom. They are
shielded.3
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The peak they produce is upfield in the spectrum, close to 0 ppm.
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Chemical shifts are measured relative to the peak produced by a standard
substance, called TMS. By definition, δ is 0 for TMS.
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This is TMS, tetramethylsilane. Why is it chosen for the reference peak?
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These are some reasons why TMS is chosen.
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It produces a single, intense peak.
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Its n.m.r. peak is upfield of most other peaks.
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It will not react with the sample material.
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End of section one, return to menu by clicking "Reset".
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How many peaks should appear in the proton n.m.r. spectrum of methoxyethane?
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Methoxyethane has a methyl group here.
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It also has a methyl group here, but it is in a different chemical environment.
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It has a methylene group, CH .3
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There are three different chemical environments, so there are three peaks in the
spectrum.
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If we count up each equivalent hydrogen atom in the three chemical environments,
we expect peak areas in the ratio 3:2:3.
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Propan-1-ol is an isomer of methoxyethane. How many peaks should appear in its
proton n.m.r. spectrum?
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Propan-1-ol has a hydrogen atom in its hydroxyl group.
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It has two methylene groups, CH , but they are in different chemical environments.3
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It has a methyl group here.
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There are four different chemical environments, so there are four peaks in the
spectrum.
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If we count up each equivalent hydrogen atom in the four chemical environments,
we expect peak areas in the ratio 1:2:2:3.
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Propan-2-ol is an isomer of methoxyethane and propan-1-ol. How many peaks
should appear in its proton n.m.r. spectrum?
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Propan-2-ol has a methyl group here.
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It also has a methyl group here.
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Both methyl groups are in the same chemical environment. Their hydrogen atoms
are all equivalent and will produce a single peak in the n.m.r. spectrum.
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Propan-2-ol has a hydrogen atom in the hydroxyl group.
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It has another hydrogen atom here, but this is in a different chemical environment
to the one in the hydroxyl group.
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There are three different chemical environments, so there are three peaks in the
spectrum.
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If we count up each equivalent hydrogen atom in the three chemical environments,
we expect peak areas in the ratio 6:1:1.
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End of section two, return to menu by clicking "Reset".
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Some of the peaks are split into clusters of smaller peaks in high-resolution
proton n.m.r. spectra, because of spin-spin coupling.
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This happens if non-equivalent hydrogen atoms are adjacent to each other. No
splitting occurs otherwise.
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Without spin-spin coupling a single peak forms, called a singlet, just as in a low-
resolution spectrum.
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If hydrogen atoms in one chemical environment are adjacent to one hydrogen atom
in another chemical environment, the peak they produce will split into two.
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This is called a doublet, with a ratio of peak areas of 1:1.
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If hydrogen atoms in one chemical environment are adjacent to two hydrogen
atoms in another chemical environment, the peak they produce will split into three.
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This is called a triplet, with a ratio of peak areas of 1:2:1.
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If hydrogen atoms in one chemical environment are adjacent to three hydrogen
atoms in another chemical environment, the peak they produce will split into four.
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This is called a quartet, with a ratio of peak areas of 1:3:3:1.
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This is ethyl ethanoate.
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Its low-resolution proton n.m.r. spectrum would show three peaks in the ratio 3:2:3.
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The hydrogen atoms in this methyl group have no adjacent non-equivalent
hydrogen atoms.
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In a high-resolution spectrum, they would still produce a single peak.
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The hydrogen atoms in this methyl group have two adjacent non-equivalent
hydrogen atoms.
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In a high-resolution spectrum, they would produce a triplet of peaks, with a ratio of
1:2:1.
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The hydrogen atoms in this methylene group have three adjacent non-equivalent
hydrogen atoms.
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In a high-resolution spectrum, they would produce a quartet of peaks, with a ratio
of 1:3:3:1.
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This is a simplified high-resolution proton n.m.r. spectrum of ethyl ethanoate.
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These are the equivalent hydrogen atoms in the different chemical environments
and the peaks they produce.
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These are the equivalent hydrogen atoms in the different chemical environments
and the peaks they produce.
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These are the equivalent hydrogen atoms in the different chemical environments
and the peaks they produce.
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