NMR spectroscopy is a form of spectroscopy that identifies molecules
through their reaction to radio waves (100 MHz to 800 MHz). While in the
presence of an external magnetic field, 13C atoms will absorb
radio wave radiation. NMR machines can measure the exact amount absorbed
to determine the environment which the carbon atom is in. Absorbing
radiation causes the atom to undergo a “nuclear flip”, thus the name
“Nuclear Magnetic Resonance”. NMR is the most widely used tool for
determining the structure of molecules.
13C NMR spectrum of 1-bromo-2,3,3-trimethylbutane
Five different carbon environments are labeled a - e. Each of the
different environments represents how much radiation a given carbon
absorbs (measured in ppm). Despite having 7 carbons, the molecule only
has five clusters of peaks due to multiple carbons existing in the same
environment. Carbons share an environment if their chemical bonds are
identical
Exercize 1
Next to each carbon in the structure, indicate the number of
hydrogens attached to it.
Label each peak cluster with the amount of hydrogen
bonds carbons in that environment have.
Explain why the ‘e’ carbons are in a different environment than
the ‘d’ carbon.
Peak Multiplicity
The three hydrogen atoms attached to the carbon cause the peak
generated to split. Splitting shows the possible spins
of the hydrogen atoms. the amount of post-split peaks
is equal to N+1 where N is the number of bonded hydrgogens.
Hydrogens bonded to carbons can have either a positive or negative
spin, the peaks spit in a way to show the probability of a given spin
set.
Naming Conventions
A cluster with one peak is a singlet (s)
A cluster with two peaks is a doublet (d)
A cluster with three peaks is a triplet (t)
A cluster with four peaks is a quartet (q)
A cluster with five or more peaks is a multiplet (m)
Exercize 2
Label each peak cluster as a singlet, doublet, triplet, quartet, or
multiplet.
Which of the following best explains the ppm values of a given peak
cluster
The number of hydrogens attached to a carbon atom
The total of non-hydrogen bonds the carbon forms
The distance from the bromine (number of bonds in between the carbon
and bromine)
13C NMR Ranges
The x axis of the spectrum measures the chemical
shift of a carbon environment. Chemical shift (measured in ppm)
is equal to the frequency of resonance normalized by the frequency
rating of the NMR machine used. This allows machines with different
frequency ratings to produce the same chemical shift. The exact ppm of
an environment is difficult to predict so they are usually represented
in general as a range of values.
The chemical shift of a carbon environment is related to the density
of the electron cloud. Therefor, things such as double bonds, and bonds
to electron hungry elements effect the ppm of a given carbon
environment. These effects are cumulative so a combination of a double
bond and a bond to a halogen will both effect the chemical shift
Exercize 3
Label each peak with the number of its corresponding atom(s) and
identify the multiplicity of the peak
Coupled and Decoupled Spectra
A decoupled
13C graph of 2-bromobutane
As molecules get more complex, sometimes their peak clusters will
overlap, making interpretation very difficult. To deal with this, the
graph is decoupled, removing the effects of the
hydrogen atoms from the carbons. This turns the peak clusters into
singular peaks
Excercize 4
What information is lost in decoupling?
What structural information remains?
Is the hight of the peak correspondent to the amount of carbons in
the environment?
Sketch the skeletal structure of 3-methyl-pent-1-ene, then sketch the
coupled and decoupled 13C spectrum.
NMR Reference and Solvent Peaks
Often in NMR spectra there will be a peak at 0.0ppm and three at
77.00. This is due to the machine reading resonance from additives that
act as solutes or as consltants for the machine to tune to. The most
common additives are TMS and CDCl_3_
DEPT NMR
DEPT NMR spectrum of
2-bromobutane
A DEPT spectrum consists of four spectra:
The first spectrum shows carbons with three hydrogen bonds
The second shows carbons with two hydrogen bonds
The third shows carbons with only one hydrogen bond
The last shows an overlay of the previous three as well as any peaks
with more than three hydrogens
Despite the peaks not being split, the mutliplet type can still be
determined based on the layer it appears
Singlets on DEPT Spectra
Because DEPT spectra only show carbons with attached hydrogens,
singlets do not appear.
Exercize
Label each peak with the letter q, t, d, or s, and assign the peaks
to carbons 1-4 on the structure of 2-bromobutane on the spectrum.
Explain why the TMS peak appears on the bottom and top levels of the
DEPT spectrum.
Label each carbon on the molecule butan-2-one with what type of
multiplet it is
Label the peaks on the DEPT spectra with their corresponding
carbons (1, 2, 3, or 4)
Which of the carbons does not appear on the DEPT
spectra?
What ppm range do you expect carbon 2 to have?
Mirror Planes
Some two dimensional objects have mirror planes in which a line can
be drawn splitting the object into two identical objects.
Draw all lines on symmetry in the following objects
Mark all planes of symmetry on the following molecules
Identifying Carbon Equivilence Through Symetry
To determine if C_a_ and
C_a_^ are chemically equivalent draw two coppies of the molecule. in the first, replace C~a~ with an X and in the second, replace C_a_^
with an X. Using rotation and mirroring across lines of symmetry, if you
are able to make the two molecules identical, then the two carbons are
chemically equivalent.
Example
Number the carbons in the above molecules, giving chemically
equivalent carbons the same number label.
Label each carbon with its multiplet type.
Spectra to Structure Infference
The decoupled 13C spectrum for C_6_H_12_ is shown above. Draw
a possible molecular structure that would produce this spectrum.
The 13C spectrum for C5H10O is shown
above. Draw a possible molecular structure that would produce this
spectrum.
The 13C spectrum for another C5H10O
molecule is shown above. This molecule has five carbon environments
instead of the previous molecule’s three. Draw a possible molecular
structure that would produce this spectrum.
Conclusion
NMR (nuclear magnetic resonance) spectroscopy is the most
powerful tool that scientists have for looking at the structure of
organic molecules
Medicine makes extensive use of this technique–though they drop
the word “nuclear” and call it MRI (magnetic resonance imaging)
The physics behind NMR is very complex, although it is
essentially similar to other spectroscopy such as IR—except that radio
frequency radiation is used instead of infrared light
The very low energy radio waves excite a property called nuclear
spin
Though many organic chemists do not have a deep understanding of
the theory and physics behind NMR, it is critical that a chemist be an
expert at interpreting NMR spectra
Interpretation of a (decoupled) carbon NMR spectrum involves two
variables: number of peaks, and peak location
The challenge is figuring out how many peaks are expected for a
candidate structure; in other words, figuring out which carbons are
unique and which carbons are equivalent
The two key tools for this are identifying mirror planes and
visualizing the molecule in motion
Important Points
ppm or chemical shift
If a C is close to an electronegative element or involved in a
multiple bond, or both, you will find the corresponding peak at higher
ppm (farther left on the spectrum)
Each chemically distinct C should have a unique chemical shift,
though in practice different peak clusters sometimes overlap just by
coincidence - this can make interpretation of coupled spectra very
difficult, which is why decoupled spectra are usually taken
You do not need to know the exact ppm value associated with a
carbon on a given structure - You need only know the ppm ranges for
common types of carbon atoms
Peaks
Peak height is not a reliable measure of the number of carbons of
a given type in C-13 NMR
Multiplicity in a proton-coupled 13C NMR spectrum tells you the
number of H’s attached to a given carbon - For example, a C that
produces a doublet must have one H attached to it
Coupling
Very often chemists record “proton-decoupled” 13C NMR spectra -
Such spectra have a singlet for each chemically unique carbon. This is
useful for complex molecules for which the peak clusters would
overlap
A decoupled 13C NMR spectrum tells you the number of different
carbon atoms present in the sample, but not the multiplicity - This is
where a DEPT spectrum can be very useful as it tells you the
multiplicity of each peak appearing on the 13C NMR spectrum
DEPT spectra do not include singlets - this means it is usually
necessary to use a DEPT in conjunction with an ordinary 13C NMR
spectrum
Using the DEPT, you can assign each peak on the ordinary 13C NMR
spectrum as a singlet, doublet, triplet, or quartet
Keep in mind that an NMR spectrum is a “snapshot” of the molecule
over a number of seconds
Molecular motion such as rotation about single bonds gets
averaged
This means the OH group in phenol does not break the symmetry of
the molecule
Label each peak with the number of its corresponding atom(s) and
identify the multiplicity of the peak
The decoupled 13C spectrum for C_6_H_12_ is shown above. Draw
a possible molecular structure that would produce this spectrum.
The 13C spectrum for C5H10O is shown
above. Draw a possible molecular structure that would produce this
spectrum.
The 13C spectrum for another C5H10O
molecule is shown above. This molecule has five carbon environments
instead of the previous molecule’s three. Draw a possible molecular
structure that would produce this spectrum.