Transcript NMR-Understanding.pps

Understanding 13C NMR
spectroscopy
Why carbon–13?
Nuclear magnetic resonance is concerned with the
magnetic properties of certain nuclei. In this course we are
concerned with the magnetic behaviour of 13C nuclei.
About 1% of all carbon atoms are the 13C isotope; the rest
(apart from tiny amounts of the radioactive 14C) is 12C.
Carbon-13 nuclei fall into a class known as "spin ½" nuclei.
The effect of this is that a 13C nucleus can behave as a
little magnet. 12C nuclei don't have this property.
13C
NMR relies on the magnetic properties of the
nuclei.
13C
Since only 1% of carbon atoms are 13C, only a fraction of
the molecules in a sample will contain any 13C.
However, even a single drop of a compound will contain
billions of molecules. Among all of those, there will be
plenty of 13C atoms in every possible position within the
molecule.
Nuclear spin
Protons and neutrons in nuclei can be regarded as
spinning about their axis.
In many atoms these spins are paired against each other
and so the nucleus has no overall spin (e.g. 12C, 16O).
In other atoms, including
overall spin.
13C
and 1H, the nucleus has an
A nucleus that spins (a moving electric
charge) generates a magnetic field.
The direction of the magnetic field
depends on which way the nucleus spins.
Spin state opposed to
applied field
Energy
Energy gap corresponds to
frequency of radio waves
Applied
magnetic
field
Spin state aligned
with applied field
Normally the two possible spin states of the nucleus have
the same amount of energy.
However, in a magnetic field, the two spin states have
different energies.
The nucleus of a 13C atom has a very weak magnetic spin
(represented as a tiny magnet S N ).
If a molecule containing 13C is placed in a strong magnetic
field, the magnetic 13C nucleus can line up with the field or
line up against it.
S
S
N
N
S
N
N
S
S
S
N
Low
energy
N
High
energy
Which of these is the higher energy orientation?
Excited state = High energy
N
N
S
S
Add energy
N
S
N
Energy released
S
Aligned = Low energy
N
S
N
S
Back to low energy ground state
When the spin falls back into line with the magnetic field it
releases energy.
We detect this energy and it provides information on the
environment of the carbon atom in the molecule.
In physics, resonance occurs when a system is able to
easily oscillate between two different energy conditions.
The flipping of 13C nuclei from one spin state to the other
is nuclear magnetic resonance.
The energies involved, which depend on the exact
conditions around each carbon nucleus, are those
corresponding to radio waves – about 25–100 MHz.
When a nucleus in a particular magnetic field has exactly
the right amount of energy to flip, it has reached the
resonance condition.
In NMR spectroscopy we normally supply a constant
amount of energy (eg 40 MHz) and change the magnetic
field.
Electron shielding
So far we've been considering 13C nuclei in isolation, but
these nuclei are normally surrounded by electrons. These
electrons help to shield the nucleus from the magnetic
field.
You need a stronger field to flip a 13C nucleus inside an
atom than you would if that nucleus had been stripped of
all its electrons.
Most 13C nuclei aren't found in discrete atoms though,
they're found in molecules whose atoms are bonded
together by covalent bonds. The amount of shielding
provided the electrons around each nucleus depends on
which atoms it is bonded to, and which atoms they are
bonded to.
What happens if a 13C atom is bonded to an oxygen atom?
Oxygen is more electronegative than carbon, causing the
shared electrons to spend more time with the oxygen atom
and less with the carbon.
This reduces the amount of shielding around the 13C
nucleus, allowing the nucleus to resonate in a weaker
magnetic field than if it were bonded to another carbon
atom.
Tetramethylsilane
Tetramethylsilane, usually called
TMS, is used as a calibration
standard in NMR spectroscopy.
Silicon has a lower electronegativity
than carbon, (carbon's is 2.5 and
silicon 1.8) causing the carbon nuclei
to have slightly more electron shielding than they would
have when bonded to most other atoms.
Thus the carbon nuclei in TMS (and all four nuclei are in
the same environment) require a stronger magnetic field to
reach the resonance condition than those in other
compounds.
The field required for TMS is set to zero, with all other
carbon atoms compared to it.
Scale and units
What we're measuring in NMR spectroscopy is the
difference in magnetic field required to bring each 13C
nucleus into resonance. This difference is called the
chemical shift, symbol δ.
Even in the powerful magnetic fields used in NMR
spectroscopy, the differences in field required to flip a 13C
nucleus in TMS, and one bonded to a C or O atom is very,
very small – so much so that we need to multiply this
difference by a million to get a useful number!
This equation produces a ratio, which has no units. Because
we've multiplied by a million though, the unit for chemical
shift is given as ppm (parts per million).
Chemical shift measured the difference in the magnetic
field required to reach resonance, compared to TMS.
But virtually every carbon atom we study will have a smaller
field than required for TMS.
The scale for chemical shift, like that for IR spectroscopy,
runs backwards.
200
150
100
50
δ chemical shift
0
The peak for TMS – if shown – will appear on the right
hand edge.
A peak at a chemical shift of 120 is said to be downfield of
TMS.
Deuterated chloroform – CDCl3
CDCl3 is often used as a solvent in
13C
NMR.
Chloroform is a useful solvent in organic chemistry, but
the 1H atom in normal chloroform, (CHCl3) has a magnetic
spin like 13C, so it has been replaced by deuterium, 2H,
which has no magnetic spin.
The peak from CDCl3 appears at a chemical shift of
77 downfield of TMS. Like the TMS peak, it has been
deleted from the spectra in the Spectral Database of
Organic Compounds.
This is the 13C NMR spectrum for ethanol.
It has two peaks, because there are two carbon atoms.
The peak at 60 ppm is from the right-hand C atom.
CH3
|
CH3—C—CH2—CH3
|
OH
2-methylbutan-2-ol has 5 carbons, but there are only 4
peaks on this 13C NMR spectrum. Why?
Two of the carbon atoms are in exactly the same
environments, so their chemical shifts will be the same.
Peak height
On this spectrum, we have
one tall peak and three
shorter peaks.
CH3
|
CH3—C—CH2—CH3
|
OH
It is reasonable to suggest
that the tall peak
corresponds to those two
identical CH3 groups.
However, because of the random distribution of 13C atoms
through the sample, you cannot be certain that any extratall peak is caused by multiple carbons in the same
environment. That particular sample may simply have had a
lot of molecules containing 13C in the same position.
Carbon environments
Recognising equivalent carbon environments is a critical
step in interpreting 13C NMR spectra.
Can you see why 2-methylbutan-2-ol
has 4 carbon environments and not 3?
Why is the final CH3 group different
from the other two?
It is attached to a CH2, whereas they
are attached to a carbon bonded to an
oxygen.
Who the neighbours are – and who
their neighbours are – changes the
environment.
CH3
|
CH3—C—CH2—CH3
|
OH
Experienced chemists use tables of chemical shifts to
identify every peak on a 13C spectrum.
All you will be required to do is to use a 13C NMR spectrum
to distinguish between small numbers of possible
compounds, mostly by matching the spectrum to the
correct compound by its number of carbon environments.