The simplified world of''

1
The simplified world of..
2
Nuclear magnetic resonance spectroscopy is the
use of the NMR phenomenon to study physical,
chemical, and biological properties of matter.
3
They changed the name for use in the medical
field because the word nuclear might scare some
people. No one wants to get nuked! There's no
need to fear, NMR and MRI use harmless radio
waves to acquire their data, not the gamma rays
that "nuke ya." In fact, radio waves are on the
opposite end of the electromagnetic spectrum from
gamma rays. Take a look
4
NMR is also a characterisation technique where a
sample is immersed in a magnetic field and hit
with radio waves. These radio waves encourage the
nuclei of the molecule to sing a song for us that
can only be picked up on a special radio
receiver.
5
NMR is a phenomenon which occurs when the nuclei
of certain atoms are immersed in a static
magnetic field and exposed to a second
oscillating magnetic field. Some nuclei
experience this phenomenon, and others do not,
dependent upon whether they possess a property
called
spin
6
Scientists can't just call it spinning though.
They have to try and sound smart and call it
resonance. As the positively charged nucleus
spins, this moving charge creates a magnetic
moment. You can think of it as a spinning
subatomic bar magnet. When no magnetic field is
present, these tiny magnets are aligned randomly,
but when they are placed in a homogenous magnetic
field, the magnetic moments will line up with the
magnetic field
7
the nuclear spin is not simple and flat like the
spin of a merry-go-round. The thermal motion of
the molecule creates a torque which makes the
magnetic moment "wobble" like a child's top.
8
Here are a few water molecules.
9
Each water molecule has one oxygen and two
hydrogen atoms
10
If we zoom into one of the hydrogens past the
electron cloud we see a nucleus composed of a
single proton. The proton possesses a property
called spin
11
Think of the spin of this proton as a magnetic
moment vector, causing the proton to behave like
a tiny magnet with a north and south pole.
12
When the proton is placed in an external magnetic
field, the spin vector of the particle aligns
itself with the external field, just like a
magnet would. There is a low energy configuration
or state where the poles are aligned N-S-N-S and
a high energy state N-N-S-S.
13
When the proton is placed in an external magnetic
field, the spin vector of the particle aligns
itself with the external field, just like a
magnet would
There is a low energy configuration or state
where the poles are aligned N-S-N-S
and a high energy state N-N-S-S.
14
This particle can undergo a transition between
the two energy states by the absorption of a
photon. A particle in the lower energy state
absorbs a photon and ends up in the upper energy
state
photon
Ehv
15
(No Transcript)
16
Different nuclei resonate at different
frequencies. This means that you must hit a
carbon atom with a different frequency radio wave
than a hydrogen atom to get it to flip. It also
means that similar atoms in different
environments, such as a hydrogen attached to an
oxygen and a hydrogen attached to a carbon, flip
at different frequencies. By seeing at which
frequencies these different nuclei flip, one can
determine how a molecule is put together, as well
as many other interesting properties of the
molecule.
17
I understand that two different atoms, like
carbon and hydrogen, resonate at different
frequencies because they are different, but why
do two similar atoms in different environments
resonate at different frequencies?
18
That is a very good question. The answer is
shielding. What is shielding you ask? Well I'll
tell you. The electrons which surround the
spinning nuclei are also charged and spinning,
and if you've been paying attention, you know
that a spinning charge creates a magnetic field.
which is in opposition to the applied magnetic
field.
Wendy says..
19
This decreases the magnitude of the applied
magnetic field which reaches the nuclei. In other
words, the electrons "shield" the nuclei from the
full magnetic field. Because the resonant
frequency of a nuclei depends on the strength of
the magnetic field it "feels" . . . I think you
get the idea.(NOT!!!)
20
Okay, I know what an NMR spectrometer does, but
how does it work?
21
The majority of the machine is just a big
"cooler" filled with two very cold liquids,
liquid helium and liquid nitrogen.
This is where you put your sample
The liquid helium is in the innermost part of the
"cooler" to cool the super conducting coil which
creates the magnetic field to -269oC
22
Electromagnetic radiation
Big Magnet
Rotating sample
23
Now that our sample is in a magnetic field,
locked and spinning, we can acquire a spectrum.
First, an RF (radio frequency) generator
"pulses" the sample with a short burst of radio
waves. These waves are absorbed and transmitted
through the sample to the receiver which detects
the signal from the sample. This information is
then transmitted to the computer next to the NMR
where it is translated and analysed
24
A Typical NMR Spectrum
Absorbance
chemical shift
25
There are four aspects of an NMR spectrum that
will allow us to determine the identity of the
molecule. The number of NMR signals The
position of each signal relative to a reference
signal The relative area under each signal The
spin coupling or spin spin splitting pattern of
each signal
26
aspirin
(CH3COOC6H4COOH)
27
(No Transcript)
28
butane
(CH3CH2CH2CH3)
29
...ratio of height of peaks
46 or 23
30
(No Transcript)
31
Consider methanol, CH3OH, which has two types of
hydrogen atoms. The three methyl hydrogens have
one chemical shift while the lone hydroxyl
hydrogen has a different chemical shift. The NMR
spectrum for this compound shows two peaks. Above
each peak is an integration line, the height of
which represents the area under the peak
32
(No Transcript)