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>> All right so listen today what I want to talk
about is something that's more conceptual.
We've been sort of talking nuts and bolts
and today practical stuff, interpretive stuff.
Today I want to talk a little bit more
about how some stuff works in NMR spectroscopy.
It's not going to be anything too fancy
and it gives you a little perspective on my take
and understanding of NMR experiments,
which is very different say
than Professor Shaka's [phonetic] take
or interpretation or Professor Martin's take or interpretation.
These are NMR development people and my group
and you will be users of NMR spectroscopy as a research tool,
but one of the things is you can't just take NMRs
as a black box.
As you're already seeing when you're going
down using the instrument you kind of have to keep your head
about you, oh, what am I doing, why am I locking it,
why am I shimmying it, what's the shimmy doing,
what does it do if I don't do this?
And what I would like to be doing is giving you a feel
for some parameters and things
that you may not be understanding in spectra.
So we've already, for example, had instances where people look
at their own DEPT spectra and say wait a second why do I see
in the DEPT 90 a little peak for say a methyl group
or a little peak for a CH2 group?
A lot of this has to do with parameters
and so that's what I want to give you a feeling about.
We're going to start with 1 experiment that's kind
of like a DEPT but a lot less fancy
and I think it's one that's easy to understand.
It's called the APT.
So it was a precursor.
That stands for attached proton test.
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And the more NMR spectroscopic name for the technique
which actually is going to make a lot of sense to you
when we start to talk about how it works is called the J
modulated spin echo technique.
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And then we'll talk a little bit about DEPT.
We won't go, and understand it as well,
but you'll see how the parameters that we talked
about for the APT relate to the DEPT experiment.
DEPT if you don't know is Distortionless Enhancement
Bipolarization Transfer.
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From a practical point of view what the depth does
as you all know is allows you to identify the CHs, CH2s and CH3.
So in other words, the methines, the methylenes
and the methyl groups.
What the APT does is it's a less sophisticated experiment.
It gives you your Cs, your quats and CH2s and distinguishes them
so I'll say distinguishes let me maybe put it that way.
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Quat and CH2 from CH from your methine and methyl.
So in a way, the DEBT experiment is more
of a Litmus test experiment.
So we're going to start to get a feel
for complex pulse sequences.
I'm going to introduce some concepts like the rotating frame
and the effect of various pulses.
We talked a little bit about this
when we introduced NMR spectroscopy
and we'll see a spin echo experiment.
So I just want to review some of the spin physics that we learned
when we started to talk about NMR spectroscopy.
So remember you start and you've got this difference
in population between the alpha and beta states and that leads
to a net magnetic vector along the X axis and remember
that vector is a bunch of vectors that are precessing
and we talked before about the concept
of applying a 90-degree pulse.
So remember 90-degree X pulse and you think
about the right hand rule
that means you have a quail along the X axis you're applying
essentially a 4 sign to the vector,
which is bringing the vector down into the X, Y plane.
That brings it along the Y axis.
Now remember what that means.
Having the net magnetization in the X, Y plane means
that we've now equalized the alpha and beta state populations
by having a differentiated population of alpha
and beta states you have net magnetization along the Z axis,
but when the alpha states are equal to the beta states,
then you have no net magnetization along the Z axis.
What you do have is magnetization on the X, Y plane
and because you've done this as a pulse all
of your vectors start out initially along the Y axis
and then remember you have your precession that leads to motion
in the X, Y plane of the vectors and that gets picked
up by the coil and gives you a cosine wave
that when you Fourier transform gives you a peak.
So the other pulse I'll give you an idea on is just remember
if we apply instead of a 90-degree pulse,
180-degree pulse along the X axis,
now that flips the population of the alpha and beta states.
So, instead of dragging the magnetization down into the X,
Y plane you dragged it all the way along
to the negative Z axis.
I'm trying to represent things in 3 dimensions here.
So that brings our magnetization to the negative Z axis.
If you think about it, this means now
that you've inverted the population of alpha
and beta states so if we had more nuclei in the alpha state
than the beta state before then after more nuclei in the spin
down state in the, if you've had more nuclei in the alpha state
than the beta state, you flipped the population over here.
So, that's basically all the spin physics that we need to get
to a point to really think our way through an experiment
that ultimately is going to allow us
to distinguish our quats and our methylenes
from our methyls and methines.
So now I want to develop that idea.
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So let's take this situation here and I'm just going
to project the X, Y plane into the plane of the blackboard
because it's going to help us to look down on the system rather
than for me to try to keep making these horrible 3
dimensional drawings.
So, okay, here we are in this situation
where we've got our X axis, we've got our Y axis
and now we're looking down the Z axis
and we have our magnetization along the Y axis.
Now, remember the magnetization along the Y axis is precessing
and so if you just think of this as a single line,
it's going to precess at an angular velocity omega.
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Like if it's 500 megahertz you're going to have precession
at 500 million times per second in the X, Y plane.
That's this angular velocity if you're interested.
It's just my way of representing that it's moving.
So in other words as you wait, if we're precessing
around in the X, Y plane as you wait, sometime T now
when we look at the X, Y plane now your vector is over here
and it's continuing to precess.
Remember this is what gives rise to your signal that's your FID
where you get this cosine wave
over here that's basically the coil picking
up your precessing vector and generating electricity,
generating alternating current in the coil
that ultimately you amplify and use the analog
to digital converter on and then Fourier transform.
All right what I want to do at this point though is
to introduce a concept that NMR spectroscopists use
to make their lives easier and the idea is the rotating frame
and it's basically saying let's just have our axes precess
for conceptual purposes actually works
out for the detection purposes
with various electronic techniques,
but let's have our axes rotate at the angular velocity.
So that means we'll have the axes rotate at omega
and so we'll have our rotating frame and we'll call
that X prime, Y prime.
So now if you think about it if you're in the rotating frame,
if your frame is rotating, your frame of reference is rotating
with angular velocity omega and we wait time T
from the rotating frame how does our magnetization look
at time T?
Exactly the same and that's the whole big concept
of the rotating frame is simply we go ahead and we adjust
so we're not spinning around it 5 million times per second.
All right now we're ready to introduce the concept
of the spin echo and that's going to be the basis
for differentiating our methines and methylenes, our quats
and methylenes from our methines and methyls
in the DEBT experiment.
Okay, so the idea behind the spin echo is as follows.
We're going to be it the rotating frame and so we start
with a 90-degree pulse and I'll call it a 90-degree X prime