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.

[ Writing on board ]

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.

[ Writing on board ]

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.

[ Writing on board ]

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.

[ Writing on board ]

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.

[ Writing on board ]

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.

[ Writing on board ]

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

pulse just to show that we're working in the rotating frame.

Imagine for a moment now that I have some vectors

and they start along the Y prime axis, but now let's say

for a moment that some of those vectors are moving a little bit

faster than the Larmor frequency than the velocity omega and some

of them are moving a little bit slower.

So, in other words, we've got our frame rotating

at 500 million cycles per second

but some vectors are going a little faster

and some vectors are going a little slower.

So now if you imagine on this thought experiment

that you wait time T, now what's going to happen after time T

to after some time to those vectors?

The ones that are rotating slower will appear

to go clockwise and the ones that are going, right,

clockwise and counterclockwise.

So now our vectors are going to fan out with respect

to the rotating frame and those

that are going one direction are going to be going this way.

I think it's actually counter clockwise in this axis,

axis system, but the point is

that they're diverging in velocity.

So now we've got some that are headed this way and some

that are headed this way.

Now, imagine for a moment we now at this point

after time T apply a 180-degree X prime pulse.

In other words, now we do the same thing that we did before

and we apply a 180-degree pulse.

Now, when you do that, again just think right hand rule

and remember any component

of your magnetization that's along the X prime axis

when you apply a pulse to it right hand rule,

it doesn't do anything.

Any component of your vector that's along the Y prime axis

flips over 180 degrees.

Does that make sense?

So when we take this vector

and we apply our 180-degree pulse the Y prime component

comes on over to the negative Y prime like so and we do

that to all of these different vectors and they've all flipped

around the X prime axis and they've all gone

to the negative Y prime axis and yet they are still precessing

at the rotating frame but now they're headed inwards

and that's the key to this whole thing.

So because they're still going

in the same direction these ones are a little bit faster,

these ones a little bit slower

than the rotating frame they converge inward

and if we wait another time T, that same time increment

so I'll say T again now what's our picture at this point.

They're all on the negative Y axis so they are all

like so along the negative Y axis.

This is the basis for many, many sorts of NMR experiments.

This is called the spin echo and one of the reasons

that people invariably do spin echoes is in addition

to what we're talking about here which I'll show you in a second

which is J modulation, you've also got T2 relaxation

where vectors fan out in the X, Y plane

and what the spin echo experiment does,

what the spin echo does is a refocusing of those vectors.

So the ones that are inadvertently due

to T2 effects moving a little faster and the ones

that are moving a little slower

by applying this 180-degree pulse halfway

through they refocus and so almost every experiment you see,

the ones I'm talking about now

but also 2D experiments are going to involve some sort

of thing that involves a weight and then an equal weight.

So if I write out this simple pulse sequence,

what it is is we apply a 90-degree pulse, we wait,

we apply 180-degree pulse, we have an equal weight and at

that point the spin is refocused.

[ Writing on board ]

And so if you've already started to notice as you're working some

of the 2D experiments, there are various delays

and you may see this in the handout about various delays,

some of the delays you've learned

about in the 1D experiments may be relaxation delays

where it's just for your magnetization to return