Right, so we're going to move away from the theory,

move into more now about the analysis of organic or

biological compounds and how you interpret spectra.

And as I say here, that most organic molecules are made

up of carbon 12 Protons, Oxygen and Nitrogen.

That's a fair approximation.

Now we saw the last day, because you have even numbers of protons and

even numbers of neutrons on carbon 12, and oxygen 16, they have spin I=0,

so you had no, you don't have the property of angular momentum for these nuclei.

So they won't have a moment when they're placed in a magnetic field.

So you don't have any more transitions.

And the one H spin which we talk about mainly is I=1/2 14N has I=1.

So you do have NMR absorption.

But nitro 14 is a different gyromagnetic ratio than the proton so

it will absorb in different regions.

And then you have Itirium 2H.

That has spin I=1, so that will have anymore transitions.

And then you have a very important nucleus carbon-13 which is only 1%

abundance compared with carbon-12, but it does have a I=1/2 and

it's a very important nucleus that people look at using Inamori spectroscopy,

because it's got a spin and therefore it will give spectra.

And even though it's in the very low, lower abundance, with modern instruments

you can observe natural abundance carbon tardeme and it's very useful.

But the vast majority, at least on the basic level and

litical level is done with the proton nucleus.

So just a little bit again about what spectrometer or schematic.

So, it's again, you have to have the magnet.

Here you have the magnet.

Here you have a sample tube.

[COUGH] And, again you have to size and

the difference set between UV and the infrared spectroscopy.

What you have there is you have a sample and you have something that will give you

the electromagnetic spectrum or betas in the range of the electromagnetic spectrum.

And then you get your transitions.

As I said, the key in this inamori experiment the magnet,

as well, because it's the magnet that creates the levels.

So again, you have a magnet, you have a sample tube, again, it says not to scale,

obviously, because the sample tube is going to be smaller than the magnets.

Then you have some way of generating a radio frequency [COUGH] field.

And you have a detector with electronic systems.

You can get a spectrum from your sample.

Okay, so again, so you have a magnetic field, so say the 7T field.

You know that that proton comes into rest at around

300MHz from the equation we just talked about.

So the old spectrometers there's two ways you can do it.

You can hold the magnetic field fixed.

And you can scan the radio frequency field and

then find out where you get your absorptions.

And that's how we do a UV vis and an infrared spectrum.

But, alternatively, again, in the old days,

you can actually keep the radio frequency fixed.

So you keep this fixed and you'd scan.

You'd have some way of changing the magnetic field because we change

the magnetic field, you're changing the energy levels.

What you do, modern spectrometers now used what's called

where you keep the magnetic field fixed and you pulse with a range of frequencies.

But that's a more advanced technique.

Okay so as I was emphasizing there this approach is different to other

spectroscopies because you can adjust the gap if you like between the energy levels

by changing the magnetic field.

Okay so, so far we've said so this hydrogen and as I say here is that it?

And you might say well all we've done, we know if we have a certain

value of the magnetic field, so seven tesla we talked about in the last slide,

then it's if we induce we can have a radio frequency field of 300 MHz.

Then we should get a resonance or

we should get say detect it we get an absorption.

So at the moment all we said is well we can detect hydrogen in a sample.

But that seems to be not very

comprehensive if there was other ways of detecting hydrogen.

So there must be something else to the technique.

And indeed there is, because you have an NMR spectrum,

you may have seen this in school.

If you look at an NMR spectrum of this molecule right here is 3.5-demethylbenzoic

acid, and so far we expect there's a lot of protons on this sample.

And so far you just expect one peak,

but as you can see we talk about the actual way of measuring.

The moment that you can see that you have peaks for different positions.

Here they're color coded within the molecule.

So clearly there's something else that we haven't considered.

These four distinct hydrogens and

there's four lines up there on the spectrum, right?

So what's going on?

And what's going on is that the protons in this molecule

there's protons but they're in different regions of the molecule.

And in addition of course as you know in the molecule the protons of the nuclei

you have also got electrons, and

electrons are particles very similar to nuclei or protons.

They're spinning as well.

They've got angular momentum, they'll spin in half.

And so therefore when you put them in a magnetic field

they also generate a magnetic mold.

Okay?

So and also you know in molecules that you can have electron rich regions

of the molecule and you can have electron-poor regions of the molecule.

So therefore, what's happening is

that the field that the proton sees from the magnet.

The magnets set up the field, magnetic field, that the proton sees will be

different depending on how much electron density has around it.

So I say here electrons moving around the nucleus so they change the total field.

So What you have

then is you have the proton and you have the electrons near it.

And the proton, the magnetic field is a fixed value that you

put in your spectrometer, but those electrons that are rotating around,

are moving around, that they have their own magnetic fields.

And what happens with electrons, in general, is they create a magnetic

field that opposes, opposes the applied field.

So what this is called, it's called shielding.

So here we try to schematically do it.

Here you have your applied field.

Now these electrons most generally said is that,

they setup a Bsh or a shielding field.

And this shielding field is proportional to the strength of the magnetic field

you apply and sigma then is some proportionality constant.

But the key is for most electrons, it opposes the external

field B0.

So when we talk about Beff, we're talking about the actual field

that the nucleus in the molecule feels if you like.

So, say you have your proton here, and

here's your B0 field that your magnet creates.

If you have electrons they will oppose

that field or they shield it from the outside field.

So the field that that nucleus fills is less.

So if the field, so what it means then if you have molecules, you have electrons in

a molecule, and they have different amounts of electron density around them.

If you have electron with a high electron density around it,

then it will be highly shielded, so the field it sees will be quite less.

If you have another one that's not as shielded,

say, then it'll feel a greater field.

And we know from the relationship that the frequency is proportionate to the field so

these will have different resonance frequencies.

So now what we're finding is that depending on the environment of

the nucleus within the molecule, whether it's an electron rich region or

an electron poor region, it will have a different resonance frequency.

And you can detect that in a experiment.

Okay, so sigma is a shielding constant and you can see our B0 is this magnitude

if Bsh is this magnitude, then the effective field that the actual

proton or particular nucleus fields is smaller.

Okay, so I think this is just emphasizing the of what I've said that

hydrogen nuclei they'll have different electrons that surrounds them depending on

what chemical group.

And usually, if you say hydrogen is attached to fluorine which is highly

electronegative then the fluorine will call an electron density away from there.

So, it will be deshielded.

Whereas if you attached it, say to a CH3 group, the hydrogens in the CH3 group

flourine is more electro negative so that will be less shielded.

So different groups of different shielding constant and then because we know

the relationship between the field that the nucleus fields and

its resonance frequency you'll have slightly different absorption frequencies.

So here we have a nice, simple example here.

We're going to vary frequency in that way.

And [COUGH] here we have a deshielded nucleus.

So these protons will feel a much higher

magnetic field than these ones.

So we know that nu is proportional to B0 so

therefore they'll have a higher frequency value.

You'll need a higher frequency value to bring them into resonance.

Whereas these ones, they're shielded, they'll feel a smaller magnetic field.

So therefore they'll need a smaller frequency value c.

Right, okay.

So just because this is a crucial point in a more spectroscopy,

I'll just go through it and emphasize it again to you.

So let me get this program up.

So let's say we have, So we're talking about the chemical shift here.

So we have B0 is equal to 0.

So here we have our plus and minus a half.

And then the axes off the magnetic field they're degenerate.

This is energy on this scale here.

Now if we apply a magnetic field and we have a constant magnetic field,

we're going to have the plus a half down here and

the minus a half up here.

Okay? So here B0 > 0.

So what we have then is we've created an energy gap here.

And this DE which is equal to h nu and

that's equal to h gamma, Over 2 pi B0.

So, let's assume here we have the barren, which doesn't happen,

we have just a barren nucleus with no electron density around it.

So this is like a totally deshielded nucleus.

It's got no electron density to shield it.

Now what we could imagine is we have a nucleus that is likely shielded.

Now if it's shielded, what it means

is that this gap is less.

So we have another energy here, delta E,

and this is equal to h nu as well.

But delta E is smaller so

therefore the frequency of this transition is smaller as well, okay?

And you could think of another one, say it was even shielded any more.

And it brings this gap down even more.

Okay?

So if you're measuring that spectrum then and

as we say, say you're measuring the absorbance versus the frequency and

we're going to increase the frequency in this fashion.

The smallest frequency is going to be for

the green one here because that's the smallest.

So that's going to say occur here.

And then as we increase the frequency, we'll get the red one into resonance.

Let's just draw them all the same height here.

And then as we increase the frequency even more,

as we scan the frequency even more we'll get the blue one.

So what you often hear, I'm just trying to get some terms,

what you'll hear is this is the high frequency, or the deshielded

Region, and this is the low frequency or the shielded,

Region, okay?

So it's important to get this into your heads at this stage

what high frequency with low frequency, shielded and deshielded means.

Another thing you might try to think of is say we kept the frequency fixed.

Say we have a fixed frequency.

So we have that amount.

We are already a sample with that amount of frequency.

What will happen now?

Say we have, with that amount of frequency when we go a shielded case and

we could vary the magnetic field,

how will that change for a shielded case?

Okay.

So we have this gap, we know the key relationship here is B0, and nu.

So we have to match for resonance to occur,

we have to match this precession frequency with this gap here.

Okay?

So when we deshield, or when we shield, like we have here with

low energy the external field will have to increase it.

because don't forget the electrons are shielding from the outside magnetic

field and you're going to have to increase that external magnetic field to

overcome the shielding if you like.

So as you deshield you need to go upfield.

So sometimes again it's called upfield and

the opposite for deshielding it's downfield.

So perhaps difficult concepts to take in at the beginning.

But you need to understand in a more correctly.

You just need these concepts in your head at this stage and

then it's a little bit clearer.

Chemical shift

Introduction to Molecular Spectroscopy

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