So we talked about the chemical shift.

And we can see how different positions within the molecule can be assigned,

depending on the chemical shift value that we measure.

But we also want to take in, there is another characteristic of intermol

spectroscopy that's quite useful in assigning particular band positions,

and this is called spin coupling.

So the best way to explain this is to take a simple molecule spectrum,

here we have one two dichloroethane and here you have four protons.

So the four equivalent hydrogens,

that can't be distinguished they're in the same position within the molecule and

you expect and here you have your tetracycline standard at zero ppm.

Then you would expect 1 peak here and

you get 3.73 and of course it's a bit high.

Mid chemical shift value.

Mid to high frequency range, and that's due to the fact that the chlorine is

literally negative in polyelectron densities away from the hydrogens.

So anyways, you get a shift value of 3.73.

So your stage of understanding more at this stage,

you can understand that the spectrum from this molecule.

When you look at the related molecule well

now you've changed one of the chlorine to carbon,

so now you have 1.1 dicholoroethane.

Now if you're looking at this molecule, you would say,

well you have a CH3 group here and

all the hydrogens on that are the same or equivalent.

And then you have another one here that's joined to this carbon and

that's distinct and that's bound closer to the chlorines.

So you would expect from the chemical shift alone,

that you would get a peak for the CH3 protons.

And you'll get a peak here for the hydrogen here to just [INAUDIBLE].

So you'd expect two peaks in this spectrum.

And based a little bit on what we're talking about, just what you'd expect that

the CH3 proton would have a lower chemical shift and this

proton here that's near to the chlorines, would have a higher chemical shift.

It would be more deshielded.

And that's what you'd expect.

You expect two lines, then it's also you'd expect, because there's three protons for

this group, the intensity would be three times this one here.

So instead, what you get is you get a doublet here.

You get two peaks together, close together here.

And also, you get this one.

You get four peaks.

So what's going on here really?

Because we're getting, based on our knowledge so far,

we're getting these extra peaks in the spectra.

So we have to try and understand why this is happening.

So the expected peaks, as I say, instead of having a singlet,

you have a doublet at delta 2.06 and you have a quartet at four lines at 5.89.

And basically, what we're gonna teach is that such spin-splittings,

you frequently observe them.

When you go to higher resolution, you see that, instead of getting,

where you expect one peak from the coming shift, you observe multiple instead.

So first we're gonna describe what you expect with these multiplers.

As I said, this is quite common and basically the patterns are symmetrically

distributed on both sides after central position.

The central lines are always stronger than the outer lines.

And then you observe common patterns.

And what you observe commonly is a doublet two equal intensity

lines like we saw in the spectrum previous.

You can get a triplet and you have an intensity ratio of 1:2:1.

You can get a quartet, which we saw in the last spectrum, 1:3:3:1.

And you can get a quintet and so on and you can get sextets as well.

So you're getting this splitting if you like,

off where you expect from at least from a chemical shift argument.

We would just expect one line.

So what you get is typically like this.

We get a doublet, you have a triplet, 121,

1331 and that's 12421 in this one, quintet.

So the lines separation is always constant between these multiplets.

And what we call this is, we call this J.

And when we measure the magnitude of J, it's usually in hertz.

Remember, our input frequency is a megahertz.

Well these are quite small couplings.

They're in hertz, and typical chemical shifts are in kilohertz.

So these are quite small, and also it doesnt depend on the magnetic field.

So again, if you remember back to the chemical shift,

chemical shift does depend on the magnetic field.

But these couplings that we see here don't depend on a living field.

So basically these are small splittings, and

they range from a fractions of a Hz, up to 18 Hz.

And for the example we just saw this splitting,

if you measure this, it will come out to be 6 Hz.

So they're quite small, small, small splittings.

Spin-spin coupling

Introduction to Molecular Spectroscopy

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