Name: 24.高階效應、動力學和核磁共振時間尺度。 (24. Higher-Order Effects, Dynamics, and the NMR Time Scale)
Uploaded: 2021-01-14T05:21:37.000Z
Duration: 51 min 13 s
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J. MICHAEL MDBRIDE: So, we'll finish up on

about more dynamic things involving decoupling.

Then a little bit about C-13 NMR and double labeling

And then we'll get onto another reaction,

Remember that the proton is four times as strong a magnet

as the Carbon-13, so it precesses much faster.

So it's easy to do a pulse that will tilt the Carbon-13

net magnetization down into the plane, so that it'll

precess and be an antenna that can tell us its local field

without doing the same thing to the proton.

And if we do that, then we see the C-13 NMR spectrum.

Right? it can be either up or down, 50:50

Now suppose at the same time you're doing that, when you do

this pulse and listen to the C-13 to see what its local

That is, if we look in the rotating frame, we can see a

weak field that's horizontal, about which the

So the protons will go down, and then up, and then down,

And what determines how fast it goes up and down?

The stronger it is, the faster it will precess.

And if it's fast enough, then you won't see a doublet for

the C-13 anymore, because you'll see the average.

So you'll see a single peak, you'll proton-decouple the

OK, so it depends on how much power you put in.

Now here's a C-13 spectrum of a given compound.

Where we're listening to the C-13 and we see all these

But we're irradiating the proton at the same time, but

This number up there, 40 decibels, is an inverse ratio

of how strong that horizontal field is for the protons.

So it's 10-4th weaker than what you'll see you

And if we look at the carbon, we see that some of the carbon

signals are split and some are not split.

The ones that are split are the ones that have

So if we start from the left, we see there's one with no

So we're looking at the carbon of the CDCl3.

Then there's a carbon with two hydrogens, a carbon with three

Underneath, a carbon with two hydrogens, it's a triplet.

So very low power in the proton, that's what we see.

Now we're going to reduce the size of that.

And see what happens, is we start increasing the power.

See how the patterns are changing their multiplicity

Because it turns out that when you irradiate the protons by a

complicated mechanism it strengthens the signal of the

And that's called nuclear Overhauser enhancement, as

OK. So now you see the splitting is going away when

we get down to two decibels or one decibel.

Now we see that each carbon gives just a single peak, we've

And notice, incidentally, that the CDCl3 didn't get stronger

Because what strengthened the signal was irradiating the

protons and CDCl3 doesn't have hydrogens on it.

OK, notice also that the carbon without a hydrogen on

it, on the far left here, didn't get strengthened as

OK, so that's decoupling, and it can be useful.

There's two things, you get rid of the spin-spin splitting

And that could be bad if you're interested in measuring

the size of the peaks to get how many carbons there are,

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24.高階效應、動力學和核磁共振時間尺度。 (24. Higher-Order Effects, Dynamics, and the NMR Time Scale)

molecule

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