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So, we have stopped here at the number,

we talked a bit about the theory of infrared spectroscopy.

And then we had where you have the number of vibrational modes.

There are a number of types of vibrations you can have from molecule, and

we can move a linear molecule within atoms,

the number of vibrational modes is three and minus five.

And for a non-linear molecule, it's equal to 3N minus 6 or N is the number of atoms.

So therefore something like [INAUDIBLE] CH4, n is equal to five.

So you've got 9.

9 vibrational modes in the spectrum.

So, in theory, if you looked at that, the infrared spectrum of methane

you should see 9, 9 bands in the spectrum.

So what happens as you can imagine when you get the number of atoms

in a molecule increases.

Then the complexity of the spectrum increases.

So, if you have a very complex spectrum.

The following is given here.

So you've got something like a protein,

then you're going to have a lot of vibration on modes.

For Fullerene C60 you're going to have 174.

So it's going to be quite a complex spectrum.

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So the whole idea of infrared spectroscopy is to be able to use the position of

these bands to be able to tell you something about the structure

of the molecule.

So, if we go on to move a bit from what kind of,

we've talked about the simple diatomics.

Stretching, you've got stretching mode for the bond.

And we moved on to talk about,

we looked at Tuesday CO2 and water and you see you have a symmetric stretch.

Where the 2 bonds stretch together or you have a nice symmetric stretch or

you have one bond compressing and the other bond stretching or lengthening so

you have symmetric and asymmetric but you also then have bending vibrations.

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Through the years people have tried to classify these types of modes.

Way the atoms move, if you like, in the molecule and

you have scissoring, rocking, wagging, and twisting.

And basically, I have a little animation here, that shows you what these

types of modes are, so here you have an in-plane rocking on the left there.

Scissoring motion, out of plane wagging, out of plane bending.

So there's various types of this basically actual movements.

You need to be able to classify these types of movements in some way where

you're assigning your infrared spectrum.

And also you have to remember I'm just going to pass on through this Is that,

we talked about the last.

Just because the frequency of the electromagnetic radiation

corresponds to the frequency at which the mode is occurring.

It doesn't mean you're going to have a nice infrared band,

because, we talked about this the other day,

towards the end of the lecture, that you also need a change in the dipole moment.

That movement needs to cause a change in the dipole moment.

Otherwise, you won't see any absorption of the infrared band.

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So first of all well just a bit of detail on what spectrum it involves.

So, we talked about the stretching, and the bending, and just to clarify,

that with each of these stretching, and different bends in the molds, you have

the frequency of the electromagnetic ratio coming in, if it's that frequency,

then you get an absorption of the radiation,

so long as the dipole moments changes.

So, in terms of practical spectroscopy,

you had different functional groups in your molecule.

You had a CC single bond, CC double bond, C double bonds O, N H and so forth.

And the thing about it, is, we saw the [INAUDIBLE].

The frequency at which they absorb depends on the force constant for that bond.

And also the atomic masses.

So they'll have different frequencies.

And a lot of infrared interpretation

is based on what we call characteristic frequencies for these particular bonds.

So as we'll see in a minute, people have worked out

where you expect CC stretches come in certain regions of the spectrum.

C double bond C come in other regions, C double O, so forth.

And therefore when you run your infrared spectrum self in the known sample

then you can say oh that band corresponds to the region where you

would expect to see say a C double bond O.

And therefore you can say well there's a C double bond O in that molecule or

C double bond C.

So in practical use of infrared spectroscopy.

That's what people do.

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So therefore, the next thing on the slide there is just saying exactly what I said,

that you can determine from the spectrum where it is at.

And then the last thing is, basically, it's not as easy as it sounds.

It doesn't mean you can just look at the spectrum and say, well,

that's C double bond C.

A little bit more in spectrum especially for

large molecules can be quite complex.

So let's look at a simple case where you have a gas phase spectrum.

And the gas phase spectrum is usually easier to interpret.

And this is for a fairly simple molecule, formaldehyde.

So you have H2C double bond O.

And you put that into your infrared spectrometer,

we're not going to go into the details of how you run a infrared spectrum but

let's just assume you can do it.

You compare your sample.

And this is a spectrum.

if you measure the gas phase spectrum for formaldehyde, that's what you will get.

And you can see the infrared region here is running from about four.

It should be four.

But usually in infrared spectras [INAUDIBLE] is centimeters for minus one.

The wave number in centimeters is minus one.

And reduce the infrared rays also about four tiles.

And wave number down to be minus one.

4,000 down to about four or five.

Four or five hundred, that's the infrared region on the spectrum.

So, we're going from high energy here, high wave number down to low wave number.

Traditionally, that's the way an infrared spectrum is presented.

And also, in this one here I have percent transmission, and again traditionally

in the oldest spectrometers they have percent transmission.

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So we have the peaks, or where it absorbs.

It's like you have an inverse peak, if you look in the spectrum.

Nowadays you can also get them displayed as absorbents, as well.

So what you have in this anyways, no matter how you represent it,

you've got six peaks ago.

1, 2, 3, 4, 5 so you've got six peaks.

And of course you know from what we did there before, that you expect for

a non-linear molecule three and a six vibrational modes, and

you expect six modes.

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And also you have the C double O stretch around here.

And we have these bending, which always [INAUDIBLE] the way infrared spectroscopy

is classified and you have scissors or rocks or and there at lower energy.

I think I mentioned that the last day, it's a very simple explanation.

If you try to stretch something,

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it's going to take more energy than just to bend it.

So you generally find bending modes down at the lower energy region and

you find stretching modes up here at the higher region.

And where they actually occur as stretching modes will depend,

as we're talking in the last day, about the force constant.

Some bonds are stronger, and, also,

it will depend on the mass of the atoms involved.

Remember, we worked out what the reduced mass was.

So it's quite difficult to assign a spectrum like this.

And what is increasingly being used now is,

you would do what's known as an electronic structure calculation, and

you would calculate, theoretically, the infrared spectrum.

You can do that fairly easily.

And then you can predict where these various modes would occur.

And then, that way, you can assign the spectrum quite easily.

So this is almost like an ideal case, in some way,

where you've worked out all the vibrational bands for

the molecule corresponding to each vibrational mode.

Okay, so this is about the transmittance that I talked about.