0:08

So, we have stopped here at the number,

Â we talked a bit about the theory of infrared spectroscopy.

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

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

Â we can move a linear molecule within atoms,

Â the number of vibrational modes is three and minus five.

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

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

Â So you've got 9.

Â 9 vibrational modes in the spectrum.

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

Â you should see 9, 9 bands in the spectrum.

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

Â in a molecule increases.

Â Then the complexity of the spectrum increases.

Â So, if you have a very complex spectrum.

Â The following is given here.

Â So you've got something like a protein,

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

Â For Fullerene C60 you're going to have 174.

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

Â 1:24

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

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

Â of the molecule.

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

Â we've talked about the simple diatomics.

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

Â And we moved on to talk about,

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

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

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

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

Â 2:13

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

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

Â you have scissoring, rocking, wagging, and twisting.

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

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

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

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

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

Â you're assigning your infrared spectrum.

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

Â we talked about the last.

Â Just because the frequency of the electromagnetic radiation

Â corresponds to the frequency at which the mode is occurring.

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

Â because, we talked about this the other day,

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

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

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

Â 3:42

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

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

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

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

Â then you get an absorption of the radiation,

Â so long as the dipole moments changes.

Â So, in terms of practical spectroscopy,

Â you had different functional groups in your molecule.

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

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

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

Â And also the atomic masses.

Â So they'll have different frequencies.

Â And a lot of infrared interpretation

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

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

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

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

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

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

Â would expect to see say a C double bond O.

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

Â C double bond C.

Â So in practical use of infrared spectroscopy.

Â That's what people do.

Â 5:19

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

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

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

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

Â that's C double bond C.

Â A little bit more in spectrum especially for

Â large molecules can be quite complex.

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

Â And the gas phase spectrum is usually easier to interpret.

Â And this is for a fairly simple molecule, formaldehyde.

Â So you have H2C double bond O.

Â And you put that into your infrared spectrometer,

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

Â let's just assume you can do it.

Â You compare your sample.

Â And this is a spectrum.

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

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

Â It should be four.

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

Â The wave number in centimeters is minus one.

Â And reduce the infrared rays also about four tiles.

Â And wave number down to be minus one.

Â 4,000 down to about four or five.

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

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

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

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

Â in the oldest spectrometers they have percent transmission.

Â 6:57

So we have the peaks, or where it absorbs.

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

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

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

Â you've got six peaks ago.

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

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

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

Â you expect six modes.

Â 7:49

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

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

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

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

Â If you try to stretch something,

Â 8:09

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

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

Â you find stretching modes up here at the higher region.

Â And where they actually occur as stretching modes will depend,

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

Â Some bonds are stronger, and, also,

Â it will depend on the mass of the atoms involved.

Â Remember, we worked out what the reduced mass was.

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

Â And what is increasingly being used now is,

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

Â you would calculate, theoretically, the infrared spectrum.

Â You can do that fairly easily.

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

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

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

Â where you've worked out all the vibrational bands for

Â the molecule corresponding to each vibrational mode.

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

Â