 Okay, so we've seen that for diatomic molecules, if we include various corrections for non-ideality like anharmonicity and centrifugal distortion and so on, we can predict the spectrum, the infrared absorption spectrum for diatomic molecules pretty accurately. Nail every single peak down to hundreds of a wave number or so. As we move on to triatomic molecules, they have the same sorts of peak, but those bands of peaks begin to overlap and the spectra get a little more complex. And the same pattern continues as we move on to more polyatomic molecules, molecules with even more atoms in them. So for example, I can show you the spectrum for ethane molecules, which I'll put on the screen right here. And so this is the spectrum for ethane, I'll draw the structure of ethane so we can talk about the different types of bonds and angles in an ethane molecule. So this spectrum for ethane has the same types of rovibrational absorption bands that we've seen for smaller molecules at relatively high frequencies. So these correspond to the CH stretch type of absorption. And the reason we can't see as many individual distinct peaks here is partially because there's many different types of CH vibrations. There's six identical, symmetric or identical hydrogens, but there's symmetric and anti-symmetric stretches of various different sorts. So there's a lot of overlapping rovibrational bands for different types of CH stretch that appear in this region. This set of bands would be the angle bending modes. For example, those HCH angle bending modes. So there's different regions in the spectrum that absorb light of different types, but because we have only carbons and hydrogens, only a few different types of bonds, only a few different types of angles, there's only a few different bands in which this molecule absorbs light. If I also show you a similar spectrum for a more complicated molecule, propane. So this molecule again just has carbons and hydrogens in it, has HCH angles, has CH spawns. So not surprisingly there's still some CH stretches, there's still various bending modes that show up at the same sorts of vibrational frequencies as for ethane. The only difference is the propane molecule has more of those. It also has a few, if you look in this region, it begins to have a few types of motions that are not as prominent in the ethane molecule or not present at all in the ethane molecule like, for example, the carbon-carbon-carbon bending angle. That's a type of motion that's not present in the ethane molecule. But in general, alkanes, as we add more atoms, the spectrum doesn't get terribly more complex. We just get more absorption in the regions where we have more different CH stretches and more different angle bends and so on. On the other hand, if we move to a more complicated molecule, that's not just an alkane. So next I'll pull up the spectrum for acetic acid. So acetic acid has not just methyl groups and not just carbon-carbon bonds and carbon-hydrogen bonds, but it's also got oxygens, it's got OH bonds, it's got carbonyl bonds, it's got a larger variety of different types of chemical substituents and chemical groups in that molecule. So consequently we see a larger variety of different types of absorption bands in this molecule. So for example, in this broad band around 3,000 wave numbers, we see not just the CH stretch in roughly the same position as it was for the alkanes because this molecule does indeed still have some CH stretches, but we also see absorption at a wider range of frequencies that is associated with OH stretches. So that OH stretch is a much broader absorption band than the CH stretch. This sharp peak right here is the carbonyl stretch and there's various bending modes and so on. So if we're interested we can go through and identify each one of these individual peaks as corresponding to some particular vibrational mode, whether it's a bond stretch or a collective motion of several different atoms or a bending mode or something like that. The main point however is the spectrum ends up looking different for different molecules not because of the number of atoms of the particular type they are, but as a diagnostic of what types of functional groups there are. Molecules with carbonyl stretches, carbonyl groups will tend to have an absorption peak here around 1,700 wave numbers. Molecules with alcohol groups will have an OH stretch and they'll have this broad absorption band around 3,000 wave numbers. So we see that as the molecules get more complex from just a handful of atoms for ethane, more atoms, more complicated still, the spectra get less and less well defined. So for ethane we can still see the remnant of several individual small peaks. We can identify if we were to zoom in on this portion of the spectrum, we can see the individual peaks at specific frequencies that are caused by absorption of individual row vibrational excitations, but as the molecules get larger and more complicated those sharp peaks blur and get smoothed out to form these perhaps very broad peaks or perhaps still fairly narrow but smoothed out peaks where we no longer see individual row vibrational excitations but a complete band. So these line spectra have become band spectra by the time the molecules get more complex. There's two reasons for that smoothing out of the spectrum. One feature is that as we gain more different types of the same stretch, these CH stretches, sorry CH stretches over here, they different row vibrational collections of row vibrational peaks overlap with one another and becomes harder to distinguish the individual lines and also there's a feature called heterogeneity of the environment of these molecules which we'll talk about more in the next video lecture but that also broadens these line spectra into band spectra. So as a summary as molecules get larger, more complicated, more functional groups you should expect to see more different types of peaks in the infrared spectrum corresponding to those functional groups and in fact that's what infrared spectroscopy is often used for is as a sort of fingerprint or diagnostic technique. If you take the infrared spectrum of a molecule and you observe that it's got a sharp peak near 1700, you know that that molecule probably has a carbonyl group in it. If it has a broad absorption band near 3,000 wave numbers, you know it probably has an alcohol in it. And you can get more precise and depending on whether that peak near 1700 is red shifted by a little or blue shifted by a little, you can use that as a signal perhaps of the electronegativity of the group surrounding the carbonyl group, is it in a carboxylic acid, is it in a ketone. So you can use infrared spectroscopy very usefully as a diagnostic or fingerprint technique to identify different types of molecules but you're typically not going to use it to identify individual raw vibrational peaks going from N equals 1 to N equals 2 or L equals 5 to L equals 6 or something like that. So that's summary of molecular infrared spectroscopy. The next thing we'll talk about is what these spectra look like and how they're different for gas phase molecules as opposed to liquid phase molecules.