 Previously we looked at molecular rotations and these produce very simple spectrum like we have region where every line is spaced quite evenly by two times the rotational constant. But now we're going to add more energy and enter the infrared. Again, let's cut to the punchline because you're probably aware of some of this already. Infrared light induces vibrations in molecules. It's not enough energy to move electrons around but it's far more energy than is needed to simply begin rotating them. And this should make some sense because atoms are held together by electrostatic forces and rotations don't really push a pull against that but molecular vibrations do. And now this also gives us a little bit of distinction between the idea of stretching the bond and bending them. The idea of bending obviously does not push a pull against the bond itself. But as you'll see when we discuss vibrational modes in a bit more detail, that distinction is fairly arbitrary. The underlying physics of vibrational spectroscopy is the same whether it's a stretch or a bend. And vibrations aren't always compliant to functional groups. We may talk about an OH bond in a spectrum but in reality the whole molecule will be vibrating in some way. But to consider vibrational spectroscopy in depth, we're going to boil it down to the simplest molecule once again. In order to be seen in a vibrational spectrum and interact with infrared light, a molecule's dipole must change on a vibration. This is a very different consideration to rotational spectroscopy where the dipole must be permanent. In a heteronuclear diatomic molecule it's quite easy to see that the dipole would change. There's only one bond to change and it stretches and changes that dipole moment. With a homonuclear diatomic molecule there's no overall dipole when we distort the molecule. The requirement for a dipole to change during the vibration means that molecules that normally don't have a dipole can still have a vibration visible in the infrared if the change happens to create one. We can see this in carbon dioxide. Overall both oxygen atoms pull the charge equally in both directions. No overall dipole. And this remains true if the vibration is symmetrical. If it's asymmetrical however, a dipole is created as the bond lengths are no longer equal. The charge will be pulled in each direction differently because of this. Now think about why this might occur. If the electromagnetic field is oscillating backwards and forwards, one with the other, at a fixed frequency and that frequency happens to coincide with the frequency that this molecule would vibrate at because of the bond strength and their mass then that electromagnetic radiation is absorbed and it would induce that vibration. Now that's a very classical picture and things do get a little bit more complicated when we try to ask why is this quantised and we try to do some calculations with it. But to finish the introduction from now, let's ask how many vibrations are in a molecule. Clearly this can be quite a complicated question. But we do need to abandon this idea of an individual stretch or an individual bend and instead look at degrees of freedom. A degree of freedom is how many ways we can move each of the atoms in a molecule. In 3D space there are three different ways of doing it for each atom, the X, Y and Z directions. Three times the number of atoms is a good start but there aren't three N vibrations. Consider a lone atom, if it moves in three dimensions it's simply a translation, there is no vibration. So we need to at least subtract three degrees of freedom. Now consider a pair of atoms, some combinations of movement actually induce rotations so we need to subtract two more. So for any linear molecule we need to subtract five degrees of freedom to remove translations and rotations. For a non-linear molecule we need to subtract six. Another way to justify the number of vibrations is to build up to it. A lone atom cannot vibrate, a pair of atoms has only one additional coordinate, the bond length between them. A third atom adds a new bond length and also an angle that can change so there must be three distinct vibrational modes. A fourth atom adds another bond length, another angle and then a torsional or dihedral angle which makes six. Each additional atom will add three degrees of freedom and therefore three new vibrational modes. I do put a pin in that idea of building up molecular degrees of freedom because it also gets used in computational chemistry and molecular modelling. But for now that's it, in the next video though we are going to make this quantum.