Good afternoon and welcome back to the second in our installment of organic analysis techniques going to look at infrared spectroscopy today. Bit more involved than mass spectroscopy this one. Let me find the outcomes on it. Basically what you do is you take yourself a molecule and you fire a low below what we can see infrared energy at this molecule. And unlike mass spectroscopy, we're not breaking it apart this time. What you are doing is making the bonds vibrate. They go twang like elastic bands. They can actually do a couple of things. They can stretch and they can actually bend, but you are not depending on the wavelength of infrared light. But the key thing here is of course that a C-C bond does not vibrate or stretch or bend at the same wavelength as a CH bond, which enables you to try and map different bonds to different wavelengths of infrared light that go missing. Because this is absorption spectroscopy basically folks. Let me show you a typical infrared spectrum here and let me just check the outcomes. So this is what it looks like. So because I said its absorption spectroscopy basically a flat line if you passed it through nothing would just look like that. Certain wavelengths go missing because they're absorbed by these bonds as they vibrate and that's why you get drops in the spectrum. If you have a look at here you'll find transmittance up the side and this what sort of unit is that? Wave number as well because I called it wavelength early on. Wave length and wave number are the reciprocal of each other. So wave number is basically one over a wavelength, a lambda. With this weird unit here and I'll show you how to calculate the affiquant loved question is to turn that wave number into wavelength and there's a trip while waiting for you here but we'll come back to that in the very near future. In the meantime let's have a look at a typical spectrum here. This spectrum here is peachy because it's got two of the favourite things they love asking about on it. It's got this big massive crystal clear spike around 1700 centimeters to the minus one today's ridiculous unit and it's also got this big broad blob here around 3,000. Very different shapes and very different bonds that you're searching for. This corresponds to the carbonyl vibration and this big blob here across here when it's wide and spread like that, that corresponds to OH. On a technical basis it's actually because the OH is interact with each other and you get hydrogen bonding and that's why it absorbs across a huge variety of wave numbers. We're looking for the table on. Let me just find it for you, page something of the data book. Here we go, we're on page 14 of the data book and here are your wave numbers. Here are the individual bonds that are stretching. Sometimes they clarify them a CH stretch in a double bond or a CH in a triple bond. Sometimes the carbonyl kind of have slightly different wave numbers depending on what type of molecule it's in but as I said those two are the favourites for them asking about. The last thing I'll do is have a look at calculation of wave numbers and wavelengths on the other side of this. If you had a wave number of 1700 cm to the minus 1 and we wanted to do some maths with that, we wanted to turn it into a wavelength then for example what I would say is I would get you to realise that is 1 over 1700 cm. That is not an SI unit so let's fix that right now. Let's turn that into meters. Excuse me just two seconds. Sorry my apologies. Let's do the calculation first. The wave number, get it right here. I've been doing videos all morning, I do apologise. Let's start again, wave number equals 1 over lambda. We've got 1700 cm is 1 over lambda so therefore if we do 1 over 1700 that will give us the wavelength in centimetres. So let's do that first, where's my calculator? Excuse me. So that will be 2.946 times 10 to the minus 4. So our wavelength is that but that's in centimetres, that's your classic mistake. At this point here we want to change it into meters last. So do the reciprocal of it first and then the second stage is change that into meters because that's what they're asking for the wavelength in which will be divided by 100 of course, so we'll put another two on that, that will become 2.946 times 10 to the minus 6 meters. It's just one mark that but it's easy to make a mistake. As you saw, nearly minimum mistake first. So a quick recap there, do the reciprocal flip it first, that gives you the wavelength in centimetres and the last change it into meters. So anything I haven't covered on infrared, excuse me. Nope, that's it folks. So very quick recap, infrared spectroscopy is all about making bonds go buoying, make them vibrate or bend and stretch at specific wavelengths. And nowingly, it's not actually shown in wavelength, it's shown in wave number. If you're interested, why is that? It's because as you go, do you know what, we're not even going to go there. We're not even, it's spectroscopy, it's a full of historians. So it's shown in wave number, which is one over the wave length. The two common ones, the ASCUE, about 1700 and this is what Broad Stretch here for OH and carbonyl groups. But there are other groups and they might ask you about them, so they're on page 14 of the day at book. And lastly, conversion from wave number to wave length is, do the reciprocal of the wave number first, which gives you wavelength in centimetres and in turn in centimetres. And we're done. Thank you for listening.