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Current time:0:00Total duration:9:25

Introduction to infrared spectroscopy

Video transcript

if you shine infrared light on a molecule it's possible for the molecule to absorb energy from the light and energy from the light can cause a bond to stretch and we call that a stretching vibration you can have other kinds but we're only going to focus on stretching here and the stretching vibration of a bond is like the oscillation of a spring so you can think about a bond as being like a Springs let's let's think about this bond right here so the bond between the carbon and the hydrogen we're going to model that bond as a spring so I'm going to attempt to draw a spring in here so here's the spring and then let's put in the carbon on one side so we have the carbon on one side and the hydrogen on the other side and so the stretching vibration of the bond is like the oscillation of this spring so if you had a spring and you had two masses on either end of the spring if you put some energy right you put some energy in you can stretch that spring so you could you could pull the carbon this way you could pull the hydrogen that way so that's just like the stretching of this bond here and we know that Springs also contract right so then the spring could pull back in this direction and you get an oscillation of a spring which is once again how we model the stretching vibration of a bond so let's let's look at the IR spectrum for this molecule so we're talking about one octane here and if you if you shine a range of infrared frequencies through a sample of this compound some of the frequencies are absorbed by the compound and you can tell which frequencies are absorbed by looking at looking at your infrared spectrum here so for right now let's think about these numbers like 3000 or 4000 let's think about those as representing frequencies of light and so over here we have percent transmittance and so if you had a hundred percent transmittance let me go ahead and draw a line on up here so a hundred percent transmittance so let's say we're talking up this frequency of light so I look at this frequency of light I go up to here alright and I can see I have a hundred percent transmittance 100 percent transmittance means all that light was transmitted through your sample if all the light went through your sample nothing was absorbed and so this particular frequency was not absorbed by your compound so if you're talking about less than 100 percent transmittance all right so let's say let's say for this frequency right here so for this frequency we have that we have this signal here appearing at this frequency we don't have 100 percent transmittance so that means not all of the light went through the compound some of it was absorbed all right so some of them so this specific frequency was absorbed by the molecule and that energy can cause a bond to stretch and we get a stretching vibration and actually it this signal corresponds to the bond that we've been talking about so this signal indicates the stretching of this bond right here all right let's uh let's think about let's thing about wave number next so we just talked about percent transmittance let's think about wave number so I've been calling all these things frequencies different frequencies of light let's see how wave number relates to the frequency of light and also the wavelength of light so the definition of wave number so a wave number here's a symbol for wave number the definition of wave number is it's equal to one over the wavelength in centimeters all right so if we had a wavelength of light of point zero zero two centimeters so let's go ahead and plug that in so a wavelength of light of point zero zero two centimeters what would be the wave number so let's get out the calculator here and let's do that math one divided by point zero zero two is equal to 500 all right so that's equal to 500 units would be one over centimeters or you could write that meaning the same thing so that's the wave number that's that's the wave number so if we go over here write a wave number 500 you could think about this this corresponding to a particular wavelength of light and of course this also relates to frequency because we know that wavelength and frequency are related to each other so let's get some more room down here and we know that the wavelength times the frequency wavelength times the frequency so lambda times nu is equal to the speed of light that's equal to C all right so if I want to solve for the frequency z so solve for Nu the frequency is equal to the speed of light divided by lambda divided by the wavelength and that's the same thing that's the same thing as 1 over lambda alright times the speed of light 1 over lambda was our definition for wave number so you could say that the the frequency of light is equal to the wave number all right times the speed of light so let's go ahead and do that calculation here so for we talked about this as being as a particular wavelength all right we found the wave number let's plug that wave number into here and let's see what we get so the wave number was 500 units for 1 over centimeters multiply that that by the speed of light we need to have the speed of light and centimeters so that's three times approximately 3 times 10 to the 10th centimeters per second is the speed of light notice what happens to the units the centimeters cancel and let's do the math so let's get some room over here so we take the wave number and we multiply that by the speed of light in centimeters so 3 times 10 to the 10th and we get 1.5 times 10 to the 13th so the frequency all right the frequency would be one point five times ten to the thirteenth the units would be this would be one over seconds or you could say or use Hertz for that so a wave number right corresponds to the wavelength and you can also get the frequency from that so we had let me just rewrite this really quickly so frequency is equal to wave number times the speed of light and so wave number is equal to the frequency divided by the speed of light and what we'll use this in a later video so we'll come back to this idea but for right now the frequency of light is is directly proportional to the wave number and so you could look at an infrared spectra I'm going to go back up here you could look at an infrared spectrum and and call this down here you call this wave number you could refer to it as a frequency oh you could call it whatever you want as long as you understand what's going on here let's look more in detail at this infrared spectrum and let's draw a line at approximately 1500 wave numbers right here and the left side to the left side of that line so we've divided our spectrum into two regions the region on the left is called the diagnostic region so this is called this is called the diagnostic region of our spectrum and it's because a signal in this region can be diagnostic for a certain functional group for example this this signal right here if we go down here to the wave number that signals out approximately approximately 2,100 right for this wave number here and and and that that's corresponding to the triple bond here so this tells us a functional group right this tells us that a functional group is present this triple bond is present and so it gets diagnostic right it helps you figure out the structure of their molecule so you can figure out different functional groups present in molecules using IR spectra so the right side the right side this line is called the fingerprint region so this is the fingerprint region and it's harder to interpret the fingerprint region it's much more complicated it's not it's not as easy to see different signals it's extremely complicated but it is unique to each molecule and so it's like a fingerprint for the molecule and so you can match up IR spectra up here if you're given unknown you can look at the fingerprint region and again it's unique all these different lines are unique to that molecule so we have a the diagnostic region and the fingerprint region we're not going to we're not going to deal much with the fingerprint region maybe a little bit we're going to focus in on the diagnostic region we're going to focus where the signals appear all right so I look at the signal I go down to here and I get a specific wave number so the location of the signal is pretty important I I did want to point out that if you look at what I use for the wave number here right I changed I kind of changed how I did everything so the spacing is different it doesn't really matter I Allen did this too to fit to fit this video and to make it work for this video I'm also going to hand draw all my IR spectrum so it's certainly not going to be perfect the idea is is is not worry too much about what give you here for your scaling for the wave number but think about where the signals appear so the location of this signal approximately 2100 wave numbers you also want to think about the intensity of the signal and the shape of the signal we'll talk much more about those in later videos on the next video we need to we need to develop this idea of bonds as Springs a little bit more so we'll think a little bit about some classical physics