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Current time:0:00Total duration:9:18

Video transcript

in this video we're going to talk about diamagnetic anisotropy so some pretty fancy words there we talked about diamagnetism in an earlier video and we used current in a loop of wire as an analogy so if current is moving in this direction in a loop of wire so I represents current a magnetic field is created and at the very centre of this loop the magnetic field is pointing straight down as you move away from the center I can draw in some more magnetic field lines so we didn't do this in the earlier video and as you get closer to the edge of this loop right inside of the loop the magnetic field will be pointing down but outside of the loop of wire the magnetic field will be pointing up same thing on this side so pointing down inside and the magnetic field points up on the outside of the loop so when you're talking about current you're thinking about positive charges moving but that's not what's happening we know that electrons are really what are moving and moving charges create a magnetic field the electrons are moving in a direction opposite to to how we define current so we have electron density moving this way and we get a magnetic field if we think about benzene benzene has six PI electrons so up here is benzene so let's go ahead and identify the pi electrons 2 4 and 6 and if we put benzene in an applied magnetic field so that here is our applied magnetic field B naught so it's pointing up those six pi electrons of benzene are going to circulate to create an induced magnetic field so let me go ahead and draw a picture of the pi electrons in benzene circulating so the PI electrons are going in this direction at the PI electrons going in that direction then we know the induced magnetic field will be pointing down here so at the very centre the induced magnetic field will be pointing down so induced magnetic field points down as you move away from the center once again we can draw in some more magnetic field lines and then as you get to the edge of the ring edge of the benzene ring here once again inside of the ring the magnetic field points down but outside of the ring the magnetic field is going to point up so same thing on this side right inside it points down outside the ring the magnetic field points up so let's think about the magnetic field experienced by this proton all right so that proton experiences the applied magnetic field B not but it also feels this induced magnetic field which is the same it's in the same direction as the external magnetic field right so this is the direction of the induced magnetic field outside of the Ring so the effective magnetic field felt by this proton you'd have to add the induced magnetic field to the applied magnetic field to find the effective magnetic field so outside of the ring we get a larger we get a larger magnetic field so we get a large magnetic field we get a large difference between the Alpha and the beta spin States in terms of energy and a greater difference in terms of energy means means a higher frequency absorbed and therefore you get a higher chemical shift and so the proton on benzene has a chemical shift of approximately seven point two seven parts per million so this is just for any any proton on any kind of benzene ring here your general range is going to be six point five to eight and so if there are several molecules that demonstrate this effect very dramatically and let's let's take a look at one of them so how do we know that this effect is even true so if I look at this if I look at this molecule right have a giant ring here so let me go around so you can see the outline of this giant ring all right so much bigger ring than benzene we have a lot of pi electrons so more pi electrons in benzene so I'll just highlight some of them two four six eight and so on you can see we have alternating single single double bonds here in this molecule and so if you put this molecule into an external magnetic field you're going to get the same situation as benzene so let's think about these inner protons here's we have six inner protons if we look at the diagram for benzene right if you have an applied external field B not in the center right in the center of the Ring the enter protons experience an induced magnetic field that's down it opposes the external magnetic field let me go ahead and draw that out here so if we apply if we apply an external magnetic field be not the inner protons right have an induced magnetic field caused by caused by the movement of those pi electrons the induced magnetic field opposes the applied field and so the effective magnetic field felt by those inner protons is smaller right so we get a smaller we get a smaller effective magnetic field smaller effective magnetic field means a smaller energy difference between the Alpha and the beta spin States therefore we get a lower frequency signal and a lower chemical shift and the chemical shift for these six inner protons turns out to be negative two parts per million so think about what that means negative two is passed TMS right so if I go back up here alright TMS was at zero so negative two would be to the right I don't even have room to show it on this chemical shift right here so way past TMS all right so a pretty pretty dramatic effect we can look at the protons outside of the ring as well so let me go ahead and highlight those so we have twelve protons outside of the Ring since those protons are outside of the Ring right the induced magnetic field is now in the same direction as the applied magnetic field so therefore we get a larger effective magnetic field felt by one of those protons a larger a larger magnetic field means a greater difference in energy between your alpha and your beta spin States so you get a higher frequency signal and a higher chemical shift the chemical shift is about nine parts per million so the dramatic difference between these chemical shifts for these inner and outer protons shows you how powerful this effect can be let's uh let's use this effect to explain the shift for a proton on a triple bond so if we think about acetylene and so here's acetylene and we think about the signal for this proton right let's think about the carbon it's attached to so this carbon right here is s P hybridized and in the previous video we talked about the fact that an SP hybrid orbital has more s character than an sp2 or sp3 hybrid orbital and therefore the electron density is going to be closer to that carbon so you can think about an SP hybridized carbon as being more electronegative than an sp2 or sp3 hybridized carbon so the electron density is closer to this carbon here which you would think would de shield this proton and give you a higher chemical shift than a proton on a double bond but that's not what we observe the shift for this proton turns out to be approximately approximately 2 to 2.5 so it's actually a lower chemical shift than a proton on a double bond and let's see if we can explain why so if we apply an external magnetic field so B naught is our applied external magnetic field we know that causes pi electrons to circulate and if we have an upright orientation of acetylene so it the the orientation of the molecule matters so if it's if it's facing in this direction alright the PI electrons are going to circulate like this and just like we talked about in benzene at the PI electrons circulate like that we get an induced magnetic field right down in this direction like that so we can draw a few more magnetic field lines like that and think about the magnetic field experienced by let's say this proton so this proton is feeling the applied magnetic field it's also feeling the induced magnetic field but the induced magnetic field is in the opposite direction of the applied magnetic field so we can draw the induced magnetic field opposing the applied magnetic field so that proton that I circled there actually feels a smaller effective effective magnetic field here so if you have a smaller effective magnetic field right you're decreasing the energy difference between your alpha and your beta spin States so you get a lower chemical shift than expected due to this effect and so that's currently how we explain the chemical shift of some around 2.5 for a proton on a triple bond and so that this effect holds true anytime you have pile electrons right that can circulate when you when you put a molecule in and applied magnetic field and so we could also use this to explain for example the proton on a double bond right so here are some PI electrons or the proton and here we have next to a carbonyl here so we have PI electrons here so anytime you have PI electrons this effect can be present and as we've seen it can be a very powerful effects and really affect the chemical shift