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Current time:0:00Total duration:10:22

Video transcript

before we get into nuclear shielding we need to review some physics so let's say we have current in a loop of wire so on the left is our loop of wire and let's say that current is going in this direction so in physics you represent current by I and let's say we're looking down on this loop of wire and so over here this would be the top view and if we're looking down current would be going in a clockwise fashion so around this loop and physics current is thought of as being moving positive charges so even though that's not really what's happening but the moving charges a moving charge creates a magnetic field and so the current is going to create a magnetic field we can figure out the direction of the magnetic field by using a variation of the right-hand rule so if you think about our right hand being at being right here on our loop we point our thumb in the direction of the current so the current is going to the left at this point so we we point our thumb to the left and this is going to be the back of my right hand here so if using your right hand there's only one direction for your fingers to curl and in this loop your your fingers would curl down so you're so in this loop and this loop your fingers curled down and that's the direction of the magnetic field created by the current and so from a top view the magnetic field is going into the page and if you're looking at it from this orientation the magnetic field would be going down so a magnetic field represented by B here is created by current in our loop of wire in reality it's the electrons that are moving and since the electrons are negatively charged electrons move in an opposite direction from the current so the electrons are actually going around this way so if you look at if you look at a top view the electrons will be going around counterclockwise and so this is important the idea of moving electrons creating a magnetic field now let's look at let's look at a situation where we have a proton involved so proton NMR in the last video I talked about how in proton NMR you apply an external magnetic field so this this vector here represents our external magnetic field B naught and in the presence of an external magnetic field electron density around our proton circulates so if you think about think about this is being a proton and you think about some electron density going around the proton so here's some electron density that's circulating the electron density that circulating creates an induced magnetic field alright so if the electrons are moving this way you could think about this situation here and ND and the induced magnetic field would go down so the induced magnetic field opposes the applied magnetic field so here's the induced magnetic field I mean is a different color here for that so this is the induced magnetic field which is in a direction this vector is in a direction opposite to this magnetic field all right this is effect called diamagnetism and so the proton the proton right here experiences a smaller overall magnetic field so let's let's think about that so if we have if we have an applied magnetic field of a certain magnitude so B naught and the circulating electron density produces an induced magnetic field that opposes the applied field the proton is going to feel an overall smaller smaller magnetic field so let me go ahead and draw that in here so the proton experience is a smaller magnetic field which I will call be effective so the effective magnetic field that the proton experiences or you can think about it like this right if you start the effective magnetic field experienced by the proton to be equal to the original magnetic field the applied magnetic field minus the induced magnetic field and so this proton this proton this nucleus is shielded from the external magnetic field by electrons right so this proton here is said to be shielded and if you increase the electron density around a proton you would therefore increase the shielding of that proton so shielding shielding has the effect of lowering the effective the effective magnetic field experienced by the proton so let's let's think about two examples now so first let's start with let's start with just a bare protons over here we have just a proton all by itself it's completely d shielded there are no electrons around it let me go and write that so we have a completely d shielded proton here because there are no electrons therefore this D shielded proton going to experience the full effect of the applied magnetic fields alright so and we know from the previous video that the applied the applied magnetic field the external magnetic field right causes your alpha and your beta spin States to be separated by a certain distance here so here's the Alpha spin state and here's the beta spin state and and this would be a certain energy difference between our two spin states so this is an energy difference right here now let's move to the example on the right so the example on the right this proton here is a proton in a molecule it's shielded there's there's electron density around this proton right so this is a shielded proton let me go ahead and write that shielded proton and we've just talked about what that means a shielded proton right has circulating electron density that creates a magnetic field that opposes the applied magnetic field and so and so the proton feels a smaller a smaller effective magnetic field so we decrease we decrease the magnetic field experienced by this proton in the previous video I talked about what happens when when you have a decreased magnetic field the magnetic field strength corresponds to the energy difference between the Alpha and Beta States and so if we're decreasing the magnetic field compared to the example on the Left we're going to decrease the energy so decreasing the magnetic field decreases the energy difference between the Alpha and the beta states so I can go ahead and write that I can show the Alpha and the beta states here and I can show a smaller gap between them all right so there's a decrease in energy and we know that that energy difference right is equal to H nu so if we decrease the energy we're going to decrease the frequency all right so the energy and the frequency are directly proportional so if you decrease the energy difference you decrease the frequency and so therefore a shielded proton absorbs at a lower frequency than ad shielded proton so ad shielded proton Adi shouldered proton the energy difference would correspond to a higher frequency because there's a larger difference in energy and so that's what we need to think about when we're looking at an NMR spectrum and so I just I just went ahead and drew a just generic NMR this isn't a real NMR we're just trying to think about this example these two protons here so we have we have one spectrum up here so this would be this would be let me go ahead and mark this so this is the D shielded spectrum and then this one down here represents the shielded spectrum again not a real NMR spectrum just helping to think about what's happening here and for the example on the left for the D shielded protons let's think about this really fast alright so as you go to the left on an NMR spectrum you get more and more D shielded and if you're more and more D shielded you experience a greater magnetic field right so greater magnetic field a greater magnetic field corresponds to a greater difference in energy and a greater difference in energy corresponds to a higher frequency absorbed so a higher frequency absorbed here and so therefore as we go to the left we're talking about increasing in we're talking about an increasingly D shielded proton and this signal that appears that your NMR right here so this is the signal for this D shielded proton we're talking about a high-frequency signal so moving to the left on an NMR spectrum we're talking about we're talking about higher frequency signal all right let's think about the shielded proton over here on the right so we're thinking about the shielded proton now and as you move to the right on your NMR spectrum right so we're moving to the right on our NMR spectrum we're getting more and more shielded so this signal is the signal for this proton so it's more shielded than the one on the left so move to the right you're talking about increasing shielding and increasing shielding decreases the effective magnetic field decreasing the effective magnetic field decreases the energy difference between the Alpha and the beta states and therefore decreases the frequency absorbed alright so as you move to the right you're talking about a lower frequency signal so as you move to the right you're talking about a lower frequency alright so this is the idea of ft NMR which I briefly introduced in the previous video so in ft NMR you're holding the external magnetic field constant and you're hitting the sample with a short pulse that contains that contains a range of frequencies and so these frequencies correspond to the energy differences so one frequency might correspond to this energy difference and and when and when the proton goes back to the lower energy state the NMR machine you this signal another frequency you might correspond to this energy difference and once again the NMR would give you this signal and so that's the idea about F T NMR you do all this at once and the NMR machine gives you your NMR spectrum for older NMR's you would hold the frequency constant and vary the strength of the magnetic field and for older NMR's it turns out that as you go to the right you needed a higher you need the higher magnetic field strength and so we call this upfield so this is B this would be a upfield if you will and as you go to the left on an NMR spectrum you need a lower magnetic field strength and so this is called downfield so up fuel in downfield are our two terms that you might hear and they're they're older - older terminology that relate to an older kind of NMR but you'll still hear them and I'm sure I will use those terms sometime as well so in this video we've we've talked about I've talked about two protons with different amounts of shielding so a completely bare proton completely D shielded and a shielded proton here so two protons with different amounts of shielding are in two different environments and we get two different signals right two different signals in two different having different frequencies on our NMR so if you have two protons in the same environment you should only get one signal and we'll talk we'll talk more about this in the next video