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Current time:0:00Total duration:10:27

Video transcript

the nucleus of a hydrogen atom is a proton and has a property called spin so you could think about just as a visual aid you can think about this proton spinning this way a spinning proton right is it like a rotating sphere of charge and any moving charge creates a magnetic field therefore you can say a proton is a tiny magnet so like a bar magnet or a compass needle so over here on the right let's look at a compass needle right which has two poles so we have the North Pole which I'll color in red here and the South Pole so the compass needle is like a tiny bar magnet too and so we could draw the magnetic field right so magnetic field lines go from the North Pole to the south so I could draw in a magnetic field line here so going from the north into the south so going from the north to the south for our magnetic field line all right we could also think about the magnetic dipole moment of the of the compass needle right the magnetic dipole moment is also called the magnetic moment and it's a vector that points in the direction of the dipoles magnetic field right so we have two poles North Pole and a South Pole and the magnetic moment is going to point in this direction alright so using the same idea we can go back to the proton and think about it like a like a compass needle alright so if it's if it's spinning this way it's going to have a North Pole and a South Pole so let me go ahead and cover color the North Pole red here we could draw magnetic field lines all right so we could draw a magnetic field line going from the North Pole to the South Pole and go ahead and do it over here - like that and therefore we could also draw in the magnetic moment of the proton right so the magnetic moment right points in the direction of our dipoles magnetic field and so this is this is how we're going to think about a proton like a tiny magnet with a magnetic moment alright let's go back to the idea of the compass needle now because we know that a compass needle right if you put into the Earth's magnetic field the compass needle is going to point north and so that's what I have down here right so the magnetic moment right the compass needle is pointing north like that and we know that opposite poles attract so if this is the North Pole of our little bar magnets of our compass needle this must be the magnetic South Pole and so I'm sure some of you are like well that's the geographic North Pole and it is the geographic North Pole but if you're talking about magnets it's actually the magnetic South Pole because opposite poles attract so if this is the South Pole down here this must be the magnetic north pole of the earth all right so this is just what happens when you put a compass needle into the magnetic field of the earth and so if you wanted to make the compass needle point in the opposite direction right so here I have the compass needle pointed pointed in this direction you would have to put energy in all right so here's my finger and so I had to rotate I had to rotate the compass needle I to put energy in in order to get the compass needle to point in this direction and hopefully you can see this tiny little mark right here on the table that I left in so you can see that I'm actually moving the compass needle with my finger and so it took energy and so this right so that having the pickup's needle point pointing this direction is higher in energy than than this one and so if I let go right if I just let go with my finger the compass needle would automatically swing back and end point in this direction again so this is the lower energy state and this is the higher energy state because I had to put energy in to make the compass needle point down and so that's how we're going to think about our proton alright so we could have we could have a proton and if we have an external magnetic field let me go and identify that so this right here I'm saying is an external magnetic field that we're applying something call this B naught and if you put the proton in this external applied magnetic field there's a quantized interaction between the magnetic moment of the proton and this external magnetic field and the magnetic moment of the proton either aligns with the external magnetic field or it aligns against the external magnetic field so let me go ahead and draw that in alright so here it would be our magnetic moment aligning with the external magnetic field and then here would be the magnetic lining against the external magnetic field we could think about that relating to the spin of the proton right because I said if it's spinning this way right this was the North Pole and this is the South Pole so I could color in my North Pole here red and the magnetic moment the magnetic moment was in this direction and so for the other one if the magnetic moment is now aligned against the applied magnetic field the proton must be spinning in the opposite direction so we could imagine right even though this isn't exactly what's happening we could imagine the proton spinning this way making this the North Pole and this the South Pole all right so that's why this compass needle analogy helps so much because there's an energy difference between these two spin states so when the when the magnetic moment is aligned with the magnetic field this is the Alpha spin state and when the magnetic moment is aligned against the applied magnetic field this is the beta spin state and there's a difference in energy between these two spin States just like there's a difference in energy between between these states of the compass needle right so this one this one was higher in energy than this one it's the exact same idea or you could think about it as being the same for our proton right so we have a difference in energy so this spin state is higher in energy than this spend state all right let's go back to the analogy of the compass needle if we were somehow able to increase the magnetic field of the earth it would take me more energy in order to make the compass point down right so I would have to I would have to put more energy in and in order to change the direction of the compass needle same idea with the proton if you increase the the applied magnetic field so I'm now going to draw a bigger a bigger magnetic field right so here's a bigger magnetic field so a bigger B notch so I've increased B naught all right I'm going to increase the energy difference between the two spin States so I can draw a greater difference in energy between the Alpha and the beta spin state so now now this difference in energy and I'll just go ahead and draw it in here so this difference in energy this difference in energy is greater than this difference in energy because we have we applied a stronger magnetic field and again the compass analogy helps us understand that all right so now we've now we've now we've learned that we have these two different spin States and turns out a proton can absorb energy and flip from the lower spin state the Alpha spin state to the beta spin state so let's take a look at a diagram showing that so here we go down here all right so if we apply once again if we apply an external magnetic field there are two possible spin States for our proton for our nucleus right so the nucleus could be in the Alpha spin state or the beta spin state and let's say we have a proton or nucleus in the Alpha spin State all right so there's a certain difference in energy between the Alpha and the beta spin state so there's certain difference in energy and and the proton can absorb energy and flip to the higher energy spin stage right so if we apply the right amount of energy this proton can flip from the Alpha spin state to the beta spin States let us draw them here this is alpha and this is beta and when that happens the nucleus is said to be in resonance with your applied magnetic field and hence the term hence the terms nuclear magnetic resonance and so this energy difference this energy difference between your two spin States corresponds to a frequency because E is equal to H nu where E is energy right and and nu is the frequency and this frequency falls in the radio wave region of the electromagnetic spectrum and so now we know enough to to think about how an NMR works and I should point out that I'm really only going to talk about F T NMR all right let me go ahead and rewrite that so I'm only going to talk about F T NMR and it in these and the set of videos here in this tutorial and in F T NMR you take a sample of your compound and you put it in an external magnetic field and the nuclei can either be in the Alpha spin State or the beta spin State there's a slight excess of nuclei in the Alpha spin State and so you hit the sample with a short pulse that contains a different range of frequencies and that those excess nuclei can absorb the energy and flip from the alpha spin state to the beta spin state when the nuclei fall back down from the fall from the beta spin state back down to the alpha spin state so just like if I took my finger off the compass needle the compass needle flips back to the lower energy State the NMR machine can detect the energy that's given off and it gives us a signal on an NMR spectrum and so down here down here I'm showing you just a very simple NMR spectrum and we get a signal we get a signal right let me go ahead and draw that signal in here so a signal looks like this like a peak right here and this peak occurs at a certain frequency right so if you drop down to here this represents a certain frequency over here this is the intensity so the number of absorptions so so how high or I'll talk about this in more detail later your your door peak is here on your NMR spectrum and so it's possible to get different signals at different frequencies let me go ahead and draw on undraw on another signal right down here like that and so this signals that this frequency and this signal is at this frequency and if you have different frequencies if you have different frequencies right you have different differences in energy here and and this is what helps us understand understand the structure of molecules and so I will get more into this in the next video right how you can have how you can have different frequencies which correspond to different energy differences between the Alpha and the beta state