- Introduction to proton NMR
- Nuclear shielding
- Chemical equivalence
- Chemical shift
- Electronegativity and chemical shift
- Diamagnetic anisotropy
- Spin-spin splitting (coupling)
- Multiplicity: n + 1 rule
- Coupling constant
- Complex splitting
- Hydrogen deficiency index
- Proton NMR practice 1
- Proton NMR practice 2
- Proton NMR practice 3
The basic physical principles underlying proton NMR spectroscopy. Created by Jay.
Want to join the conversation?
- At8:46, he says "a slight excess of nuclei in the alpha-spin state", why is that?(6 votes)
- The energy from thermal collisions is sufficient to place many nuclei into higher energy spin states.
The number in each energy level is given by the Boltzmann distribution formula.
The spin states are so close together that there is almost the same number of nuclei in each state, but there are always a few more in the lower energy state.(10 votes)
- why we talk about proton[hydrogen nucleus] only ? there are also negatively charged particle [electrons] and also other atoms other than hydrogen.so why we don't talk about them?(4 votes)
- there has to be an uneven number of protons (and or?) neutrons in order for the nucleuss to have spin. You can only detect nuclei that have spin with the NMR machine. It so happens that over 99% of Hydrogen is H-1 (one is an odd number) and over 99% of C is C-13 (odd number again). So it makes sense that these would be easy to detect. They are also found in lots of molecules. So I think NMR detects common isotopes that are easy to detect becaues they have nuclear spin.(1 vote)
- Is the energy released as a photon?(3 votes)
- "And When that happens, the nucleus is said to be in resonance with your applied magnetic field and hence the term nuclear magnetic resonance."
AFAIK Magnetic resonance is created when there is shift in energy state from alpha to beta... But its still vague to me!! can anyone clarify...?(2 votes)
- I think Jay may have misspoke – my understanding is that (in a magnetic field of given strength) a nucleus resonates with a particular frequency of radio waves. This means that electromagnetic radiation of that frequency has the correct amount of energy to 'flip' the nucleus from a spin that results in a magnetic field aligned with the external field to one that is the reverse orientation.
However, this description appears to be a gross simplification of what is actually going on, see:
Though I recommend starting here:
- at8:45, what do you mean by there is a "slight excess of nuclei in the alpha spin state?" How do you have an excess of nuclei?(1 vote)
- There's no excess of nuclei. Like you quoted, Jay is saying that there are more nuclei in the alpha spin state than nuclei in the beta spin state, and having more alpha than beta is important to NMR (the machinery can only be so sensitive, so increasing the number of nuclei that flip from alpha to beta allows for better detection when energy is released as they flip back). You can do this by increasing the magnitude of the external magnetic field, as is discussed at6:08, because doing so increases the difference in energy between spins.(3 votes)
- If a static magnetic field can cause protons to change spin states and light has an alternating magnetic field that could change to having a north to south affect many times per second, does the proton absorb the light similar to an electron and use the energy to change its spin state or is it changing spin states with the changing magnetic polarity of the light applied?(1 vote)
- The magnetic field is steady, but the sample is being irradiated with radiofrequency energy (light).
The proton absorbs the energy and flips to a higher-energy state, similar to the way an electron absorbs energy and goes to a higher energy state,(2 votes)
- what if a compass has multiple magnetic forces(1 vote)
- If there's multiple magnetic forces acting on an object then we would evaluate it similarly to any other force. Each of the forces can be represented by vectors and we can do vector addition to find the net total force acting on the object.
Hope that helps.(2 votes)
- I thought it was the electrons spinning or is this only the case in H-NMR ? :)(1 vote)
- Almost all particles have spin states, though remember they're not really spinning; it's an analogy.(2 votes)
- I have a compound with the formula: C8H8O2
I have a NMR spectre which shows me a singlet at 10ppm, a doublet between 7.5-8ppm, a doublet at 7ppm and a singlet between 3.5-4ppm.
I also know, from an IR spectre, that there is an aromatic ring in the structure, and that there is no hydroxyl group.
What would be the name of the compound? Which is it's structure?(1 vote)
- IHD = 5. Aromatic ring has IHD = 4. ∴ 1 more ring or double bond.
Only 4 NMR peaks. ∴ Must be symmetrical (para-disubstituted?).
δ 10(s) = aldehyde CHO.
dd at δ 7.8 and 7 = aromatic C-H (para).
δ 3.8(s) = OCH₃.
∴ 4-methoxybenzaldehyde, CH₃O-C₆H₄-CHO.(2 votes)
- At7:44"And when that happens, the nucleus is said to be in resonance with your applied magnetic field and hence the term nuclear magnetic resonance"
Can't get it, what does resonance refer to here ?(1 vote)
- I think Jay may have misspoke – my understanding is that (in a magnetic field of given strength) a nucleus resonates with a particular frequency of radio waves. This means that electromagnetic radiation of that frequency can be absorbed by that nucleus.(1 vote)
- The nucleus of a Hydrogen atom is a proton and has a property called spin. So you can think about, just as a visual aid, you can think about this proton that's spinning this way. A spinning proton, right, is 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's like a tiny bar magnet, too, and so we can draw the magnetic fields, right? So magnetic field lines go from the North pole to the South. So I can 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. Alright, we could also think about the magnetic dipole moment of the compass needle. The magnetic dipole moment is also called the magnetic moment and it's a vector that points in the direction of the dipole's magnetic field. 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 this same idea, we can go back to the proton and think about it like a compass needle. So if it's spinning this way it's going to have a North pole and a South pole, and so I'm going to go ahead and cover-- Color the North pole red, here. We can draw magnetic field lines. So we can draw a magnetic field line going from the North pole to the South pole and then go ahead and do it over here, too, like that. And, therefore, we could also draw in the magnetic moment of the proton. So the magnetic moment points in the direction of our dipole's magnetic field. And so, 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 because we know that a compass needle, if you put 'em to the Earth's magnetic field, the compass needle is going to point North and so that's what I have down here. So the magnetic moment, 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 magnet, of our compass needle, this must be the magnetic South pole. And so I'm sure some of you are like, "Whoa, 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. Alright, 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... So here I have the compass needle pointed in this direction. You would have to put energy in, right? So here's my finger, and so I had to rotate, I had to rotate the compass needle. I had 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... So having the compass needle point in this direction is higher in energy than this one. And so if I let go, if I just let go with my finger, the compass needle would automatically swing back and 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 gonna think about our proton. So we could have a proton, and if we have an external magnetic field, let me go ahead and identify that, so this right here I'm saying is an external magnetic field that we're applying. So I'm gonna call this B naught. And if you put the proton in this external applied magnetic field, there's a quantize 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. So here it would be our magnetic moment aligning with the external magnetic field and then here it would be the magnetic moment aligning against the external magnetic field. We could think about that relating to the spin of the proton, because I said if it's spinning this way, this is the North pole and this is the South pole, so I can color in my North pole here red, and 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 can imagine, 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. 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 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 these states of the compass needle. So this one was higher in energy than this one, and it's the exact same idea where you could think about it as being the same for our proton. So we have a difference in energy. So this spin state is higher in energy than this spin state. Alright, 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 put more energy in in order to change the direction of the compass needle. Same idea with the proton. If you increase the applied magnetic field, so I'm now gonna draw a bigger magnetic field. So here's a bigger magnetic field. So a bigger B naught. So I've increased B naught. 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 states. So now this difference in energy, let me 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've applied a stronger magnetic field. And, again, the compass analogy helps us understand that. Alright, so 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. Alright, 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. So the nucleus could be in the alpha spin state or the beta spin state. And let's say we have a proton or a nucleus in the alpha spin state, so there's a certain difference in energy between the alpha and the beta spin states. So there's a certain difference in energy. And the proton can absorb energy and flip to the higher energy spin state. So if we apply the right amount of energy, this proton can flip from the alpha spin state to the beta spin state. So let me just draw it in 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 nuclear magnetic resonance. And so this energy difference between your two spin states corresponds to a frequency because E is equal to h nu, where E is energy 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 think about how an NMR works, and I should point out that I'm really only going to talk about F T NMR. Let me go ahead and rewrite that. So I'm only going to talk about F T NMR in this 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 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 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 I'm showing you just a very simple NMR spectrum and we get a signal, right? So let me go ahead and draw that signal in here. So a signal looks like this, so like a peak right here. And this peak occurs at a certain frequency, so if you drop down to here, this represents a certain frequency. Over here, this is the intensity, so the number of absorptions. So how high, or I'll talk about this in more detail later, your 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 in another signal right down here like that. And so this signal's at this frequency and this signal is at this frequency, and if you have different frequencies, if you have different frequencies you have different differences in energy here. And this is what helps us understand the structure of molecules. And so I will get more into this in the next video, how you can have different frequencies which correspond to different energy differences between the alpha and the beta state.