# Do nuclei spin WITHIN A MAGNETIC FIELD or do they spin all the time??

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#1
I'm doing nmr spectroscopy right now and I don't quite understand whether the nucleus of an atom spins all the time or only when we apply a magnetic field ???
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2 years ago
#2
(Original post by foxstudy)
I'm doing nmr spectroscopy right now and I don't quite understand whether the nucleus of an atom spins all the time or only when we apply a magnetic field ???
They spin all the time. When you apply an external magnetic field their magnetic moments (of magnetically active nuclei) align in or against the direction of this externally applied magnetic field.
Last edited by Plantagenet Crown; 2 years ago
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#3
(Original post by Plantagenet Crown)
They spin all the time. When you apply an external magnetic field their magnetic moments (of magnetically active nuclei) align in or against the direction of this externally applied magnetic field.
Right so nmr only works with atoms that have an odd number of protons and neutrons in the nucleus of the atom. This is because they have a magnetic field. Therefore atoms that have an even number of protons and neutrons don't have a magnetic field hence do not spin?
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2 years ago
#4
(Original post by foxstudy)
Right so nmr only works with atoms that have an odd number of protons and neutrons in the nucleus of the atom. This is because they have a magnetic field. Therefore atoms that have an even number of protons and neutrons don't have a magnetic field hence do not spin?
Correct, the even nuclei’s individual magnetic fields of protons and neutrons cancel each other out, meaning there’s no net magnetic moment and thus are invisible to NMR.
Last edited by Plantagenet Crown; 2 years ago
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#5
(Original post by Plantagenet Crown)
Correct, the even nuclei’s individual magnetic fields of protons and neutrons cancel each other out, meaning there’s no net magnetic moment and thus are invisible to NMR.
Ahh, so the protons and neutrons have their own mini magnetic fields but when there are an even number of protons and neutrons, these magnetic fields cancel each other out so there's no overall magnetic field? This makes more sense now.
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2 years ago
#6
(Original post by foxstudy)
Ahh, so the protons and neutrons have their own mini magnetic fields but when there are an even number of protons and neutrons, these magnetic fields cancel each other out so there's no overall magnetic field? This makes more sense now.
Yep, it’s the NET magnetic moment of the nucleus as a whole that aligns with or against the external field.
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#7
(Original post by Plantagenet Crown)
Yep, it’s the NET magnetic moment of the nucleus as a whole that aligns with or against the external field.
So once radio waves are passed through the nucleus causing it to line itself AGAINST the magnetic field, how does it then go back to aligning itself WITH the magnetic field?
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2 years ago
#8
(Original post by foxstudy)
So once radio waves are passed through the nucleus causing it to line itself AGAINST the magnetic field, how does it then go back to aligning itself WITH the magnetic field?
It’s not as simple as that, both the with and against fields are titled in the xy plane and then net magnetisation is returned to the z axis via T1 and T2 relaxation. You might want to google them as they can be quite complex to get your head around.
Last edited by Plantagenet Crown; 2 years ago
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#9
(Original post by Plantagenet Crown)
T1 and T2 relaxation. You might want to google them as they can be quite complex to get your head around.
Alright, I'll probably leave it there because i'm in year 12 and my textbook goes as far as talking about the absorption of energy.

So to summarise,
- nmr only works for atoms with an odd number of protons and neutrons e.g 1H, 13C
- this is because the nuclei have a magnetic field so their nuclei will spin
- a strong magnet is used to align the nuclei in the sample
- some nuclei will spin with magnetic field and some will spin in the opposite direction to the magnetic field
- radio waves are then passed through the sample and if its at the correct frequency the nuclei will absorb the energy and spin in the opposite direction
- then the nucleus will go back to its original spin and emit the same amount of energy that it absorbed or resonance? (what actually does resonance mean?)
- this energy is now measured to give us chemical shift values
Last edited by foxstudy; 2 years ago
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2 years ago
#10
(Original post by foxstudy)
Alright, I'll probably leave it there because i'm in year 12 and my textbook goes as far as explaining the absorption of energy.

So to summarise,
- nmr only works for atoms with an odd number of protons and neutrons e.g 1H, 13C
- this is because the nuclei have a magnetic field so their nuclei will spin
- a strong magnet is used to align the nuclei in the sample
- some nuclei will spin with magnetic field and some will spin in the opposite direction to the magnetic field
- radio waves are then passed through the sample and if its at the correct frequency the nuclei will absorb the energy and spin in the opposite direction
- then the nucleus will go back to its original spin and emit the same amount of energy that it absorbed or resonance? (what actually does resonance mean?)
- this energy is now measured to give us chemical shift values
Yh, that seems about right for year 12. The “correct” frequency of the radio waves needed to “flip” the spins is actually called the Larmor frequency, which is unique for each nucleus. So all 1H will have same Larmor frequency, all 13C will have the same one etc.

Chemical shift is a bit more nuanced, because it depends on different chemical environments; the electrons of all protons will oppose the external field (some special cases in aromatic and cyclic molecules where electrons can augment the local field) and thus cause the nuclei in different environments to be more or less exposed to the external field. More exposed nuclei resonate more because they’re “feeling” more of the external field and then we use equations to convert this to chemical shift. I’m pretty sure resonance is just the nuclei oscillating at a particular frequency.
Last edited by Plantagenet Crown; 2 years ago
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#11
(Original post by Plantagenet Crown)
Yh, that seems about right for year 12. The “correct” frequency of the radio waves needed to “flip” the spins is actually called the Larmor frequency, which is unique for each nucleus. So all 1H will have same Larmor frequency, all 13C will have the same one etc.

Chemical shift is a bit more nuanced, because it depends on different chemical environments; the electrons of all protons will oppose the external field (some special cases in aromatic and cyclic molecules where electrons can augment the local field) and thus cause the nuclei in different environments to be more or less exposed to it. More exposed nuclei resonate faster and more intensely because they’re “feeling” more of the external field and then we use equations to convert this to chemical shift.
Ah ok so they convert energy emitted into chemical shift.

Is resonance the same as the energy emitted then?

and the amount of energy absorbed/emitted gives us an indication of its environment?
Last edited by foxstudy; 2 years ago
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#12
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2 years ago
#13
(Original post by foxstudy)
Ah ok so they convert energy emitted into chemical shift.

Is resonance the same as the energy emitted then?

and the amount of energy absorbed/emitted gives us an indication of its environment?
Not quite. I was taught that NMR doesn't directly measure the energy absorbed, what actually gives the signal is the net magnetisation in the xy plane. The difference in energy between the spin up and spin down states leads to the intensity of the signal. This partly explains why proton signals are generally more intense than carbon ones: because the difference between spin up and spin down protons is greater than the difference between spin up and down 13C nuclei, for example. Natural abundance of the elements also contributes.

No, resonance is the oscillation of nuclei relative to the amount of external magnetic field they're exposed to and this fundamentally depends on the different chemical environments. The electrons around each environment cause those environments to experience the external field more or less, leading to the different resonances.

I don't think the energy absorbed has anything to do with the environment, the environment is caused by the types of atoms and electrons nearby.
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