NMR spectroscopy

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When a molecule, lets say butane is placed in an external magnetic field and its hydrogens align parallel or antiparallel to the direction of the magnetic field, do 50% of the H atoms align parallel to the B field and the other 50% align antiparallel or what? If so, when they radio waves are applied would it only be the 50% of the H atoms that are aligned parallel that would be flipped to a higher energy level and recorded by the spectrometer?
 

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  • #2
Borek
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Can't help you much, but you may want to read about relaxation in NMR spectroscopy.
 
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ZapperZ
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When a molecule, lets say butane is placed in an external magnetic field and its hydrogens align parallel or antiparallel to the direction of the magnetic field, do 50% of the H atoms align parallel to the B field and the other 50% align antiparallel or what? If so, when they radio waves are applied would it only be the 50% of the H atoms that are aligned parallel that would be flipped to a higher energy level and recorded by the spectrometer?

No, it depends on the temperature.

In a magnetic field, the degeneracy of the two energy level is removed, so one will be higher than the other. The lower energy level (spin parallel to the field) will tend to be more populated than the other. How much the higher state is populated depends on the temperature of the material. This means that as 0K, only the lowest state is populated. At room temperature, a fraction of the higher state is populated. How much it is populated depends on the energy difference and the temperature, i.e. statistics.

Zz.
 
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Borek
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In a magnetic field, the degeneracy of the two energy level is removed, so one will be higher than the other.

I wonder - in the case of organic molecules, are they really degenerated before magnetic field is applied? After all, it is a little bit like ortho and para hydrogen - they differ enough to be separated. Sure, hydrogen is a specific case, so the difference can be much more pronounced, but is there any reason for the energy difference to disappear in more complicated molecules?
 
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The magnetic fields applied in NMR are around 10 - 15 T, which is enormously strong compared to spin magnetic fields in molecules. I don't think the molecular structure has any measurable effect on the spin preference of protons. At least I can't find anything about it on Google.
 
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Borek
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Methane molecule has 3 possible spin orientations - do you think each of them has the same energy? I think not. Could be differences are small enough to be negligible, but at least they should be possible to estimate.
 
  • #7
alxm
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I wonder - in the case of organic molecules, are they really degenerated before magnetic field is applied? After all, it is a little bit like ortho and para hydrogen - they differ enough to be separated.

Yes, there exists a "nuclear hyperfine splitting" if the electronic system is paramagnetic. But AFAIK they don't usually need to take it into account in NMR.
Methane molecule has 3 possible spin orientations - do you think each of them has the same energy? I think not. Could be differences are small enough to be negligible, but at least they should be possible to estimate.

Well, what happens is that the nuclear-spin dipole coupling averages out to zero, because the molecules are all moving and rotating relative the magnetic field. But you can indirectly measure it through the Nuclear Overhauser Effect, hence giving you information about the relative locations of the nuclei (since the spin polarization is obviously distance-dependent). This is the basis of structural NMR methods.

To answer the original question: Yes, the populations are very near 50-50% at room temperature; as the energy difference is very small. As you said, the splitting is in the radio-wave region - which region of the spectrum do you associate with heat? (This is a blessing in disguise; at higher energies the NMR signal would likely have been drowned out by thermal noise. As it is, there's relatively little going on in that region of the spectrum) NMR spectrometers are just very sensitive. We're talking about a population distribution of 50.001%-49.999% or around that order.
 
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  • #8
DrDu
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Borek, yes, even without magnetic field, there are states of different energy. Without a magnetic field, these states can be classified by total Spin S. The radio frequency field cannot induce transitions between states with different S but only between states differing in M_S. However, the states with same S and different M_S are degenerate, so no NMR signal will result. An even more fundamental reason is the time reversal invariance of the Hamiltonian of the system without magnetic field which guarrantees that to every possible transition there exists an equally probable transition in the other direction.
 

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