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Homework Help: NMR, Nuclear & Electron magnetic moments

  1. Jan 20, 2009 #1
    I have got myself very confused. I am trying to understand Nuclear Magnetic Resonance but am running into issues.

    The basic idea of NMR, resonant excitations between Zeeman split nuclear energy states, I understand. The semi-classical picture of precessing moments also makes some sense to me.

    I think basically I am having trouble distinguishing between the influence of the nuclear and electron magnetic moments on NMR. In a recent course on magnetism, we were taught all about para/dia/ferromagnetism etc. The basic conclusion I now make is that magnetism is related to the moment of the electron itself, or due to its orbit, NOT the nucleus. But that is where I get lost. If magnetism is defined as the alignment of electronic or atomic moments, where do nuclear moments fit in? Aren't NMR experiments supposed to manipulate NUCLEAR moments?

    I’m finding it hard to explain exactly what I’m having trouble with, but if anyone can see what I’m grasping at, or has an idea of a possible mistake in my understanding, please post. Maybe this will help me to explain what I think further.

    Thanks in advance
  2. jcsd
  3. Jan 20, 2009 #2
    Ok, quick after thought.

    Does the solution lie in the fact that although the magnetising field acts on both moments, subsequent NMR pulse routines etc, are at frequencies so as to only act on the nuclear moments? The magnetic dipole moments of nucleons and electrons are significantly different...
  4. Jan 20, 2009 #3


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    That would be part of my guess. The Larmor frequency of the electron should, naively, be on the order of 1000 times greater than the Larmor frequency of the proton.

    I would also suggest that, since electrons are on the outside of the atom, they interact with other electrons more readily, whereas the nuclei are much more isolated from each other. So, probably the electronic moments decay more rapidly due to their thermal interactions with each other, and the lasting signal that is picked up is that from the longer decay of the protons. This notion is further supported by the relative strength of the signal from water compared to substances with less hydrogen density.
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