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Trying to Understand Attractive Interactions

  1. Jul 27, 2007 #1
    Before someone accuses me of being a half-wit attemtping to explore fields way beyond my intelligence, I'll just come straight out right now and say: "My highest level of education is that of a highschool graduate"!

    I am however, on a very serious trip to learn as much as I can about the attractive interactions between molecules and subatomic particles. So if you can help, please do so by answering my questions below. If my questions don't make sense, don't call me a half-wit, simply invite me to re-ask the question or elaborate:

    1-Is Nuclear Magnetic Resonance and Proton Magnetic Resonance essentially the same thing? (i.e. in both instances, the participating particle is the proton). If no, what are the defining differences? Can you clearly define NMR? and PMR if different?

    2-Does Electron Magnetic Resonance trigger a similar event as Nuclear Magnetic Resonance? If no, what is the defining difference? Can you clearly define EMR?

    3-Systemically describe the events leading up to achieving magnetic resonance?

    4-Is the following statement accurate: "every chemical element has a unique NMR frequency that distinguishes them apart!"? If this is true, does this also mean that a precisely transmitted RF pulse will dictate whether work performed is work associated with NMR, PMR, or EMR?

    Thank you in advance for your thoughts, and comments.

    Please note: If an example-molecule is necessary to help answer my questions, let's use "water".
    Last edited: Jul 28, 2007
  2. jcsd
  3. Jul 28, 2007 #2
    Hi lvegaskiwi,

    I am not an expert in this stuff, but I heard other people talking about that many years ago. So my memory may be failing me. I'll give it a try.

    Proton has nuclear spin, but there are other nuclei with non-zero spin as well. So, in principle, NMR need not to be limited to PMR, though it seems that PMR is used more frequently. Does anybody know why?

    Yes, there is a close similarity between NMR and the electron resonance. Its proper name is ESR (electron spin resonance) or EPR (electron paramagnetic resonance). ESR is possible when the electron shell of a molecule has nonzero spin. It occurs all the time when the number of electrons is odd. Even some even-electron-number molecules may have nonzero total spin in the ground state. If I remember correctly, O2 molecule (oxygen) is of that kind.

    The idea is simple. Suppose for simplicity that your system (a nucleus or an electron shell) has total spin 1/2. This means that there are two states (nearly) degenerate by energy: spin-up state and spin-down state. If you place this system in a magnetic field the energy separation between these two states increases. If the temperature is sufficiently low, the higher-energy state will be less populated. So, your system will be able to absorb radio-frequency EM radiation. The goal of NMR or ESR experiment is to find this resonance condition (when the frequency of radiation matches the energy separation between levels).

    If the total spin is higher (1, 3/2, ...) there are many energy levels and the spectrum of different transitions becomes rather complex.

    The power of NMR in chemistry stems from the fact that NMR signal depends not only on the type of nucleus, but also on the electron density surrounding this nucleus. For example, the NMR signal (the dependence of level-splitting on the applied magnetic field) of the proton is different in different molecules, and even at different locations in the same molecule. So, the answer is yes, you can often identify a molecule by its NMR signature. ESR is less informative, because it provides just one number (the g-factor) for each (paramagnetic) molecule.

    I think NMR and ESR work in different parts of RF spectrum, so they can be easily distinguished. But here my memory starts to fail.

  4. Jul 28, 2007 #3


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    Proton and C-13 NMR are popular because H & C are the skin and bones of organic molecules.
  5. Jul 31, 2007 #4
    that's correct, [tex]O_2[/tex] is a triplet in the ground state, hence it's ease in becoming free radical superoxide [tex]O_2^-[/tex], the enemy of the living cell :)
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