NMR question

  • #1
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Summary:

I understand why shielding lowers the effect of a magnetic field, but why doesn't it lower the effect of RF radiation?

Main Question or Discussion Point

I am watching this video;
at the 8:20 mark it shows that a proton in a magnetic field will align with the magnetic field very easily if there is no shielding from electrons, and therefore will require energy to shift to the opposite spin state; makes sense to me. I also understand that when there is more shielding, the proton will be less effected by the magnetic field and therefore will require less energy to move it to the opposite spin state. What I don't understand, is why don't the electrons shield the proton from the RF radiation? Electrons are effected by radiation (hence IR spectroscopy), so I'm curious as to why the RF radiation is not also shielded by the electrons. With no shielding it takes more energy to spin, but there is less shielding to block the radiation. With shielding it requires less energy to spin, but there is more shielding to block the radiation, so why isn't the same amount of radiation required to bring both nuclei into resonance? I'm guessing the answer is that RF radiation is not absorbed by electrons, whereas IR is? I don't know anything about the different types of radiation, or QM, so that's probably where my ignorance lies.

Thank you in advance
 

Answers and Replies

  • #2
gleem
Science Advisor
Education Advisor
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I only have a elementary knowledge of NMR but I think you are misinterpreting the use of the terms shielding/deshielding. I know when we talk about shielding we naturally think of the absorption of radiation or energy. But here the term is used only for the affect of nearby electrons on a proton's local magnetic environment. This "shielding" effect of the electrons reduces the local magnetic field near the proton under question. That is the shielding electrons counteract the applied magnetic field so that the energy needed to "flip" the magnetic moment is less than if the the proton is "out in the open" deshielded so to speak experiencing the full affect of the applied magnetic field. This shielding does not absorb energy. The use of the term shielding is irrelevant to the penetration of the applied RF energy to the proton.
 
  • #3
190
24
Short answer to the point and easy to understand. Thanks!
 

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