NMR - does rf cause transitions or simply rotate the magn vector?

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Discussion Overview

The discussion revolves around the nature of radiofrequency (rf) radiation in nuclear magnetic resonance (NMR) and whether it causes transitions between spin states or merely rotates the magnetization vector. The conversation includes both classical and quantum mechanical perspectives on NMR.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant suggests that rf radiation does not cause transitions between spin states but instead provides a magnetic field that rotates the magnetization vector away from the primary field.
  • Another participant argues that while a classical approach may suffice for certain applications, a more nuanced understanding is necessary for complex systems, particularly involving quadrupolar nuclei.
  • A third participant clarifies that in the quantum mechanical view, rf radiation is absorbed by the nucleus, leading to transitions between states, while the classical view describes the precession of spins around the static magnetic field.
  • A participant expresses confusion about how both perspectives can be correct and seeks clarification on the mechanism by which the magnetization vector is tipped into the xy-plane.
  • One participant references a previous thread that may provide additional insights, particularly regarding the Bloch sphere representation.

Areas of Agreement / Disagreement

Participants express differing views on the role of rf radiation in NMR, with some supporting the classical perspective and others advocating for the quantum mechanical interpretation. The discussion remains unresolved regarding the reconciliation of these viewpoints.

Contextual Notes

Participants highlight the limitations of the magnetization vector model in certain experimental contexts and the need for a deeper understanding of the underlying physics in more complex systems.

mrquantum
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Am I wrong in thinking that the rf radiation used in nmr is not "absorbed" by protons to cause transitions between spin states but rather it is only there to provide a magnetic field which can rotate the magnetisation vector away from the primary field?
 
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You can think that way. The issue is that the magnetization vector picture eventually loses the ability to really make sense of or devise the kinds of experiments people are interested in conducting.

One can get away with a fairly classical approach to magnetic resonance for spin-1/2 nuclei and imaging, to a large extent, but if, for example, one wants to work for my one former labmate - who does all kinds of stuff with quadrupolar nuclei in inorganic and biological solids, including dynamics and relaxation studies - just sticking with the vector model isn't going to cut it.

If you're interested in further reading, I'd suggest digging up the work of David Hoult on the nature of RF absorption and NMR over the last ~ 20 years. I can dig up the references, but I'll have to get back to you on that.
 
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mrquantum, despite your name you have confused the quantum mechanical and classical pictures of NMR. Quantum mechanically a photon is indeed absorbed by the nucleus, which makes a transition between states. In the classical view, RF radiation causes the spin to precess at the Larmor frequency about the direction of the static magnetic field such that its polar angle changes. Both views are correct within their model universes.
 
I don't understand how both can be correct. Is the RF radiation absorbed and re-emitted or does it pass straight through the sample.

I'm basically trying to understand how the magnetisation vector (along +z) gets tipped into the xy-plane, or, if you like, how the z component of each nuclei's spin angular momentum is transferred into spin angular momentum in the x and y directions.

What am I misunderstanding? Does anybody know any good articles that will elucidate the difference between the quantum mechanical and classical approaches to NMR?
 

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