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I am studying Magnetic Resonance Spectroscopy and I have some difficulties in understanding the j-j coupling interaction. What does actually happen from the physical point of view to the electronic cloud of the neighbouring atom? When I try to understand in books like A Complete Introduction to MODERN NMR Spctroscopy of Roger S. Macomber (but also in websites) it seems to me that there are explainations more focused on quantum theory and its rules, but what I want to know is simply what happens physically at the electronic cloud and the consequent interactions ? Therefore why does this causes that in the spectrum, for example, of the Ethanol molecule (CH3CH2OH) there are three peaks related to -CH3 group? Or why in the Acetaldehyde molecule (CH3COH) there are four peaks related to -COH group and two peaks related to -CH3 group just for the j-j coupling interaction?

I really would like to see things clearer at the basis (so what happens physically to the electronic cloud and the consequent interactions) and thus being able to understand the number of peaks (while I think I have understood the chemical-shift interaction. I want also to specify that for the moment I have to consider just these two interactions because I am considering MRS applied to brain tissues which I can approximate as liquid so, for example, the dipolar interaction is considered average to 0).

orthocresol
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HelpNeederStudent
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  • I am not exactly sure if this is what you are asking, but the J-coupling happens primarily by the interaction between the spin of nuclei which are close to each other. Consider the simple case of two inequivalent nuclei with spin I=1/2. Then, those two nuclei can orient parallel to each other, or opposite to each other. These two states will have different energy. (You can use the analogy of a dipole to think about it) – S R Maiti Jan 25 '21 at 19:09
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    @ShoubhikRMaiti, the energies of the states is one thing, but the splitting is manifested as a difference in the transition energy. Well, yes, those are just differences between energies of the states: but it's slightly more subtle than just saying that $E(\alpha\beta) \neq E(\alpha\alpha)$. Instead, the splitting arises because $E(\beta\beta) - E(\alpha\beta) \neq E(\beta\alpha) - E(\alpha\alpha)$. Or, more loosely speaking: the energy needed for the "spin flip" depends on the spin states of the surrounding electrons. This is a pretty simplified view, but it sort of works. – orthocresol Jan 25 '21 at 19:34
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    Many organic chemistry textbooks will give some handwavy explanations which you might find useful at this stage, but beyond that, I'd also strongly suggest learning the QM at some point. I suggest Keeler's book (there is an accompanying series of lectures on YouTube). From what I can tell, you're asking for a physical picture of what goes on with nuclei / electrons, and implicitly you want a classical description: so-and-so repels so-and-so. But this is bound to fail eventually. Electrons are not classical objects, and spin is decidedly not a classical phenomenon. We can do better :-) – orthocresol Jan 25 '21 at 19:52
  • @ShoubhikRMaiti I understood that the j-j coupling interaction is also called indirect polar interaction because there is not only the interaction between nuclei but also a nucleus that causes the polarization of the electronic cloud of the close atom and this causes like a sort of interaction with the nucleus of the same close atom ... but I do not know if I understood well. And why this causes different peaks on the MRS spectrum ? So I was asking for clarification. – HelpNeederStudent Jan 25 '21 at 19:55
  • Thank you @orthocresol♦ ! So you are saying that I can not interpret the number of peaks with considering the interaction between the nuclei and electronic cloud from a classical point of view ? – HelpNeederStudent Jan 25 '21 at 19:58
  • @Manuela It looks like you've already asked this question on Physics.SE. Please note that cross-posting is usually not permitted. If there is a solid reason for it, it would be nice to add a link to the duplicate question on the other network site, mention in both questions that it's a cross-post and briefly explain the motto for it. – andselisk Jan 25 '21 at 20:03
  • @andselisk♦ I did it because nobody answered in physics so I thought it was a question more related to chemistry – HelpNeederStudent Jan 25 '21 at 20:05
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    I think there are some classical explanations that may sort of make sense, but many of them actually do not stand up to scrutiny, when judged against the quantum theory. On top of that, from your question it sounds like you are doing a reasonably serious project. So I think it would be a good thing for you to eventually read some of the QM. Re. crossposting, if nobody's said anything on Physics, then I'd suggest the easiest solution now is to just delete the question on Physics. (Btw, no need to include the diamonds when @-ing us.) – orthocresol Jan 25 '21 at 20:07
  • Because for example you can understand the peaks due to the chemical shift interaction considering the electronegativity of the different atoms of molecules so I thought I could do a similar reasoning for j-j coupling interaction. Ok @orthocresol thank you again this helps me . – HelpNeederStudent Jan 25 '21 at 20:11
  • About the question: can someone simply close the question on physics ? :) @andselisk – HelpNeederStudent Jan 25 '21 at 20:14
  • @Manuela I believe you can delete the question yourself, just as orthocresol suggested. Alternatively, you can ping moderators on Physics.SE and ask how to better handle this situation. – andselisk Jan 25 '21 at 20:35
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    The thing is, you can draw empirical correlations between molecular properties and the values of the J-coupling: that would be very similar to what you say about electronegativity and chemical shift. Again, there are a number of textbooks that cover this, in particular those with a more organic chemistry focus (the specialised NMR books will not, because they simply take the J values for granted, as some experimentally determined parameter). So are you really asking about the factors that affect how large the coupling is, or are you asking about the underlying origin of the coupling? – orthocresol Jan 25 '21 at 21:41
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    In either case, I'm not going to attempt to give a full explanation (it is a bit too long for me today), but I would point you towards some of the books here https://chemistry.stackexchange.com/questions/37303/resources-for-learning-chemistry (Ctrl-F for NMR). Check them out, and consult a supervisor or tutor as necessary... – orthocresol Jan 25 '21 at 21:43

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