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I am studying nmr and the concept of multiplicity is very hard to undersand. Specifically I wanted to know where this arises form. Can anyone explain the concept of spin spin coupling in detailed and in simple terms?

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  • $\begingroup$ This question would benefit from an answer you understand IF, and only IF, you elaborately explain why you didn't understand the tutorials you have available or ones on the Net. What if an answerer repeats the same words that made you uneasy and baffled? $\endgroup$ – M.A.R. Feb 12 '17 at 19:06
  • $\begingroup$ The reason why I am asking this seemingly difficult question is because I am not sure I understand the concept of spin properly (And i think the answer to this question is related to spin of adjacent hydrogens). Most important no one seems to answer the question as to why there are two spin value when a external magnetic field is applied. What I mean by this is why do some proton align with and some against the magnetic field applied? Being opposed to external field takes energy, so why are some protons have spin against the field? $\endgroup$ – TLo Feb 12 '17 at 19:18
  • $\begingroup$ Understanding this first then would make it easier for me to understand how does coupling occur and all. $\endgroup$ – TLo Feb 12 '17 at 19:20
  • $\begingroup$ Not all protons align with the field, because of thermal energy and the Boltzmann distribution. Simply speaking, the gap between the two energies is very small, and therefore as long as there is some heat in the system, that heat is enough to excite some protons from the lower energy state to the upper energy state. (Therefore, at absolute zero where there is no heat energy, every single proton will be aligned with the field.) However, even at non-zero temperatures, there will be an excess of aligned spins and a deficit of opposed spins. $\endgroup$ – orthocresol Feb 13 '17 at 0:03
  • $\begingroup$ MAR is right; these things are covered in many existing textbooks and websites, perhaps better than anybody here can explain. I would really recommend consulting one of them, or finding somebody in real life to go through this, instead of asking a large number of questions here. The issue is that we don't understand the context or the background behind these questions. You have to explain what exactly the problem is - and that's much easier in real life than over the Internet. Apart from that, Stack Exchange is not well-suited to lots of back-and-forth discussion and clarification. $\endgroup$ – orthocresol Feb 13 '17 at 0:05
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As it stands, your question is very general. It is unlikely that a single answer here will describe spin coupling and multiplicity in a simple and detailed manner any better than any publicly available reference source. There are hundreds available. I suggest you read one or more of the following extremely useful introductory NMR references, and ask more specific questions that you do not understand from them.

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