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How is nuclear spin multiplicity determined for molecules?

While analyzing NMR spectra, the concept of nuclear spin states and their multiplicity is fundamental. I know it relates to the spins of individual nuclei, but I get confused applying the coupling rules for systems with more than two spin-1/2 nuclei or when dealing with quadrupolar nuclei.

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By Pradeep Kumar Answered 5 months ago

You determine it by coupling the spins of the individual magnetic nuclei. For each nucleus *i*, it has a spin quantum number I_i (e.g., 1/2 for ¹H). The number of its own states is (2I_i + 1). For molecules, you combine them using angular momentum addition rules. For weakly coupled systems, the total multiplicity is simply the product of (2I_i + 1) for all active nuclei. I often recommend starting with simple cases, like two spin-1/2 protons giving a triplet and a singlet, before generalizing.

 

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