The physical basis of strong coupling is subtle, having to do with complex quantum-mechanical phenomena that I won’t go into here.Īlthough second-order sounds bad – it isn’t necessarily. Otherwise, this means you’re observing strong coupling and you’ve entered second order spin system land. This condition is satisfied if the difference between two chemical shifts (Δδ) is much larger than the coupling constant (J) between them. First order spectra are awesome because they can be solved through visual inspection (i.e., chemical shifts and coupling constants can be extracted). There are a number of criteria that define a truly first order system, but basically the designation ‘first order’ is given when multiplicity follows the ‘n + 1 rule’ and these multiplets have intensities defined by Pascal’s Triangle. So the coupling that I previously described applies to first order spin systems.
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