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85

 The value of the coupling constant J depends on the
  • distance between the two coupling nucleii, as well as their orientation towards each other
    and
  • character of the bonds between the two coupling nucleii
In the case of s bonds spin-spin couplings typically only be observed across three bonds or fewer (e.g., H-C-C-H).
If there are four bonds between the two coupling nucleii, the coupling constant typically already drops towards zero.
So-called long range couplings across a larger number of bonds typically only occurs if more polarizable p-bond systems are involved. (e.g., H-C=C-C-H).

Similar as in the case of the chemical shifts, it is possible to make quantitative predictions for J based on empirical and theoretical studies. A table with the values for common functional groups can be found here.

Now let's look at the following two molecules:
A)   B)
molecule A   molecule B
  1. Which proton groups in each of these molecules will couple with each other?
  2. To what types of multiplets with these couplings give rise?
  3. What are the expected values for the coupling constants J?


Compare your answers to our solution!