# How can I figure out which coupling constants match which hydrogen?

I am looking for some guidance assigning the $J$ coupling constants to the corresponding hydrogens. Here is my data: I can obviously see that shift 3.56 is for the methyl group. I can also see that the 4.63 is for the hydrogen adjacent to the methoxy group as it can only couple ($J=\pu{0.8Hz}$) to one hydrogen next to it. Is the approach then to see that the hydrogen at shift 4.04 also has $J=\pu{0.8Hz}$ so this must be adjacent (going clockwise from C of methoxy, next C along)?

• Yes, use the connectivity provided by matching coupling constants. Note that the methylene on the C6 carbon bears a pair of diastereotopic hydrogens, so these will be different. That should also give a hint about which hydrogen is at $\delta$ 3.43. – Zhe Sep 25 '17 at 15:10

Coupling constants are mutual. If nuclei $\ce{A}$ and $\ce{B}$ couple and this coupling results in a coupling constant $J_\ce{AB} = \pu{7.4Hz}$ on $\ce{A}$ then $\ce{B}$ will also be split into two signals with a coupling constant $J_\ce{BA} = \pu{7.4Hz}$.
• @gamma1 You can’t distinguish the two diastereotopic hydrogens on $\ce{C{6}}$ unless you have some sort of external reference. Typically, one would label them H6.1 and H6.2 or something along those lines. The only small hint you have is the coupling constant which can be used together with the Karpluls curve to determine a dihedral angle. – Jan Sep 26 '17 at 10:56