Reading introductory tutorials on proton NMR for the determination of chemical structure, I was told that protons can exhibit spin-spin coupling, thus creating multiplets on an NMR graph. The coupling can be explained by the fact that (referring to the diagram below) proton $H_A$ may be in the alpha or beta spin state, thus modifying the effective magnetic field "felt" by the $H_B$ proton, supposing they are in different chemical environments. A proton attached on a carbon further apart will have no influence because of the greater distance.

My question 1 is: Is this description of the spin-spin coupling accurate?

Protons bonded on adjacent carbons

However, it seems to me that the same argument holds to justify the influence of protons in the same chemical environment on adjacent carbons or even on the same carbon. I fail to see how the same chemical environment prevents one proton from disturbing the magnetic field felt by another one. For example, in the case of methane, any given proton may "feel" 4 different configurations of neighboring proton spin states, which should be indicated by 4 distinct signals on an NMR graph. This is not what we observe.

My question 2 is: Why protons' spin states in the same chemical environment do not couple with one another?

Any insights is greatly appreciated!



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