Hello, in my American undergraduate studies we covered spin-spin coupling in 1H NMR only in terms of 3J coupling. Now, beginning my Masters in Germany, I have become aware that there is a whole lot more to the multiplicity of a nuclide in NMR spectroscopy.
So far in my reading, I have difficulty with identifying chemically equivalent but magnetically inequivalent protons. I was hoping someone here might be able to help me along with this.
I have read that if I can turn an achiral central atom into a chiral one in one step (prochiral) by switching out a proton with, say, deuterium, and there is no preference for which proton I switch, then the protons on that central atom are magnetically equivalent. Similarly, if individually substituting each proton leads to two stereoisomers, then the protons are magnetically inequivalent.
For example, if I understand this rule correctly, then I believe if I had bromochloromethane, this rule would indicate that the two protons are enantiotopic and magnetically inequivalent.
However, I have run into a few examples usually involving double bonds (or perhaps a general case might be protons that are homotopic and magnetically inequivalent) that I cannot seem to apply this rule to.
One is 1,1-difluoroethene. These are homotopic protons which are magnetically inequivalent. Here, it is explained that J(HAFA)≠J(HAFB). This makes sense to me. However, this inequality is supposed to explain why HA≠HB. That's the step I can't follow.
In my mind, if we assume initially that both hydrogens are equal, and both fluorines are equal, then even if they have trans-coupling across the double bond that is unequal to cis coupling, they are still coupling to flourine atoms that are equivalent. In other words,
J(HAFA) = J(HBFB)
and
J(HAFB) = J(HBFA)
so if the coupling constant from one of the hydrogens were provided without saying which one it were from, one would not be able to distinguish to which hydrogen it corresponded. It could be either hydrogen, just coupling to a different flourine. To me, this indistinguishability should mean the hydrogens are magnetically equivalent?
Another example I have trouble with is tetrahydrofuran. Again I understand the chemical equivalence but not the magnetic equivalence, for the same reason as the first example.
These are just a couple of examples that I have trouble understanding. I do understand an example like 2-bromopropene, where the terminal protons on the 1 carbon are not equivalent.
Is it also the rule that ALL diastereotopic and enantiotopic protons are magnetically inequivalent? Because the rule I stated above seems to only confirm whether or not a molecule is diastereotopic, enantiotopic, or homotopic. The problem is that doesn't help much if any of those conditions can mean either equivalence or not.
Thank you very much for any advice that might be provided!