[blaisem]

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(H_AF_A)≠J(H_AF_B).  This makes sense to me.  However, this inequality is supposed to explain why H_A≠H_B.  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(H_AF_A) = J(H_BF_B)
and
J(H_AF_B) = J(H_BF_A)

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!

[Babcock_Hall]

Definitions are not right or wrote; they are useful or useless (or so said my physics instructor).  Testing for magnetic equivalence or nonequivalence presupposes that two nuclei have the same chemical shift; the question of magnetic equivalence or nonequivalence never arises for two nuclei that are diastereotopic for that reason (I am not sure about enantiotopic nuclei).  Some people use the term "isochronous" to refer to two nuclei that have the same chemical shift.  I personally think that "isochronous" is a better term than "chemical shift equivalent."

Consider ortho-dichlorobenzene (http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng and look for o-dichlorobenzene)
The two protons next to their respective chlorine atoms do not couple equally to the same nucleus; therefore, they are not magnetically equivalent, despite being isochronous.  The spectrum we see is not one we could begin to understand if we did not invoke the concept of magnetic nonequivalence (in other words, the definition of magnetic equivalence or nonequivalence is useful.  You might try R. M. Silverstein's book or J. Chem. Ed. article from 1980 for more information.

[blaisem]

Definitions are not right or wrote; they are useful or useless (or so said my physics instructor).  Testing for magnetic equivalence or nonequivalence presupposes that two nuclei have the same chemical shift; the question of magnetic equivalence or nonequivalence never arises for two nuclei that are diastereotopic for that reason (I am not sure about enantiotopic nuclei).  Some people use the term "isochronous" to refer to two nuclei that have the same chemical shift.  I personally think that "isochronous" is a better term than "chemical shift equivalent."

Consider ortho-dichlorobenzene (http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng and look for o-dichlorobenzene)
The two protons next to their respective chlorine atoms do not couple equally to the same nucleus; therefore, they are not magnetically equivalent, despite being isochronous.  The spectrum we see is not one we could begin to understand if we did not invoke the concept of magnetic nonequivalence (in other words, the definition of magnetic equivalence or nonequivalence is useful.  You might try R. M. Silverstein's book or J. Chem. Ed. article from 1980 for more information.

Hi Babcock_Hall.  Thank you for your reply.  My library happens to have that book, so I will go take a look.  Hopefully it has more to say than the other 3 books I have already looked through

In the meantime, a quick question: I know that magnetically equivalent protons are also of the same chemical shift (chemically equivalent).  You mentioned that the question of magnetic equivalence is not an issue for diastereopic protons because, as I inferred from your post, the chemical equivalence is not the same.  In other words, are you saying that diastereopic protons are inherently chemically inequivalent?

Also, do you (or does anyone) have any tips on the pattern or logic behind whether a proton is magnetically equivalent or not?  That´s my sticking point right now.  I understand qualitatively and somewhat quantitatively the implications of magnetic inequivalence, but I can´t identify protons that are magnetically inequivalent without being shown a spectrum first.

Perhaps I should have posted this topic in the Organic forums?

[Babcock_Hall]

This topic could be addressed in either forum, although it might be marginally better in organic.  Diastereotopic nuclei often have very different chemical shifts, although some pairs of diastereotopic nuclei have differences in shift that are too small to detect.  Someone who is better versed in NMR may want to correct or clarify my comments, but I never even ask the question of magnetic equivalence unless the nuclei have identical shifts (via some sort of symmetry argument).  For two nuclei to be magnetically equivalent, they must couple in an identical way to all other nuclei.  If two nuclei couple identically to two other nuclei that have the same chemical shift, that is not good enough.  That is why the two hydrogens (A and A') that are adjacent to two chlorine atoms in o-dichlorobenzene are not magnetically equivalent:  Hydrogen A does not couple with the same coupling constant to hydrogen B as hydrogen A' does.