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Thursday, August 7, 2008

Chemical and Magnetic Equivalence

Many students are unclear about the difference between chemical equivalence and magnetic equivalence. The clearest explanation I have seen on this is in Robin Harris' book, "Nuclear Magnetic Resonance Spectroscopy" (1983). Chemically equivalent nuclei behave the same as one another chemically but do not have the same NMR properties as one another, whereas magnetically equivalent nuclei are chemically equivalent and they have the same NMR properties. One can determine whether two nuclei are chemically or magnetically equivalent by considering the following:

If the nuclei under consideration are not isochronous (i.e. they do not have the same chemical shift) then they are neither chemically nor magnetically equivalent. All chemically or magnetically equivalent nuclei are isochronous however, isochronous nuclei need not be chemically or magnetically equivalent as their chemical shifts may be fortuitously identical.

If the two nuclei being considered are isochronous, one should consider how they couple to a third magnetic nucleus in the molecule which is not equivalent to them. If the coupling to the third magnetic nucleus is different for each of the nuclei being considered, then the nuclei are chemically but not magnetically equivalent to one another. If the coupling to the third magnetic nucleus is identical (and this is true for every magnetic nucleus other than those being considered) then the two nuclei are both magnetically and chemically equivalent.

This is illustrated in the figure below. (The examples from Harris' book were used here.) In the molecule on the left, the two protons are both chemically and magnetically equivalent as they both have the same coupling with the fluorine. In the molecule on the right, the two protons are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the fluorines. Similarly, the two fluorines are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the protons.

14 comments:

Anonymous said...

I understand the concept but is this statement exactly correct:

"If the coupling to the third magnetic nucleus is different for each of the nuclei being considered, then the nuclei are chemically but not magnetically equivalent to one another."

Wouldn't the two protons in your example on the left (magnetically equivalent) each couple differently to, say, the chlorine on the "left" of the molecule?

Dave

Glenn Facey said...

Dave,

You are absolutely correct. I was hoping someone would make that point. In this case I think the chlorines are "self decoupled" due to fast relaxation between the energy levels so I suppose that we can think of them in this case as having no spin. There certainly are complications though if you start considering all of the 13C isotopomers....

Glenn

Anonymous said...

so you mean
we can talk about spin systems only presence of elements which shows NMR jst like C13,F19 & etc ?...
can u give another example ??

what is exactly "self decoupled".. ?

Thanks,


NBW

Glenn Facey said...

Anonymous,

Yes, it is only the magnetically active nuclei that are important when considering magnetic equivalence.

Self decoupling is observed for a nucleus coupled to a quadrupolar when the quadrupolar nucleus relaxes very quickly among its energy states. The observed spin essentially sees an average energy state for the quadrupolar nucleus.

Glenn

swapna damera said...

I didn't understand why the protans in the left example do not show the magnetic equivalence?what factor decides?How can we say directly by seeing the example with out checking the NMR spectrum?

Glenn Facey said...

Swapna,

In the example on the left, the protons are both chemically and magnetically equivalent as they have the same relationship the fluorine in the molecule. In the example on the right, the protons are chemically but not magnetically equivalent as each proton is not related in the same way to a particular fluorine in the molecule.

Glenn

Craig Grimmer said...

Dear Glenn

What about the case of Y-C-O-CH2-X, where X is aprotic and not O-C and the molecule has no chirality? The protons are chemically equivalent, enantiotopic. They appear to be magnetically equivalent - the spectrum shows a singlet for the CH2 group but in any single conformational geometry, the dihedral angle between each of the protons and the carbon of C-O is different; by the Karplus relationship, the 3J coupling should also be different and because the 3J coupling is different, the protons are magnetically different - does that make sense?

Glenn Facey said...

Craig,

I'm not sure I am clear on the structure you propose. Is it Y3C-O-CH2-X or Y-(C=O)-CH2-X. In the second case, there is no three bond coupling between the protons and 13C. For the first case, you would be correct in that the 3 bond H-C couplings could be different however this would only be true for the 13C isotopomer where the Y3C- carbon was labelled. This would only be 1% of the signal. The other 99% of the signal would be for the 12C isotopomer for which there is no 3 bond H C coupling.

Glenn

Anonymous said...

This seems to be weird nomenclature. I don't really understand the need for such definitions.

Glenn Facey said...

Anonymous,
I think the nomenclature is quite descriptive. Such definitions are needed because there is an important difference between chemically and magnetically equivalent nuclei which affects the outcomes of experimental measurement.

Glenn

Hohsuan Chou said...

Anonymous said:

How about Ph-CH=CH-Ph (cis or trans), I am so confused on these cases. The textbook stated they are magnetically equivalent and give one signal on the spectra. But if you check the chemical distance from Ph to each proton on the double bond, they are not at the same distance. Is there any explain for these?

Glenn Facey said...

Dear Hohsuan,

Thank you for the question. In both cases (cis- and trans- Ph-CH=CH-Ph) the protons on the central double bond are isochronous (chemical shift equivalent) but are not magnetically equivalent. You can justify this by considering how the protons on the double bond are coupled to the ortho- (for example) protons of one of the phenyl groups. The protons on the double bond are not coupled equally to the ortho protons of the same phenyl group. In both cases however, one is likely to observe a singlet for the protons on the double bond as the magnitude of the coupling to any of the protons on the phenyl rings is very small. Which textbook claims that these protons are magnetically equivalent?

The result is more clear in the 1H NMR spectra of cis- and trans- (H)(F)C=C(H)(F). In each of these cases the 1H spectra are complex because the H-F coupling constants much greater than zero.

I hope this is of help.

Glenn

Anonymous said...

Hi,

For 1,3 dichlorobenzene, are protons on position 4 and 6 magnetically equivalent?
Also, is chlorine nmr nuclei active, which then affects the magnetic equivalence of the protons in question?

Thanks and regards.

Glenn Facey said...

Anonymous,

The protons in the 4 and 6 positions of 1,3-dichlorobenzene are magnetically equivalent if all of the carbons are 12C and the 35Cl and 37Cl nuclides are self decoupled due to fast relaxation. If the 35Cl and 37Cl nuclides are not self decoupled, then the protons in positions 4 and 6 are chemically but not magnetically equivalent.

Glenn