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.
34 comments:
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
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
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
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
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?
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
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?
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
This seems to be weird nomenclature. I don't really understand the need for such definitions.
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
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?
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
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.
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
Dear Glenn,
Thanks for your kind reply about " the issue on both cases (cis- and trans- Ph-CH=CH-Ph) of the protons on the central double bond are isochronous".
In the textbook" Introduction to Spectroscopy, 5th. Pavia , Page 398
It states that A molecule that has a symmetry element(a plane or axis of symmetry) passing through the C=C double bond dose not show any cis or trans splitting since the vinyl protons are chemically and magnetically equivalent. Are these correct?
Besides, The books describe in the beginning of the Chaper 7.3 Magnetic equivalenece. It showed that fumaric acid do not showspin-sping splitting on the trans vinyl hydrogens. Does it mean they are also magnetically equivalent?
Thanks for your kind explanation.
Hohsuan,
The statement (as you have written it) [if] "A molecule that has a symmetry element(a plane or axis of symmetry) passing through the C=C double bond dose not show any cis or trans splitting since the vinyl protons are chemically and magnetically equivalent." is not correct. If you take into account 4 bond couplings, you see splittings in the vinyl protons in both the cis and trans cases. In both cases the vinyl protons are chemically but not magnetically equivalent. If the 4 bond coupling is very small, it may not be observed and there would be no apparent splitting. This is also true for fumaric acid. Your statement is correct if the X substituents on the double bond have no spin. I suggest that you take a look at Robin Harris' book "Nuclear Magnetic Resonance Spectroscopy".
Glenn
I understand this but one thing that is still a doubt fr me , i.e, can two chemically non equivalent protons be magnetically non-equivalent alwayss
Anonymous,
Chemically nonequivalent protons are always magnetically nonequivalent. They may be isochronous but always magnetically nonequivalent.
Glenn
Let's say you have hydrogens attached to a centre with a formal charge, would those hydrogens be chemically and magnetically equivalent or just chemically equivalent?
Unknown,
You haven't provided enough information for a conclusion. What is the "center"? What is the entire structure?
Glenn
for example if you were looking at the 1H nor for BH4-, would all the hydrogens be chemically and magnetically equivalent? whereas if you are looking at the 11B NMR, would all the hydrogens still be chemically and magnetically equivalent?
Unknown,
Yes and Yes
A group of spins are magnetically equivalent if they have the same chemical shift, δ and are coupled equally to all other groups of magnetically equivalent spins in the molecule. The only exception to this rule is that if there is only one group of spins in the molecule then they are magnetically equivalent (eg the protons of benzene, methane, water etc...).
Glenn
Hello,
Levitt's description of magnetic equivalence:
Spins must have the same chemical shift
AND
Must have identical couplings to all other spins in the molecule
OR
There are no other spins in the molecule
However, going by that criteria if you were to look at ethene:
the coupling of the top left proton to the proton trans to it would be different than the cis coupling.
How are these considered magnetically equivalent from that definition? I know it is magnetically inequivalent but ethane technically have different coupling if its cis vs. trans.
Anonymous,
In the case of ethylene all of the chemically equivalent protons belong to the same group of spins just like all of the protons of benzene are in the same symmetry related group. Each proton of ethylene has a cis, trans and syn related proton. They are magnetically equivalent just as the protons for water, methane and benzene are magnetically equivalent.
Glenn
Thank you for this post. Sometimes we forget the simple things, but we need to remember these to move forward in our study.
What is the rule for determining if nuclei are isochronous?
any reference to the answer is welcomed
thanks in advance
armando
Anonymous,
Nuclei are isochhronus if they are chemically equivalent. Nuclei are chemically equivalent if they behave the same chmically. They are usually related by molecular symmetry.
Glenn
Hello,
I read your explanation about ethylene above. If we think similarly, why the 1-methoxy-4-nitrobenzene is chemically equivalent but magnetically inequivalent?
Thank you in advance
Anonymous,
Label the protons ortho to the methoxy group Ha and Ha'. Label the protons meta to the methoxy group Hb and Hb'. Ha and Ha' are chemically equivalent. Hb and Hb' are chemically equivalent. Ha and Ha' are magnetically nonequivalent since Ha is not coupled to Hb in the same way that Ha' is coupled to Hb. Similarly Hb and Hb' are magnetically nonequivalent.
Glenn
reading these detailed and patient answers from Glenn. Great guy!! enjoy the retirement
Hey Glenn, just read through all these questions and answers. It's really impressive how you have responded throughout the years.
Hello,
I am a little confused by the example provided. You mentioned that the H in the right structure are chemically but not magnetically equivalent. But according to the given definition, nuclei that are magnetically equivalent are chemically equivalent. So if H in right structure are not magnetically equivalent then how can they be chemically equivalent?
Shiva,
Magnetically equivalent nuclei are always chemically equivalent. Chemically equivalent nuclei are not necessarily magnetically equivalent.
Glenn
That makes sense. Thanks!
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