smuv
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ESR/EPR Coupling question
How do I assign the hyperfine coupling constants to an esr spectrum (NOT predict the hyperfine coupling)?
It should be very simple but it is something I was never taught, and I havn't been able to find an answer through research and asking my peers.
An example of one of my ESR spectra is attached. The spectrum is AIBN with a PBN (N-tert-Butyl-alpha-phenylnitrone) radical trap.
Anyone who can help will gain a great deal of my gratitude.
[Edited on 11-3-2008 by smuv]
Attachment: epr.pdf (66kB)
This file has been downloaded 608 times
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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watson.fawkes
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I haven't done this sort of thing since college, when I was a physics student. But I remember enough to recommend that you read this experiment from MIT, which has a perfect good and fairly concise treatment of the nucleon-electron coupling.
What seems to me, even though I'm rusty, is that the three bursts are the ordinary Zeeman triplet splitting of an S=1 electron, and that the bursts
are a finer triplet splitting from a pair of protons in different nuclei (this is a triplet because the center state has a doublet degeneracy, so four
states have only three energy eigenvalues). The N=N bond at the center of AIBN seems to satisfy this, the N≡C bond at the edge doesn't.
I should point out that you're dealing with <sup>14</sup>N, whose nuclear spin has quantum number I=1. Interestingly, the bits of noise,
particularly between the second and third bursts, look qualitatively like they're due to uncommon isotopes, perhaps <sup>13</sup>C or
<sup>15</sup>N (both I=1/2).
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smuv
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Many thanks, that is a wonderful reference for ESR newbies.
It is possible that those bursts are due to uncommon isotopes (and not impurities) because esr spectra of this compound from literature show similar
features.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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watson.fawkes
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| Quote: | Originally posted by smuv
It is possible that those bursts are due to uncommon isotopes (and not impurities) because esr spectra of this compound from literature show similar
features. |
It's pretty likely, I'd say. <sup>13</sup>C ordinarily has about 1% abundance,
accounting for the magnitude. With I=1/2, it will show doublet splitting, which is what that little figure on the graph looks like.
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smuv
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So just to report on the internet for anyone in a simular situation as myself.
The magnetic field difference between the peaks of the multiplet in G or T is the coupling; exactly like NMR.
So basically the simplest problem in the world stumped me...
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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watson.fawkes
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| Quote: | Originally posted by smuv
The magnetic field difference between the peaks of the multiplet in G or T is the coupling; exactly like NMR. |
I have to say, the unit "G" stumped for a while, until I remembered Gauss. D'oh. And to be precise, the difference between the peaks
gives the total strength of the coupling, but since the ESR measures the first derivative, it's the distance between zeros in that graph that's you're
measurement.
Incidentally, did you notice that the maxima of the first derivative were in the ratio 9:16? It seems to me that's significant (since it's correlated
with the width of the peak), but I don't see immediately what it's about.
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smuv
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| Quote: | | Incidentally, did you notice that the maxima of the first derivative were in the ratio 9:16? It seems to me that's significant (since it's correlated
with the width of the peak), but I don't see immediately what it's about. |
I'm not certain of the significance of this either; I'll tell you in a year after I take my spectroscopy class  .
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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Quibbler
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The spectrum shows a triplet. This is due to the 14N (MI=-1,0,+1).
The triplet is further split into doublets this is due to coupling to the nearest proton (MI=-1/2,+1/2)
The difference in heights is due to the closeness of the doublets (and possibly the slow response of the chart recorder).
The magnitude of the coupling is the distance between the peaks (like NMR), but is usually quoted in Gauss.
[Edited on by Quibbler]
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