Magelia
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NMR Intensity Ratios
Hi,
What are some reasons that a multiplet does not appear to have the correct set of intensity ratios? (ie: A triplet has 1:2:1, and a quartet 1:3:3:1).
I know one of the reasons would be second order effects, while another possible reason would be peak overlap. For example... in a ddd if one only saw
seven peaks instead of eight... that would be because the two central peaks overlap to give a larger central peak.
What would be another reason for not getting the correct set of intensity ratios?
Thanks!
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Dr.Bob
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If you are talking about comparing two peaks to each other, then you normally need a long relaxation time to get quantitative integrations, but I
don't know if that applies to integrations within a single peak, it does not seem like it would, but I am not sure.
Within a single peak, I would think that the ratios would be reasonably consistent, but the baseline correction, small impurities, noise and other
issues routinely alter the integrations by plus or minus 5-10% of the signal in quickly acquired spectra, unless you are doing a lot of work to
correct them. So I don't expect to see even a triplet be exactly 1:3:1, but anything within a reasonable tolerance is considered OK by me, like
1:2.5:1 or 1:3.5:1 in that case. If you are trying to publish a spectra, or have questions about the identity of the compound that cannot be
verified by another separate method like MS, then I would run a long relaxation time experiment with many scans to remove noise, and improve the
integrations, then check the baseline carefully, and integrate it manually to make sure the cuts are correct. Then if you see non-ideal results, I
would ask the NMR nerds.
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fledarmus
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Broad peaks are one reason - the ratios are actually integrations rather than intensities, but if the peaks are very sharp the intensities come out
the same. If they are broader, it isn't always the case.
Also, if two protons are very close together in chemical shift compared to the J values, the actual population of the two peaks gets distorted. For
example, if two protons only split each other and their chemical shifts are quick different, you will see a 1:1 distribution. As the chemical shifts
get closer together, however, the distribution will tilt so that the inside peak from each doublet gets larger and the outside one gets smaller. Under
the right conditions, it can look very much like a quartet. One of the assumptions built into the Pascal's triangle model is that although neighboring
nuclei may split the spin states, they will not actually influence the populations of those spin states, and that assumption breaks down as the
chemical shift difference gets smaller.
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