What will happen if my sample is contaminated with other compounds, how it will show on NMR spectroscopy? How pure should be sample when doing NMR?phlogiston - 21-1-2016 at 13:28
[Edited on 21-1-2016 by phlogiston]morsagh - 21-1-2016 at 13:42
And is there any way how to predict spectrum of mixture? Or is it just sum of peak integrals?fluorescence - 21-1-2016 at 14:41
It really depends on the molecules. NMR shows functional groups or at least functional groups tend to be at certain positions. So the more equal
functional groups you have the worse the will it be. But if you have a clean product and literature values you can find your peaks and just calculate
how much other stuff is in there. Attached is an example of a 1H-NMR I did last week for a compound I made.
I recrystallized it from Benzene that's why there isn't much left in there
- On the very left you see the solvent
- The peaks from 3.5 - 4.5 are my product peaks
- And at around 2 ppm is something in there that shouldn't be there. My guess is that it is TMEDA, I used that, too. For the one at 1.5 we have no
idea what it could be. But the element analysis (separate meassurement) shows it's quite pure so might be some impurities in the solvent.
So since all these have different groups, one seems to me a methyl group, one is this TMEDA with Nitrogen in there and the product peaks are somewhat
complicated to explain you see that like this if you know what is in there you can just take the spectrum apart and tell what is your compound and
what is not.
If you have no clue what you have as sample it might be quite complicated, I have know idea what you would do then. Maybe use other methods.
phlogiston - 22-1-2016 at 04:13
The molecules of the contaminant and main compound are not near one another and therefore do not split or otherwise affect each others peaks.
The different contaminants each make an independent contribution to the total spectrum. As fluorescence says, if you are unlucky and the peaks overlap
it may be difficult to interpret the spectrum.
NMR is usually not the best way to quantify impurities, but if you know what the contaminants are exactly you can calculate the relative amounts, but
don't expect very accurate results.
[Edited on 22-1-2016 by phlogiston]BromicAcid - 22-1-2016 at 04:36
And is there any way how to predict spectrum of mixture? Or is it just sum of peak integrals?
There is NMR prediction software available that can help for a single compound and is fairly accurate, there are also NMR libraries that can help.
Further, most chemical education includes a bit of NMR study which enables one to give a general idea of what the specrum will look like, i.e., this
alkane should gives a septet in this region, a singlet around here, etc. Those who have taken the time to 'master' NMR usually have no issues giving
you a complete (or nearly complete) overview of the molecule from the NMR or vice versa. It always amazes me just how much a skilled individual can
pull from a NMR, it's like any language I guess.
As for quantifying impurities, I have come across several compounds where purity is determined from the vendor by NMR using the sum of integrals of
the product peaks vs all other peaks. Of course if something is not NMR active then that would be a moot point, but often the NMR testing is only one
test out of the entire testing package (IR, UV Vis, GC, GC/MS. HPLC, Titration, Chlorine Combustion, etc.).DJF90 - 22-1-2016 at 05:11
Assay by 1H-NMR is a common experiment used by process chemists. It is useful for quantifying (on a w/w basis) the purity of your material, the level
of a given impurity, or the amount of compound in solution. Using the sum of integrals gives you a mol% basis, but this does not account for
NMR-inactive materials that may be present (e.g. inorganics). I will elaborate further if there is sufficient interest.fluorescence - 22-1-2016 at 08:38
I did a 19F and a 31P NMR some time ago. They are all quite similar. And it's not that complicated to calculate how one will
look like. But there is quite much literature on the spectra of all known compounds and if you need one I have acess to the database for most of the
organic spectra. What makes a spectrum quite complicated is coupling in for example ring molecules. The spectrum above shows a sulfur bridged
Ferrocene. You would expect 2 signals for the two types of Hydrogen in there but in fact the bridging sulfur is not symmetrical but a bit offset so
the hydrogen next to it have non equal distances forming more lines in the NMR. Another example would be this Furan here. I think the picture shows
quite well how the Hydrogen may couple with different constants.
http://131.104.156.23/Lectures/CHEM_207/CHM_207_Pictures_NMR/NMR_AA'XX'_furan.gif
You would only expect 2 signals here but there are so many hydrogen so close to each other that multiplets can form making it quite hard to predict.
For the question with the impurities. We did NMR, IR, MS and lot's of other things but I don't think spectroscopy will help you here. YOu might be
able to tell all the functional groups but you don't know to which molecule they belong. It would be like making a element analysis. If you are really
good in combining MS and NMR you might be able to see how some fragments and NMR-functional groups cannot overlap but if you have ever seen a basic
MS-Spectrum you know that's really hard.
One good way would be to use simple column chromatography or even better Gas Chromatography. Here the molecules move as one single thing so you can
see on the amount of compounds and with some standarts even the concentration of each. Then you could isolate them with I dunno column chromatography
followed by TLC or GC and if it's pure enough analyze it again.
What could work, too would be to change the frequency of the NMR starting from a 300 MHz to 400, 600 and if you have acess to one 750 MHz, like we
have all of them. I'm not sure but I think you can see if peaks move further away from each other or stay in a multiplet shape but don't ask me how
you would do that I never did it.
