Hello, I was hoping that someone could help with an NMR problem. We have several

Question

Hello,

I was hoping that someone could help with an NMR problem.

We have several compounds that we need to take an NMR of, however, they are only dissolving in water. We took the NMR using deuterated H20, and the water peak at around 4.7 ppm drowns out the rest of the NMR. We have tried dissolving the compounds in typical organic solvents. However, all the solvents and even DMSO is not dissolving the compounds.

1. Is there anyway I can take an NMR of a compound, that is VERY solluble in water, but not much else, and still get a readable NMR.

2. Does anyone have any suggestions of other solvents we can use to maybe dissolve the compound?

Thanks!

Explanation / Answer

Hi as per your analysis, your compound is only soluble in highly polar solvent like water only, this means your compound us under study is highly polar, probably in the form of salts like quarternary ammonium slats or hydrochloride salts. These forms are very often in pharmaceutical/drug industry as those medicines must be soluble in water to be absorbed by human body.

My opinion/suggestion for you is try to dissolve in strong bases/alkali like NaOH/KOH, if it dissolves then definitely your compound is in the form of water soluble salt.

Now take a seperating funnel and add water/dichloromethane in a 50:50 ratio, add your compound into this seperating funnel,vigorously shake it, then add dropwisely alakli, again shake it well. Then carefully add base until all the compound seperated out into organic phase.

Now seperate the two layers, collect organic layer, dried over Na2SO4, use rotavapor to remove the dichloromethane solvent from it to get organic compound. Now it will dissolve in CDCl3/DMSO etc.

I am guessing your compound is not an inorganic or polymer compound, otherwise the above technique won't work for you.

One more best one for you is try water signal suppression technique in 1H NMR (sometimes we used term selective decoupling at 4.7 ppm - water peak/signal suppression). This will improve signal-to-noise ratio and your compound signals will be observable/readable.

Refer:

http://nmr.chem.indiana.edu/nmrblog/?page_id=989

https://www.sciencedirect.com/science/article/pii/S006641030860040X

https://onlinelibrary.wiley.com/doi/full/10.1002/9780470034590.emrstm0590

Hope this helped you!

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