It is usual and actually desired not to see these H in carbohydrates, as it eliminates unwanted complexity. They are all labile and probably exchanged with the DMSO deuteriums. Did you carry out first some lyophylisation in a (small) volume of D2O? this should help you eliminate the water peak from your spectra. There still will not be any signals for the H from alcohols, amines etc. however, they'll all be deuterated and you'll have to rely on the H from C-H for identification of the functions.

You may try and see these H signals using low temperature, I remember seeing an article by Ian Coulquhoun from IFR Norwich using that.

I just dried the powder of derivatives in room temperature, and dissolved it in DMSO-d6 for taking the HNMR spectrum. 

Well, it is worth buying a few 1ml vials of about 99% D2O and perform the exchange (dissolbe the amount of polysacch that you are going to put in the NMR in 1 mL of D2O then freeze-dry (some people did it twice a&ctually for a really clean NMR specttrum, but nodays water supression programs are quite efficient) before dissolving in DMSO). You might also try to change the temeprature to shift the water peak and reveal hidden signals, as 3.3 ppm is smack in the spectrum of polysaccharide H2/H3, depending on ugar even sometimes H4.

Actually, the peak of water is not my problem. I just want to know is it logical that we don't see any signal for these kind of functional groups in DMSO-d6 (without using of D2O)? 

Yes, they are labile, it is logical that you do not see them.

If you have wet DMSO-D6, it is pritty difficult to see them due to H-D exchange. Why don't you try an alternative but less polar (less hygroscopic) NMR solvent? Or dry your DMSO carefully?

Hello Farzad. if there is a small chance of H/D exchange it will happen. I wouldn't be surprised if you get in same sort of troubles with d6-acetone. Bear in mind that sugar samples are hygroscopic too and you may keep drying your d6-DMSO without any positive effect. So Catherine's answer about freeze-drying is very useful. Cheers

The observation of OH protons in polysaccharides, whether wanted or not, deponds on the water content (mediated exchange by water is the most prominent source of th ekinetic). Sucrose eg carefully dried at 62 C for several days, dissolved in dry DMSO (activated molecular sieve) and dried as solution again over mol. sieve, sealed into a NMR tube perfectly Show all OH Protons - nice example for students.

the larger the saccharide the more possible Exchange Partners tere are (no need for water as mediator), the higher the concentration the higher the probability for inter-molecular exchange. For several Problems in structure elucidation and the Determination of the 3D structure, the non-exchanged OH are valuable. Then use

1. low concentration

2. lower temperature

3. a carefully dried sample using a similar procedure as outlined for sucrose

4. any kationic impurity will also mediate the inter-molecular (atomar at this point!!)OH exchange

n.b.: In the presence of  free acids the procedure might be more tricky (trans-esterification) but still you may try the mol sieve.

I think you don't see the protons in your sample because of chemical exchange with water (from your sample or the solvent) --> acetone or MeCN may be better.

It may be possible to examine the acidic OH and NH2 protons by 2D-spectra (COSY/TOCSY) as exchange signals with the water signal.

The chemical shift of the water signal can also be adjusted by the use of some µL of TFA if your sample is not sensitive to acidic conditions.

There is no peak of OH groups, because water molecules form hydrogen bonds with them.

In all likelyhood, Your d6-DMSO is wet and maybe even Your synthesised samples are maybe wet too. All this water amounts to some water not tightly hydrogen bonded to DMSO and in exchange with the labile R-H's in Your compounds, with enough water You get fast exchange and only one (apparently big) peak soon the NMR-time scale. If Your problems was only caused by hydrogen bonding to Our labile R-H's Your signals would shift to low field/high ppm value as the electron density on the R-H's would be reduced.

You should dry Your samples a lot longer, reduce the water content of Your DMSO with some kind of molecular sieve or some other benign drying raging (check chemicals supplier catalogue, e.g. sigma). Be warned, that it is VERY difficult to completely dry and KEEP dry a hygroscopic solvent like DMSO Go for dry enough. If You really need it dry for some reason, keep under Argon.

Hope this helps

To get any solvent dry (absolute water free) and keep it so, the best bet is to distill it first and then add a drying agent to the distillate for storage. Admittedly, distilling minute amounts of NMR solvents such as DMSO is a pain. So, to remove water traces in such instances I would recommend adding a few beads of Molecular Sieve 3Å (it has to be 3 Ångstrom). 

Wolfram, good specific advice about the molecular sieves.

Have you obtained a proton nmr of just your DMSO-d6 solvent? Can you see the water peak?  Assuming you can see the water peak, you can get as estimate of water content from the relative integration of the water and DMSO-d5 peaks using the specified deuterium content of the DMSO. Try drying the solvent and doing the experiment again to see if there is any reduction the water peak.

Really dry, acid free DMSO-d6 should slow the exchange process enough to see the OH signals in many cases.

Dear Farzad,

we reproducibly make the following observation for carbohydrates dissolved in DMSO:

When we use d6-DMSO from ampulla (highest purity & dryness) we only see a broad hump of the OH'es.

When we use bottle ware d6-DMSO (lower purity, not quite as dry as the ampulla ware) we have sharp signals.

We speculate that this counter intuitive result is explained by a difference in pH (affecting the exchange kinetics of the OH's with spurious water or between different OH'es on the carbohydrate!) between DMSO species.

A pH difference was actually measured by dissolving small aliquots of ampulla and bottle-ware DMSO in water followed by the measurements of pH's.

You may try lower-grade d6-DMSO to improve on your results. Alternatively you can acidify your DMSO (we normally spike aliquots of TFA)!

Alfred

Hello Alfred

the pH difference is an interesting obeservation and acidification one possible Workaround. Could you identify the cation responsible for the pH shift in the DMSO ? pH alone might not be the only reason for broad OH signals as many carbohydrates have been isolated from acidic chromatography and still have fast exchanging OH protons, as long as there is residual water in the solution. mol sieve for water removal works well, however not in all cases -

herbert

Dear Herbert,

no idea on the cation...but definitely nothing carrying a observable hydrogen....:-). We also see the broadening/not broadening effect if the concentration of the carbohydrate is higher than spurious water....

Cheers

Alfred