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Tuesday, October 2, 2007

Improve Your Chemical Shift Resolution Without Going to Higher Fields

Deuterated chloroform is a very inexpensive solvent suitable for many organic compounds, however, the proton spectra of compounds dissolved in CDCl3 often have overlapping peaks complicating the interpretation of the data. One solution is to use a higher magnetic field strength but, what if you are already using the highest available field? One method used a great deal in the past seems to have been forgotten - the use of an aromatic solvent like benzene or pyridine. These solvents induce shifts in the resonances of the solutes often providing dramatically better resolution. Below is an example of the 500 MHz spectrum of tropolone. This compound undergoes fast exchange between the two tautomers indicated in the figure. The bottom trace is the spectrum in chloroform-d and the top trace is spectrum of the same sample in benzene-d6. The exchangable proton is broad and not shown in the figure. The improvement in resolution in the top trace compared to the bottom trace is remarkable.

The next time you are fighting a resolution problem, try using benzene-d6 as your solvent!

5 comments:

Anonymous said...

Dear Glenn,

Why does chloroform make the signals very overlapped?
Is it because of polarity?
Thank you very much...

Michèle

Glenn Facey said...

Michele,

I don't think its chloroform that makes the signals overlap. It is just that the chemical shifts of the solute protons are dependant on a solvent-solute contribution to the shielding. The aromatic ring currents of an aromatic solvent make it very different than a solvent like chloroform and therefore the chemical shifts can be quite different in benzene compared to chloroform.

Glenn

Anonymous said...

Thank you, Glenn, and so for this very useful blog!

Corey R. Johnson said...

I have synthesized a compound with the overlapping problem, but it is not soluble in benzene. However, it is soluble in pyridine. Would pyridine afford similar results?

Glenn Facey said...

Hi Corey,

It is the magnetic fields generated by aromatic rings in the benzene solvent that drive the separation of the solute signals. You may have some luck with pyridine as a solvent.

Glenn