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!