Chemical Shift Reagents High-Field Spectra

The observed chemical shifts, however, depend not only on the structure of the molecule being studied, but also on the interactions between the molecule and the surrounding solvent molecules. If the solvent consists of nonpolar molecules, such as hydrocarbons, there is only weak interaction between solute and solvent (van der Waals interactions or London forces), and the solvent has only a minimal effect on the observed chemical shift. 

If the solvent that is selected is polar (e.g., acetone, chloroform, or dimethyl sulfoxide), there are stronger dipole interactions between solvent and solute, especially if the solute molecule also contains polar bonds. The interactions between the polar solvent and a polar solute are likely to be stronger than the interactions between the solvent and tetramethylsilane (TMS, which is nonpolar), and the result is that the observed chemical shift of the molecule of interest will be shifted with respect to the observed chemical shift in a nonpolar solvent. The magnitude of this solvent-induced shift can be on the order of several tenths of a part per million in a proton spectrum. 

The chemist can use these solvent-induced shifts to clarify complex spectra that feature overlapping multiplets. By adding a small amount (up to 20%) of a benzene-dg or pyridine-dg to the CDCI3 solution of an unknown, a dramatic effect on the appearance of the spectrum can be observed. The chemical shifts of peaks in the proton spectrum can be shifted by as much as 1 ppm, with the result that overlapping multiplets may be separated from one another sufficiently to allow them to be analyzed. The use of this benzene trick is an easy way to simplify a crowded spectrum. 

8 CHEMICAL SHIFT REAGENTS HIGH-FIELD SPECTRA 

Occasionally the 60-MHz spectrum of an organic compound, or a portion of it, is almost undecipherable because the chemical shifts of several groups of protons are aU very similar. In such a case, all of the proton resonances occur in the same area of the spectrum, and often peaks overlap so extensively that individual peaks and sphttings cannot be extracted. One of the ways in which such a situation can be simplified is by the use of a spectrometer that operates at a frequency higher than 60 MHz. Two common frequencies in use for this purpose in modem instruments are 300 and 400 MHz. 

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Chemical Shift Reagents High-Field Spectra 323... 

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