Diffusion-Ordered Spectroscopy (DOSY)
Diffusion-Ordered SpectroscopY (DOSY) is a technique in which a mixture of signals from a mixture of compounds can be separated on the basis of their diffusion coefficients. Diffusion coefficients vary according to intrinsic factors such as the size and shape of the molecule, as well as extrinsic factors such as the temperature and viscosity of the solution. In general, large, long molecules tend to have the smallest diffusion coefficients, while small, round molecules have the largest.
In this experiment, a mixture of 4-fluorophenol and clarithromycin is examined. First, a regular 1D 1H NMR is acquired. The resonances in this example are non-overlapping (by design, for clarity) with the aromatic resonances between 6.5-7.0 ppm and all the clarithromycin resonances between 0.8-5.2 ppm. The residual solvent peak (CHCl3) is also visible at 7.27 ppm.
The 2D DOSY experiment generates a 2D spectrum in which one axis is the chemical shift, and the other axis is the diffusion coefficient. 2D peaks indicate the diffusion coefficient of that particular signal.
In this case, the difference between the clarithromycin and 4-fluorophenol diffusion coefficients is large, and a small difference can be seen between 4-fluorophenol and chloroform.
Diffusion coefficients can also be generated from the magnitude data in the F1 direction, fitting to a nonlinear curve. This generates a diffusion coefficient for each signal.
Chloroform (7.27 ppm):
4-fluorophenol (6.5-7.0 ppm):
Clarithromycin (0.8-5.2 ppm, 4 representative curves shown):
DOSY provides a convenient way to separate complex mixtures of signals, as well as a method for watching non-covalent interactions in solution (as two interacting molecules will have a different diffusion coefficient than either molecule alone).
If you're interested in DOSY and how it can help you with complex 1H NMR spectra, contact Emeryville Pharmaceutical Services.