Quantitative determination of mebeverine HCl by NMR chemical shift migration

Quantitative 1H NMR spectroscopic methods are not frequently reported, but current NMR instrumentation allows ready access to such data. Mebeverine HCl is an attractive molecule for NMR spectroscopy teaching purposes as it possesses a variety of simple but significant functional groups; we fully assign its 1H and 13C NMR spectra. Using mebeverine HCl, we show that concentration changes, in water as a solvent, can lead to significant changes in the 1H chemical shifts of non-exchangeable aromatic protons and to a lesser extent to aromatic methoxy protons. An important observation is that different protons migrate to different extents as the concentration of the solute is varied, and thus the 1H NMR spectra are concentration-dependent across a useful range. This chemical shift variation of selected protons was therefore analyzed and applied in the quantitative determination of mebeverine HCl in a medicine (Colofac IBS) formulated as a tablet. Self-association phenomena in water could account for these observed chemical shift migration effects as shown by determining the hydrodynamic radius from the modified form of the Stokes–Einstein equation, and thence the apparent hydrodynamic volume, VH, for mebeverine HCl in D2O solution which is 10-fold greater than that seen in either CDCl3 or CD3OD.