Cutting Edge NMR: New Tools for Analyzing Mixtures of Fluorinated Species

Incorporating fluorine into pharmaceutical candidates has typically led to improved medicinal properties, increasing the number of fluorinated active ingredients on the market. This increase requires that robust analytical tools be developed that can identify and characterize such compounds, particularly in complex mixtures such as those formed by reactions, contamination, and degradation. Proton NMR spectroscopy is well suited to the task of providing structural information; however, spectral overlap often limits the information quality for complex mixtures. In this work, we exploit the chemical shift, relaxation, and high sensitivity properties of ¹⁹F nuclei to extract individual ¹H NMR subspectra of components in mixtures without the need for laborious and time-consuming physical separation. In challenging cases, multivariate analysis is used to extract simplified ¹H NMR subspectra. The power of the new heteronuclear relaxation-encoded selective TOCSY (het-REST) and heteronuclear SCALPEL (het-SCALPEL) methods is demonstrated in mixtures of fluorine-containing corticosteroids.