Jadeizaguirre

A sample 19F NMR spectrum of a simple organic compound. Integrations are shown under each peak.

19F NMR spectrum of 1-bromo-3,4,5-trifluorobenzene. The expansion shows the spin-spin coupling pattern arising from the para-fluorine coupling to the 2 meta-fluorine and 2 ortho proton nuclei.

Fluorine-19 nuclear magnetic resonance is an analytical technique used to identify fluorine-containing compounds. ¹⁹F is one of the most important nuclei for NMR spectroscopy.

Operational details

¹⁹F has a nuclear spin of ½ and a high magnetogyric ratio, which means that this isotope is highly responsive to NMR measurements. Furthermore, ¹⁹F comprises 100% of naturally occurring fluorine. The only other highly sensitive NMR-active nuclei spin ½ that are monoisotopic (or nearly so) are ¹H and ³¹P.

Because of its favorable nuclear properties and high abundance, ¹⁹F NMR measurements are very fast, comparable with ¹H NMR spectroscopy. Indeed, the ¹⁹F nucleus is the third most receptive NMR nucleus, after the ³H nucleus and ¹H nucleus.

The ¹⁹F NMR chemical shift range is very wide, ranging from ca. 550 to -250 ppm, however the most commonly encountered signals arising from organofluorine compounds lie between ca. -50 to -70 ppm (for CF₃ groups) to -200 to -220 ppm (for CH₂F groups). The very wide spectral range can cause problems in recording spectra, such as poor data resolution and inaccurate integration.

The reference compound for ¹⁹F is CFCl₃, although in the past a number of other compounds have been used, including CF₃COOH (-76 ppm w.r.t. CFCl₃) and C₆F₆ (-163 ppm w.r.t CFCl₃). It is also possible to record decoupled ¹⁹F{¹H} and ¹H{¹⁹F} spectra and multiple bond correlations ¹⁹F-¹³C HMBC and through space HOESY spectra.

Applications

Most commonly ¹⁹F NMR spectroscopy is used to analyze the structure of organofluorine compounds. Representative targets of this method are the many pharmaceuticals that contain C-F bonds. The technique is also used to analyze fluoride salts.