Fluorine-19 nuclear magnetic resonance

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.[1]

Operational details

19F has a nuclear spin of 1/2 and a high magnetogyric ratio, which means that this isotope is highly responsive to NMR measurements. Furthermore, 19F comprises 100% of naturally-occurring fluorine. Other NMR-active nuclei spin 1/2 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,[2] 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 mainly 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.[3]

References

  1. ↑ H. Friebolin "Basic One- and Two-Dimensional NMR Spectroscopy", Wiley-VCH, Weinheim, 2011. ISBN 978-3-527-32782-9
  2. ↑ Roy Hoffman (2007). "19Fluorine NMR". Hebrew University.
  3. ↑ Gerken, M.; Boatz, J. A.; Kornath, A.; Haiges, R.; Schneider, S.; Schroer, T.; Christe, K. O. "The 19F NMR shifts are not a measure for the nakedness of the fluoride anion" Journal of Fluorine Chemistry (2002), 116(1), 49-58. doi:10.1016/S0022-1139(02)00101-X


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