Using NMR spectroscopy for the measurement of compounds containing fluorine

Fluorine is an important element in industrial chemistry and has applications in a wide range of industries, such as pharmaceuticals, agrochemicals, polymers, surfactants and solvents. It is estimated that more than 20% of all pharmaceutical compounds contain fluorine. This includes some of the commercially important drugs such as Prozac (fluoxetine) and Paxil (paroxetine).

Nuclear Magnetic Resonance (NMR) spectroscopy is a valuable technique for the measurement of compounds containing fluorine especially organofluorine compounds. After ¹H and ¹³C NMR, ¹⁹F is the most common nucleus studied by this technique. ¹⁹F nuclei are spin ½ nuclei and a have a high gyromagnetic ratio, which means that they have a high receptivity for NMR measurements. The ¹⁹F isotope has 100% natural abundance, giving high NMR sensitivity. The ¹⁹F resonance frequency on X-Pulse, a 1.45T magnet, is 56.76 MHz which is sufficiently close to the resonance frequency of ¹H such that ¹H and ¹⁹F spectra can be measured using the same probe.

A series of spectra have been collected to demonstrate the ¹⁹F and ¹H capability and performance of the instrument using the same probe.

1. Trifluorotoluene is a useful reference material for ¹⁹F NMR spectroscopy and can be used in a similar way to tetramethylsilane (TMS) for ¹H measurements.

2. A mixture of two fluorine containing chemicals; Trifluorotoluene and trifluoroethanol.

3. The compound 5-Bromo-1,2,3-trifluorobenzene.

4. A comparison has been made of the ¹⁹F spectra of two different positional isomers of bromotrifluorobenzene.

X-Pulse is a high performance benchtop NMR spectrometer that does not require external services, such as liquid cryogens or compressed air, and can be placed in the laboratory rather than in a specialised NMR facility. Spectra can be collected within a couple of minutes using standard 5mm NMR tubes.