Fluorine-19 nuclear magnetic resonance spectroscopy
19F is an important nucleus for NMR spectroscopy because of its receptivity and large chemical shift dispersion, which is greater than that for proton nuclear magnetic resonance spectroscopy.
The 19F NMR chemical shifts span a range of about 800 ppm.
The very wide spectral range can cause problems in recording spectra, such as poor data resolution and inaccurate integration.
19F NMR chemical shifts in the literature vary strongly, commonly by over 1 ppm, even within the same solvent.
[5] An investigation of the factors influencing the chemical shift in fluorine NMR spectroscopy revealed the solvent to have the largest effect (Δδ = ±2 ppm or more).
[5] Example of chemical shifts determined against neat CFCl3:[5] For a complete list the reference compounds chemical shifts in 11 deuterated solvents the reader is referred to the cited literature.
Specifically, 19F NMR shifts are strongly affected by contributions from electronic excited states whereas 1H NMR shifts are dominated by diamagnetic contributions.
[8] For vinylic fluorine substituents, the following formula allows estimation of 19F chemical shfits:
[9] Some representative values for use in this equation are provided in the table below:[10] When determining the 19F chemical shifts of aromatic fluorine atoms, specifically phenyl fluorides, there is another equation that allows for an approximation.
where Z is the SSCS value for a substituent in a given position relative to the fluorine atom.
Other sources and data tables can be consulted for a more comprehensive list of trends in 19F chemical shifts.
Something to note is that, historically, most literature sources switched the convention of using negatives.
Other nuclei can couple with fluorine, however, this can be prevented by running decoupled experiments.
It is common to run fluorine NMRs with both carbon and proton decoupled.
Application include pH-, temperature-, enzyme-, metal ion- and redox responsive- contrast agents.
