Fluorine-19 nuclear magnetic resonance spectroscopy

Fluorine-19 nuclear magnetic resonance spectroscopy (fluorine NMR or ¹⁹F NMR) is an analytical technique used to detect and identify fluorine-containing compounds. 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.

Operational details

¹⁹F has a nuclear spin (I) of and a high gyromagnetic ratio. Consequently, this isotope is highly responsive to NMR measurements. Furthermore, ¹⁹F comprises 100% of naturally occurring fluorine. The only other highly sensitive spin NMR-active nuclei that are monoisotopic (or nearly so) are ¹H and ³¹P. Indeed, the ¹⁹F nucleus is the third most receptive NMR nucleus, after the ³H nucleus and ¹H nucleus.

The ¹⁹F NMR chemical shifts span a range of about 800 ppm. For organofluorine compounds the range is narrower, being about −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.

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.

Chemical shifts

¹⁹F NMR chemical shifts in the literature vary strongly, commonly by over 1 ppm, even within the same solvent. Although the reference compound for ¹⁹F NMR spectroscopy, neat CFCl₃ (0 ppm), has been used since the 1950s, clear instructions on how to measure and deploy it in routine measurements were not present until recently. 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). A solvent-specific reference table with 5 internal reference compounds has been prepared (CFCl₃, C₆H₅F, PhCF₃, C₆F₆ and CF₃CO₂H) to allow reproducible referencing with an accuracy of Δδ = ±30 ppb. As the chemical shift of CFCl₃ is also affected by the solvent, care must be taken when using dissolved CFCl₃ as reference compound with regards to the chemical shift of neat CFCl₃ (0 ppm). Example of chemical shifts determined against neat CFCl₃:

For a complete list the reference compounds chemical shifts in 11 deuterated solvents the reader is referred to the cited literature.

A concise list of appropriately referenced chemical shifts of over 240 fluorinated chemicals has also been recently provided.

Chemical shift prediction

¹⁹F NMR chemical shifts are more difficult to predict than ¹H NMR shifts. Specifically, ¹⁹F NMR shifts are strongly affected by contributions from electronic excited states whereas ¹H NMR shifts are dominated by diamagnetic contributions.

Fluoromethyl compounds

Fluoroalkenes

For vinylic fluorine substituents, the following formula allows estimation of ¹⁹F chemical shifts:

where Z is the statistical substituent chemical shift (SSCS) for the substituent in the listed position, and S is the interaction factor. Some representative values for use in this equation are provided in the table below:

Fluorobenzenes

When determining the ¹⁹F chemical shifts of aromatic fluorine atoms, specifically phenyl fluorides, there is another equation that allows for an approximation. Adopted from "Structure Determination of Organic Compounds," this equation is where Z is the SSCS value for a substituent in a given position relative to the fluorine atom. Some representative values for use in this equation are provided in the table below:

The data shown above are only representative of some trends and molecules. 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. Therefore, be wary of the sign of values reported in other sources.

Spin–spin coupling

¹⁹F-¹⁹F coupling constants are generally larger than ¹H-¹H coupling constants. Long range ¹⁹F-¹⁹F coupling, (²J, ³J, ⁴J or even ⁵J) are commonly observed. Generally, the longer range the coupling, the smaller the value. Hydrogen couples with fluorine, which is very typical to see in ¹⁹F spectrum. With a geminal hydrogen, the coupling constants can be as large as 50 Hz. 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. Fluorine atoms can also couple with each other. Between fluorine atoms, homonuclear coupling constants are much larger than with hydrogen atoms. Geminal fluorines usually have a J-value of 250-300 Hz. There are many good references for coupling constant values. The citations are included below.

Biochemical and medical applications

Fluorine-19 has often been employed to examine the structure and dynamics of fluorine-labeled proteins. The following fluorinated amino acids have been incorporated into proteins: 3- and 4-fluorophenylalanine, 6- and 5-fluorotryptophan and 3-fluorotyrosine, 5-fluoroleucine, trifluoroethylglycine, trifluoro- and difluoromethionine, and 2-fluorohistidine. Substitution, mediated often ribosomally, is facilitated because H and F are similar in size, even though C-F bonds are somewhat longer.

In vivo magnetic resonance spectroscopy and imaging (¹⁹F MRS / MRI)

In vivo magnetic resonance spectroscopy is a technique that closely resembles the laboratory NMR, but the sample is contained within a living organism (in vivo). In turn, magnetic resonance imaging is more complex technique that can provide the spatial distribution of the study MR signal throughout the body. Most commonly, MRI acquires ¹H signal, but it can also acquire any other nuclide with non-zero spin number.

Natural fluorine is monoisotopic - its only non-radioactive nuclide (¹⁹F) has spin number 1/2 and a very high gyromagnetic ratio (ca. 93% of that of ¹H). Because of this, ¹⁹F MRS/MRI can be acquired relatively easily even with the same hardware as ¹H MRS/MRI.

The natural fluorine background in the body is negligible; most of the body's fluorine is found in teeth and bones, where it has very short T₂ relaxation times and so is virtually invisible to most ¹⁹F MRI/MRS techniques. For this reason, ¹⁹F MRI/MRS suffers from no background interference and can be used to monitor fluorinated xenobiotics (artificial compounds).

In vivo ¹⁹F MRI/MRS are not common techniques, but these methods can both quantify and locate the fluorinated drug, as well as differentiate its metabolic states. For example, it has been used to study in vivo pharmacokinetics and metabolism of 5-fluorouracil. The estimates of biological half-life of fleroxacin using in vivo ¹⁹F MRS (14.2 ± 2.4) were well in line with those determined with other methods. A fluorinated polymer poly(2,2-difluoroethyl)acrylamide showed a biological half-life ≈200 days using in vivo ¹⁹F MRS, in line with estimated provides with in vivo fluorescence imaging (150 ± 20 days). Analogously, ¹⁹F MRS/MRI has been used to track the in vivo degradation of fluorinated materials.

Notes

References