Fluorine detected 2D NMR experiments for the practical determination of size and sign of homonuclear F-F and heteronuclear C-F multiple bond J-coupling constants in multiple fluorinated compounds

Pure shift FESTA: An ultra-high resolution NMR tool for the analysis of complex fluorine-containing spin systems

NMR measurements of molecules containing sparse fluorine atoms are becoming increasingly common due to their prevalence in medicinal chemistry. However, the presence of both homonuclear and heteronuclear scalar couplings severely complicates their analysis by NMR. In complex systems, FESTA, a heteronuclear spectral editing method, allows simplified 1 H NMR spectra to be obtained containing only 1 H signals from the same spin system as a chosen 19 F. Despite spectral simplification, signal overlap due to the presence of scalar couplings is often a problem in FESTA spectra. Here, we report a new experiment that combines FESTA and pure shift methods to provide fully decoupled ultra-high resolution FESTA spectra showing a single signal for each 1 H chemical environment. The utility of the method is demonstrated for the analysis of two complex fluorine-containing mixtures of pharmaceutical and biochemical interest.

19F-centred NMR analysis of mono-fluorinated compounds

Addressing limitations of the existing NMR techniques for the structure determination of mono-fluorinated compounds, we have developed methodology that uses ¹⁹F as the focal point of this process. The proposed ¹⁹F-centred NMR analysis consists of a complementary set of broadband, phase-sensitive NMR experiments that utilise the substantial sensitivity of ¹⁹F and its far reaching couplings with ¹H and ¹³C to obtain a large number of NMR parameters. The assembled ¹H, ¹³C and ¹⁹F chemical shifts, values of J_HF, J_HH, and J_FC coupling constants and the size of ¹³C induced ¹⁹F isotopic shifts constitute a rich source of information that enables structure elucidation of fluorinated moieties and even complete structures of molecules. Here we introduce the methodology, provide a detailed description of each NMR experiment and illustrate their interpretation using 3-fluoro-3-deoxy-d-glucose. This novel approach performs particularly well in the structure elucidation of fluorinated compounds embedded in complex mixtures, eliminating the need for compound separation or use of standards to confirm the structures. It represents a major contribution towards the analysis of fluorinated agrochemicals and (radio)pharmaceuticals at any point during their lifetime, including preparation, use, biotransformation and biodegradation in the environment. The developed methodology can also assist with the investigations of the stability of fluoroorganics and their pharmacokinetics. Studies of reaction mechanisms using fluorinated molecules as convenient reporters of these processes, will also benefit.

¹⁹F is the focal point of broadband, phase-sensitive 2D NMR experiments that provide ¹H, ¹³C and ¹⁹F chemical shifts, values of J_HF, J_HH, and J_FC coupling constants and ¹³C-induced ¹⁹F isotopic shifts to elucidate structures of fluorinated molecules.

New 19F NMR methodology reveals structures of molecules in complex mixtures of fluorinated compounds

Although the number of natural fluorinated compounds is very small, fluorinated pharmaceuticals and agrochemicals are numerous. ¹⁹F NMR spectroscopy has a great potential for the structure elucidation of fluorinated organic molecules, starting with their production by chemical or chemoenzymatic reactions, through monitoring their structural integrity, to their biotic and abiotic transformation and ultimate degradation in the environment. Additionally, choosing to incorporate ¹⁹F into any organic molecule opens a convenient route to study reaction mechanisms and kinetics. Addressing limitations of the existing ¹⁹F NMR techniques, we have developed methodology that uses ¹⁹F as a powerful spectroscopic spy to study mixtures of fluorinated molecules. The proposed ¹⁹F-centred NMR analysis utilises the substantial resolution and sensitivity of ¹⁹F to obtain a large number of NMR parameters, which enable structure determination of fluorinated compounds without the need for their separation or the use of standards. Here we illustrate the ¹⁹F-centred structure determination process and demonstrate its power by successfully elucidating the structures of chloramination disinfectant by-products of a single mono-fluorinated phenolic compound, which would have been impossible otherwise. This novel NMR approach for the structure elucidation of molecules in complex mixtures represents a major contribution towards the analysis of chemical and biological processes involving fluorinated compounds.

¹⁹F-centred NMR structure determination protocol alleviates the need for compound separation. Disinfection byproducts of chloramination were unraveled by analyzing the reaction pathways of a single fluorinated molecule.

Intramolecular H-Bond Is Formed in 2-Fluorophenol and 2-Fluorothiophenol, but It May Not Be the Main Pathway of the JFH Coupling Constant Transmission

The intramolecular CF···HX (X = O or S) H-bond and J_FH spin-spin coupling constants (SSCCs) in 2-fluorophenol and 2-fluorothiophenol were investigated experimentally by ¹H and ¹⁹F NMR and theoretically in the framework of the natural bond orbital analysis. In contrast with recent findings from the literature, the results obtained in this work showed that an intramolecular H-bond is formed in the cis conformers and that it has an electrostatic origin. Such an intramolecular electrostatic H-bond is the interaction that rules the conformational preferences on these compounds. Moreover, the natural J-coupling analysis indicated that the J_HF SSCC in 2-fluorophenol has its origin on LP(F)/σ_OH steric interactions and should be labeled as ^4TSJ_HF. On the other hand, the analogue SSCC for 2-fluorothiophenol has its origin on the intramolecular H-bond LP(F) → σ_SH^* hyperconjugative interaction and should be described as a ^1hJ_FH SSCC.

Ultra high-resolution HSQC: application to the efficient and accurate measurement of heteronuclear coupling constants

A rapid acquisition strategy in terms of enhanced resolution per time unit for the simple and efficient determination of multiple coupling constants is proposed. The use of ¹³C spectral aliasing combined with pure shift NMR techniques allows accurate measurements of the magnitude and the sign from ultra high resolved 2D cross-peaks.