The effect of solvents on the chemical shift of fluorine nuclei in aromatic compounds

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Abstract
The magnitudes of the contributions to the magnetic shielding constant of a fluorine nucleus are discussed which are important when chemical shifts are obtained by recording the spectra of liquid samples. The chemical shifts between CFCl3, C6F12, C6F4Cl2 and C6F6 in various solvents have been measured, and it is concluded that an internal reference compound for fluorine should be of the same chemical type, and similar in size and shape to the molecules under investigation. The variations with solvent of the chemical shifts measured relative to C6F6 as an internal reference of the polar molecules pentafluoropyridine, 4-nitrotetrafluoropyridine, pentafluorobenzene, chloropentafluorobenzene and nitropentafluorobenzene have been measured and discussed in terms of the reaction fields produced by interaction of the polar molecules with the surrounding medium. The results have been shown to be consistent with the equation δ = - AEz , and values of the constant A have been obtained. The data of Taft et al. [7–9] on the effect of solvents on the chemical shifts of 19F nuclei in para and meta substituted fluorobenzenes relative to fluorobenzene has been re-examined and interpreted as arising predominantly from the reaction fields produced. It is concluded that only in the case of the fluoropyridines as solutes and alkylbenzenes as solvents is their evidence of the effect of complex formation on fluorine chemical shifts.