Structural assignment of fluorocyclobutenes by 19f nmr spectroscopy – comparison of calculated 19f nmr shielding constants with experimental 19f nmr shifts

Abstract

Although the optimized reduction of perfluorocyclobutene with LiAlH4 gave a quantitative yield of the target 3,3,4,4-tetrafluorocyclobut-1-ene, unoptimized reductions led to complex inseparable mixtures of fluorocyclobutenes. These mixtures showed highly complex19F NMR spectra, the assignment of which was quite tedious. Hence, we accomplished a series of19F NMR magnetic shielding computations. Suprisingly, various DFT approaches, including both traditional and advanced functionals, gave highly diverse results with poor correlations between the experimental and computed19F chemical shifts, and the individual fluorocyclobutenes could not be identified. In contrast, the Hartree–Fock (HF) method gave shielding values which, although bearing some systematic error, enabled the assignment of all observed structures. Even better results were obtained by using a specially tailored IGLO-III basis set. The method developed was successfully employed for the assignment of the19F NMR shifts of unknown fluorocyclobutenes.