Structural Assignment of Fluorocyclobutenes by ~(19)F NMR Spectroscopy - Comparison of Calculated ~(19)F NMR Shielding Constants with Experimental ~(19)F NMR Shifts

摘要

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 complex ~(19)F NMR spectra, the assignment of which was quite tedious. Hence, we accomplished a series of ~(19)F 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 computed ~(19)F 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 the ~(19)F NMR shifts of unknown fluorocyclobutenes.