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

Although the optimized reduction of perfluorocyclobutene with LiAlH 4 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 single‐reference computations of the 19 F NMR magnetic shieldings of the corresponding fluorine atoms. Surprisingly, 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 domain‐based local pair natural orbital coupled clusters (DLPNO‐CCSD) method, developed recently as a part of the ORCA computational package, gave shielding values that enabled the assignment of all structures observed, albeit with some systematic errors. Slightly better magnetic shielding values were obtained by a simple Hartree–Fock (HF) method with 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.