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 complex 19F NMR spectra, the assignment of which was quite tedious. Hence, we accomplished a series of single‐reference computations of the 19F 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 19F 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 19F NMR shifts of unknown fluorocyclobutenes.