3934-26-7Relevant academic research and scientific papers
Structural assignment of fluorocyclobutenes by19F NMR spectroscopy – Comparison of calculated19F NMR shielding constants with experimental19F NMR shifts
Ku?nirová, Kate?ina,?im?nek, Ond?ej,Rybá?ková, Markéta,Kví?ala, Jaroslav
, p. 3867 - 3874 (2018/10/20)
Although the optimized reduction of perfluorocyclo-butene 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 of single-reference computations of the19F 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 computed19F 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 the19F NMR shifts of unknown fluorocyclobutenes.
Bimolecular kinetic studies with high-temperature gas-phase 19F NMR: Cycloaddition reactions of fluoroolefins
Shtarov,Krusic,Smart,Dolbier Jr.
, p. 9956 - 9962 (2007/10/03)
A gas-phase NMR kinetic technique has been used for the first time to obtain accurate measurements of rate constants of some bimolecular, second-order cycloaddition reactions. As a test of the potential use of this technique for the study of second-order reactions, the rate constants and the activation parameters for the cyclodimerization reactions of chlorotrifluoroethylene (CTFE) and tetrafluoroethylene (TFE) were determined in the temperature range 240-340 °C, using a commercial high-temperature NMR probe. Obtaining excellent agreement of the results with published data, the technique was then applied to the reaction of 1,1-difluoroallene with 1,3-butadiene, the results of which indicate that the use of gas-phase NMR for reaction kinetics is particularly valuable when a reagent is available only in small amounts and in cases where there are several competing processes occurring simultaneously. The major processes observed in this reaction are regioselective [2+2] and [2+4] cycloadditions, whose rates and activation parameters were determined [k2=9.3×106 exp(-20.1 kcal mol-1/RT) L/mol-1 S-1 and k3=1.2×106 exp(- 18.4 kcal mol-1/RT) L/mol-1 S-1, respectively] in the temperature range 130-210 °C.
