Quantum chemical calculation of 19F NMR chemical shifts of trifluoromethyl diazirine photoproducts and precursors

Irradiation of aryl(trifluoromethyl)diazirines may result in a multitude of products, which are difficult to assign in the 19F NMR spectrum. In this article, it is demonstrated that an average accuracy of 2.9 ppm (standard deviation) can be achieved by quantum chemical calculations at the B3LYP 6-311G++(2d,2p) level of theory, followed by a Boltzmann weighting of the optimized conformers. A set of 30 compounds was investigated both experimentally and theoretically. 19F NMR chemical shifts of precursor Z-oximes and Z-tosyloximes ranged from δF −62.9 to −61.8 ppm, whereas their E counterparts showed shifts between δF −67.2 and −66.2 ppm. Stereochemical assignments were confirmed by two X-ray analyses. Quantum chemical calculation also allowed the assignment of the configuration of an (E,E) azine. Trifluoromethyl diazirines exhibited a δF between −66.1 and −65.6, diaziridines between −76.2 and −75.9 ppm. The smallest δF values were observed for α-oxygenated trifluoromethyl compounds (δF −78.9 to −77.4 ppm). The average deviation of the calculated from the experimental values corresponds to only about 1% of the standard 19F NMR chemical shift range.