Temperature dependent conformers of perfluorocyclohexylethers

Abstract

Temperature dependent high resolution 19F NMR spectroscopic investigations of perfluorocyclohexylmethylether ( 1), perfluorocyclohexylethylether ( 2), and perfluorocyclohexyl-(2-N-morpholino)-ethylether ( 3) revealed in each case the occurrence of two conformers, one clearly prevalent at a lower temperature and the other at a higher one. The low-temperature conformers show nearly identical differences of the 19F chemical shifts between axial and equatorial F, which closely resemble the value for perfluorocyclohexane. Also, the 2 and 6 F atoms with a chemical shift difference of about 17.2 ppm are closely related to the F in perfluorohexane, indicating only a small influence of the OCF 3, OC 2F 5, and OC 2F 4N(C 2F 4) 2O groups on the ring F compared to R F, R FOR′ F, or N(R F) 2. The high-temperature conformers of 1, 2 and 3 show much smaller differences of 19F chemical shifts of corresponding axial and equatorial F atoms, and again the 2- and 6-positions behave in this respect like the 3-, 5-, and 4-positions. In the case of the ethers the oxygen atom acts as a ‘spacer’, separating the fluorinated ring from the fluorinated side chain groups.