F-F coupling constants in dichlorotetrafluorocyclopropanes

Abstract

The ¹⁹F NMR spectra of the cis‐ (1) and the trans‐isomer (2) of the 1,2‐dichlorotetrafluorocyclopropane and that of the 1,1‐dichlorotetrafluorocyclopropane (3) have been investigated at different temperatures and in several solvents. magnified image From chemical shift calculations the two geminal fluorines in the cis‐isomer (1) could be assigned and on this basis the two vicinal coupling constants of 1, J_trans (ca. 140°) and J_cis (ca. 0°), were unequivocally distinguished. By frequency sweep double resonance J_trans has been shown to be of opposite sign to J_gem, whereas for J_cis the situation has been found to be reversed. Therefore J_trans is presumably negative and J_cis positive. Only the N(J_cis + J_trans) value could be extracted from the vicinal coupling constant in the fragment CFClCFCl could be evaluated. It has been noted that J_cis is more sensitive to changes in temperature than is J_trans. The variations of J_cis and J_trans induced by solvents are, on the contrary, small and irregular and no correlation with the dielectric constant of the medium has been noted. The different temperature dependence of J_cis and J_trans can be useful for assigning the vicinal FF coupling constants in cyclopropane derivatives and also for defining their signs. This method was applied to the coupling constants extracted from the ¹³C satellite spectrum of isomer 3. The coupling constants results were compared with some literature data already known, and some rationalisation and correlations from the trends was attempted.