N.M.R. studies of¹⁹F chemical shifts and coupling constants in cyclobutane derivatives

The 19F spectra of several substituted trifluorochlorocyclobutanes which commonly occur as mixtures of two cis-trans isomers have been investigated. Substituents of different bulkiness have been introduced to shift the conformational equilibrium of the puckering motion of the cyclobutane ring system. A complete assignment of the resonances of the two isomers was possible, based on chemical shifts and H-F coupling constants. Vicinal F-F coupling constants have been investigated for a possible dependence on the dihedral angle ϕ. It is found that the coupling constants change sign twice for 0 ⩽ ğf ⩽ 1>80°, being negative for small and large angles and positive for intermediate angles. The investigations were based on fluorine spectra with completely eliminated H-F couplings by means of noise decoupling.