The NMR spectra and rotational isomerism of some 1,2-disuhstituted-tetrafluoroethanes

Abstract

The AA′BB′ fluorine magnetic resonance spectra of three nonsymmetrical 1,2disubstituted-tetrafluoroethanes, ICF 2CF 2Br, ICF 2CF 2C1, and BrCF 2CF 2Cl, have been analysed. It is shown that under certain conditions of solvent and temperature the deceptive simplicity of the spectra displayed by the-CF 2CF 2-fragment of these molecules can be lifted. All geminal and vicinal F-F coupling constants could be extracted. Using energy differences AE between trans and gauche isomers of these molecules determined by ir, approximate coupling constants of the single isomers were derived. Variable temperature studies were performed; it was not possible to obtain separate spectra from the different rotational isomers. Consideration of the results obtained from the above 1,2-disubstituted-tetrafluoroethanes in relation to some other data of fluoroethanes reported in the literature and in our hands enables us to attempt some rationalization and correlations from the trends. The spectral parameters of the individual isomers of CFCl 2CFCII obtained at low temperature are also reported and compared with those of other fluoroethanes known.