Hindered internal rotation and the nuclear magnetic resonance parameters in 1,2-dibromo-1,1,2-trifluoroethane

Abstract

The ¹H and ¹⁹F n.m.r. parameters of CF₂BrCHFBr have been obtained in various solvents at 34°C and at 0°C. It is deduced that the rotational configuration in which the geminal fluorines are gauche to the hydrogen (I) and that in which the two bromine atoms are trans to each other (II) are of comparable stability in the inert solvents at 34°C and of far lower energy than the third isomer, but (II) is favoured in the basic solvents at the same temperature and in the inert solvents at 0°C. Hydrogen bonding to the solvent molecules has the effect of (1) inducing changes in the population of the rotamers in favour of the two more polar ones, (2) decreasing the geminal H–F coupling while increasing the geminal F–F coupling, and (3) decreasing the magnitude of the vicinal F–F coupling in the individual rotamers. The geminal H–F coupling decreases as the polarity of the rotamer increases while, in contrast, the geminal F–F coupling is the largest in the most polar rotamer. Increasing the dielectric constant of the solvent has the same effect as (2).