Molecular Structure and Rotational Isomerism of Some Fluorochloroethanes

In order to investigate the molecular structure and rotational isomerism, the electron diffraction camera having a rotating sector was designed and constructed and its application to some fluorochloroethanes was made successfully. The shortening of the C–F and the C–Cl bond by the substitution of atoms attached to one and the same carbon atom with other fluorine atoms was also revealed in fluorochloroethanes as well as fluorochloromethanes, although in the case of the C–Cl bond distance the inaccuracy of the data does not permit a definite conclusion to be drawn. It was definitly confirmed that there exist two rotational isomers, the trans (or Cs) and the gauche forms, in the vapor state for the molecules in question. The energy differences between the isomers and the azimuthal angles of the gauche form were determined and further discussed from the standpoint of intramolecular potential. It was found that three fluorochloroethanes in question have almost equal and small energy differences in contrast to the remarkable variation in the case of chloroethanes. This is caused by the fluorine substitution which produces the decrease of the electrostatic part in the energy difference between the isomers.