Infra-red study of rotational isomerism in 1,2-diiodotetrafluoroethane

A theoretical study on 1,2-diiodotetrafluoroethane has been undertaken to calculate the conformational potential energy as a function of the internal rotation angle. The results have shown that one " trans" ( C_{2 h}) and two equivalent " gauche" ( C₂) forms are present with energy minima at 180°, 68° and 292° respectively. A scrutiny of the i.r. spectrum of this compound, as both a liquid and a solid, and the variation of the band intensities as a function of the temperature (40-140°C) in the vapour phase spectrum have led to the identification of the bands belonging to the more stable isomer and to the less stable one. The energy difference between the two forms in the vapour phase, as determined experimentally, is 1835 ± 100 cal/mole in good accordance with the theoretical value (1600 cal/mole).