Trans-1,2-difluoroethane

Sigma lone pair nonbonded interaction in 1,2-difluorocyclopropane presents a similar situation to that observed in 1,2-difluoroethane (see Pattern b Scheme 1). Specifically, sigma nonbonded attraction may favor the trans isomer. This can be seen in the results of CNDO/2 calculations shown above where it appears that the sigma nonbonded interaction is repulsive in the cis isomer. 

Friesen, D. and Hedberg, K. (1980) Conformational analysis. 7. 1,2-Difluoroethane. An electron-diffraction investigation of the molecular structure, composition, trans-gauche energy and entropy differences, and potential hindering internal rotation. J. Am. Chem. Soc., 102, 3987-3994. 

Figure 4.18 1,2-Difluoroethane energetically prefers the gouche conformation above) by 0.5-0.9 kcal moF, because this enables stabilizing overlap of the electron-rich carbon-hydrogen a bond and the antibonding rC orbital of a neighboring carbon-fluorine bond (box). In 1,2-di-fluoroethylene (betoiv) the cis isomer is preferred to the trans isomer by 1-2 kcal mol. ...

Figure 2.1 Preferred conformations of butane and 1,2-difluoroethane, illustrating the gauche effect. In the gauche conformation each C-F a orbital can overlap with a C-H a orbital on the vicinal carbon, corresponding to no-bond resonance as shown. Such no-bond resonance would be disfavoured if the C-F bonds were trans, since it would remove electron density from an electronegative element. The effect is strong enough in this case to overcome the electrostatic repulsion between the two C-F dipoles, which favours the trans form.

Ammonia Butane Carbon tetrachloride 1,1,1-Chlorodifluoroethane Chlorodifluoromethane Chlorofluoromethane Chlorotetrafluoroethane 2-Chloro-1,1,1,2-tetrafluoroethane sym-Dibromotetrafluoroethane Dichlorodifluoromethane cis-1,2-Dichloroethylene trans-1,2-Dichloroethylene Dichlorofluoroethane Dichlorofluoromethane Dichlorotetrafluoroethane 1,2-Dichlorotetrafluoroethane Dichlorotrifluoroethane 1,1-Difluoroethane Difluoromethane Dimethyl ether Ethane Ethyl bromide Ethyl chloride ... 

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