text: When two fluorine atoms are in vicinal (i.e., adjacent) carbons, as in 1,2-difluoroethane (HFCCFH), the gauche conformer is more stable than the anti conformer—this is the opposite of what would normally be expected and to what is observed for most 1,2-disubstituted ethanes; this phenomenon is known as the gauche effect. In 1,2-difluoroethane, the gauche conformation is more stable than the anti conformation by 2.4 to 3.4 kJ/mole in the gas phase. This effect is not unique to the halogen fluorine, however; the gauche effect is also observed for 1,2-dimethoxyethane. A related effect is the alkene cis effect. For instance, the cis isomer of 1,2-difluoroethylene is more stable than the trans isomer.
  There are two main explanations for the gauche effect: hyperconjugation and bent bonds. In the hyperconjugation model, the donation of electron density from the carbon–hydrogen σ bonding orbital to the carbon–fluorine σ antibonding orbital is considered the source of stabilization in the gauche isomer. Due to the greater electronegativity of fluorine, the carbon–hydrogen σ orbital is a better electron donor than the carbon–fluorine σ orbital, while the carbon–fluorine σ orbital is a better electron acceptor than the carbon–hydrogen σ orbital. Only the gauche conformation allows good overlap between the better donor and the better acceptor.
  Key in the bent bond explanation of the gauche effect in difluoroethane is the increased p orbital character of both carbon–fluorine bonds due to the large electronegativity of fluorine.  As a result, electron density builds up above and below to the left and right of the central carbon–carbon bond.  The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond. Of these two models, hyperconjugation is generally considered the principal cause behind the gauche effect in difluoroethane.