Origin of the Gauche Effect in substituted ethanes and ethenes

1,2-Disubstituted ethanes having strongly electronegative substituents prefer the gauche conformation over the usual trans form. This is not the result of an attractive interaction in the gauche form, but rather a destabilizing interaction in the trans rotamer. The interaction appears to be the result of the formation of bent bonds, and the lower stability of a twist bent bond over that of syn bent bond. The lower energy of cis-1,2-difluoroethene as compared to its trans form may be explained in the same way.