On the stereoelectronic effects governing the rotational isomerism of 1,2-di-haloethanes

The rotational isomerism of 1,2-di-haloethanes, namely 1,2-difluoro- (1), 1,2-dichloro- (2), 1,2-dibromo- (3) and 1,2-diiodo-ethane (4), was theoretically evaluated in this work. Our DFT calculations showed that the gauche conformer is prevalent in 1, whilst the trans form is largely favoured in the remaining compounds, at the isolated state. NBO analyses revealed that the fluorinated compound experiences the gauche effect, which is due to a CH/CF* hyperconjugation. Classical steric/electrostatic repulsion and hyperconjugative interactions are more competitive in 2, and the CX/CX* electron delocalization (X = Br and I) strongly favours the trans conformer in 3 and 4.