A DFT study on the origin of the fluorine gauche effect in substituted fluoroethanes

DFT derived conformational energy profiles of a series of β-substituted α-fluoroethanes (F–CH2CH2–X) have been explored where the substituent X was varied as NH3+, OCOH, NCO, NO2, NHCHO, F, N3, CHdouble bond; length as m-dashNH, NCS, CHdouble bond; length as m-dashCdouble bond; length as m-dashCH2, CH3, CHdouble bond; length as m-dashCH2, NC, CN, CHO, and CCH. Comparisons were correlated relative to 1,2-difluoroethane, a compound which exhibits a well known gauche preference. Only four of the compounds displayed an anti preference, with the large majority preferring a gauche conformation. In particular the influence of steric and electrostatic attraction/repulsion between the fluorine atom and the X-substituent was explored by evaluating rotational energy profiles for all compounds and separately NBO correlations were evaluated to assess the contribution of hyperconjugation to the minimised gauche and anti conformers. In the event the gauche preference for 1,2-difluoroethane was shown to have an origin due largely to σ(C–H)→σ*(C–F) hyperconjugative interactions, whereas the conformational preference for the remaining structures is rationalised by hyperconjugative as well as steric and electrostatic contributions. The anti preferred compounds 13, 14 and 16 possessed triple bonds and the preference arose due to fluorine/p-orbital repulsion.