Is hyperconjugation responsible for the "gauche effect" in 1-fluoropropaneand other 2-substituted-1-fluoroethanes?

The energies and geometries of a series of 2-substituted-1-fluoroethanes we re computed at the MP2/6-311++G**(6D)//MP2/6-31+G* level of theory for both the maxima and minima of the rotation about the C-C bond. The results did not support the predictions of a hyperconjugative model, that both 1,2-difl uoroethane and 1-chloro-2-fluoroethane would strongly prefer a gauche confo rmation, and that 1-fluoro-2-silylethane would strongly prefer an anti conf ormation. The existence of competing electrostatic interactions between the fluorine and the substituents at C-2 was indicated by the detailed geometr ies of the gauche conformers and by the calculated sensitivity of the gauch e-anti energy differences to the presence of a polar solvent. However, Four ier analyses of the torsional potential energies were wholly consistent wit h hyperconjugative electron donation into the C-F sigma* orbital contributi ng to the conformational preferences of these 1-fluoroethanes. Fourier anal yses also showed that hyperconjugation contributes to the small variations in C-C and C-F bond lengths and in fluorine atomic charges that were comput ed. The torsional potential energies, variations in geometry and atomic cha rge, and sensitivity to solvent were all in accord with the expected rankin g of hyperconjugative electron donating ability of bonds to carbon, C-Si > C-H > C-C > C-Cl > C-F.