TY - JOUR
T1 - Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion
AU - Goodman, Lionel
AU - Gu, Hongbing
AU - Pophristic, Vojislava
PY - 2005/2/17
Y1 - 2005/2/17
N2 - Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (∼25% of the anti interaction). The established significantly >60° FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60° and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72° equilibrium conformer angle. The relative minimum energy for the trans conformer is the consequence of a balance between decreasing hyperconjugative stabilization and decreasing steric destabilization as the FCCF torsional angle approaches 180°. The torsional coordinate is predicted to be strongly contaminated by CCF bending, with the result that approximately half of the trans → gauche stabilization energy stems from mode coupling.
AB - Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (∼25% of the anti interaction). The established significantly >60° FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60° and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72° equilibrium conformer angle. The relative minimum energy for the trans conformer is the consequence of a balance between decreasing hyperconjugative stabilization and decreasing steric destabilization as the FCCF torsional angle approaches 180°. The torsional coordinate is predicted to be strongly contaminated by CCF bending, with the result that approximately half of the trans → gauche stabilization energy stems from mode coupling.
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U2 - https://doi.org/10.1021/jp046290d
DO - https://doi.org/10.1021/jp046290d
M3 - Article
SN - 1089-5639
VL - 109
SP - 1223
EP - 1229
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 6
ER -