Conformational Studies in the Cyclohexane Series. 3. The Dihalocyclohexanes

Abstract

The energies of axial and equatorial fluoro-, chloro-, and bromocyclohexanes as well as diaxial, axial−equatorial, and diequatorial 1,2-, 1,3-, and 1,4-dihalocyclohexanes were calculated using the hybrid density functional methods B3LYP and B3P86 as well as MP2 and QCISD and the 6-311G* and 6-311+G(2df,p) basis sets. The best agreement with experimental data was found with QCISD/6-311+G(2df,p). Solvent effects on the relative energies were calculated using the SCIPCM reaction field model. The effect of the halo substituents on the geometry of the cyclohexane ring was examined, and it was found that the effect of an axial substituent was local flattening of the cyclohexane ring, but no effect was found at the remote ring carbons, and no evidence of 1,3-diaxial interactions between the halogen and the axial ring hydrogens was found. In the case of the 1,2-dihalides, the calculations reproduce the preference for the diaxial form with X = Cl, but also predict that the energy difference between diaxial and diequatorial will be quite small when X = F. This appears to be related to the preference for the gauche form of 1,2-difluoroethane. The calculated relative energies of the 1,4-dihalocyclohexanes are in good agreement with electron diffraction data. A calculation of the electrostatic effects in the 1,4-dichlorocyclohexane reproduced the observed preference for the aa conformer.