Hindered Rotation I. The Configuration of the Cyclohexane Molecule

A restriction of the free rotation about the carbon-carbon single bond will play an important role in the potential function of the cyclohexane molecule. Any force whose nature is such that it will produce a hindrance to the free rotation about the C–C bond must also give rise to different potential energies for the ``chair'' and ``tub'' forms of cyclohexane even if these forms are ``strainless'' in the Baeyer theory sense. According to the nature of the rotational restriction (stabilizing ``staggered'' or ``opposed'' configurations) either the ``chair,'' the ``tub,'' or even the hexagonally symmetrical form with a plane carbon ring could be the most stable form. From considerations of the obtained Raman spectra of cyclohexane, cyclohexane-d1, and cyclohexane-d12 it is shown that the only form present in measurable quantities is the form with a D6h symmetry, i.e., the form with the plane of the carbon ring as a reflection plane for the hydrogen atoms. The rotational restriction about the C–C single bond, therefore, is of such a nature that it stabilizes this D6h configuration, in which the H atoms are in ``opposed'' positions.