MOLECULAR-STRUCTURE AND INTERNAL ROTATIONS OF TETRACYCLOHEXYLMETHANE

The stereochemistry of tetracyclohexylmethane (2), obtained by catalyt ic hydrogenation of tetraphenylmethane, is analyzed. The Cahn-Ingold-P relog conformational descriptors of the ideal conformers of tetrasubst ituted methanes are reevaluated and an alternative method presented. M ethane 2 exists in the crystal in a (g/a/-g) conformation (of approxim ate Sq symmetry) in which all the rings are in a chair arrangement and are skewed from their perfectly staggered D(2)d conformation. Molecul ar mechanics (MM) calculations indicate that the crystallographic conf ormation is the lowest energy form. Analysis of the H-1 and C-13 NMR d ata indicates that, even at low temperatures, the molecule undergoes a fast rotational process which exchanges the ortho and meta groups. On the basis of the MM calculations it is concluded that the preferred r otational pathway involves libration of the four rings via a transitio n state of D(2)d symmetry in which all the rings are perfectly stagger ed.