ABINITIO CONFORMATIONAL-ANALYSIS OF CYCLOHEXANE

The inversion of cyclohexane has been studied by using ab initio molecular orbital theory. Six stationary points were found and characterized as minima or transition states. With a double-ζ basis augmented with a set of d-type polarization functions on each carbon, the geometries were optimized, and force constants were calculated. An MP-2 correlation correction was also evaluated with a much larger basis of triple-ζ plus polarization quality. The lowest energy structure is the chair conformation of D3d symmetry. The other minimum is found to be the twist-boat structure of D2 symmetry, which lies 6.9 kcal/mol higher in energy. A boat structure of C2v symmetry is a transition state, along a pseudorotational coordinate connecting two twist-boat structures. This transition-state structure lies 7.9 kcal/mol above the chair. The interconversion between chair structures via the twist boat is accomplished through either of two transition states, one of C2 symmetry, the other of C1 symmetry. Both of these transition states lie about 12 kcal/mol above the chair structure. Knowledge of the zero-point correction and the resulting entropies of activation yields values of ΔS‡ and ΔG‡ that are in good agreement with experimental values. Finally, the evaluation of the force constants at each of these transition-state structures has allowed us to analyze the torsional motion of the ring skeleton. © 1990 American Chemical Society.