Catenanes: A molecular mechanics analysis of the (C₁₃H₂₆)₂ Structure 13-13 D2

Molecular mechanics (MM4) studies have been carried out on the catenane (C₁₃H₂₆)₂, specifically 13-13D2. The structure obtained is in general agreement with second-order perturbation theory. More importantly, the MM4 structure allows a breakdown of the energy of the molecule into its component classical parts. This allows an understanding of why the structure is so distorted, in terms of CC bonding and nonbonding interactions, van der Waals repulsion, C C C and C C H angle bending, torsional energies, stretch-bend, torsion-stretch, and bend-torsion-bend interactions. Clearly, the hole in 113-membered ring is too small for the other ring to fit through comfortably. There are too many atoms trying to fit into the limited space at the same time, leading to large van der Waals repulsions. The rings distort in such a way as to enlarge this available space, and lower the total energy of the molecule. While the distortions are spread around the rings, one of the nominally tetrahedral C C C bond angles in each ring is opened to 147.9 by MM4 (146.8 by MP2). The stability of the compound is discussed in terms of the strain energy.