Molecular distortions in higher symmetric, multibridged [2(n)]cyclophanes

The molecular structures of three higher symmetric, multibridged [2(n)]cycl ophanes (n = 3,4, 6) were optimized at the second-order Moller-Plesset leve l of theory. In contrast to previous assumptions, we found minimum structur es that were distorted from the fully eclipsed conformation, similar to the twisted ground state geometry established for [2.2]paracyclophane. For all the three studied cyclophanes, potential energy curves were calculated as a function of the half twist angle between the two aromatic rings, and the saddle points that interconnect the distorted minima were localized. The la rgest energetic stabilization of the distorted conformer was found for supe rphane (n = 6), a compound with extremely large overall strain. The pronoun ced differences in barrier height and the amount of distortion between the members of the [2(n)]cyclophane series are explained by a detailed analysis of the influence of several important parameters. The major limitation to larger twists in both the n = 3 and n = 4 compounds was found to be the dis tortion of the aromatic rings, away from planarity. The differences in the resulting twist potentials were related to the larger energetic penalty for a chair-like deformation than for a boat-like deformation of the aromatic rings. (C) 2000 Elsevier Science B.V. All rights reserved.