| Abstract
| - Conformational analyses in the gas phase and the prediction of the crystal structures of 5-carboxy[3.3]orthoparacyclophane (1) and 6-bromo[4.4]orthoparacyclophane (2) were carried out by using the MM4 forcefield, and the calculated structures were compared with those determined by X-ray crystallographic experiments.The experimental crystal structure of 5-carboxy[3.3]orthoparacyclophane (1) takes on a parallel conformationwhere the two benzene rings have a parallel orientation, while the crystal structure of 6-bromo[4.4]orthoparacyclophane (2) is one with quasi-perpendicular conformation where one benzene ring tilts about60° against the other benzene plane. The MM4 conformational analyses in the gas phase based on the calculatedGibbs free energies of the conformers have shown that the MM4 force field reproduces the experimentallydetermined crystal structures of the two orthoparacyclophanes (1 (parallel) and 2 (quasi-perpendicular) ) asthe dominant conformers. With regard to the crystal structures, the unit cell parameters (a, b, c, α, β, γ)optimized by using the intermolecular potential energies developed for the MM4 force field agreed very wellwith experimental ones. The essential factors which govern the stable conformations (parallel or perpendicular)of the orthoparacyclophanes were also examined on the basis of the calculated results by the MM4 (forcefield) and DFT (molecular orbital) methods.
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