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Abstract
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The structures and relative energies for the basic conformations of cyclohexyne (1), cycloheptyne (2), and cyclooctyne (3) have been calculated by the HF/ 6-31G, MP2/6-31G, and B3LYP/6-3lG methods. The C2 symmetric twist conformer of 1 is calculated to be more stable than the planar C,. geometry. Ring inversion of the envelope conformation of 2 takes place via C2 symmetric twist transition state, which is 34.0 KJ mol-1 higher than the envelope form. The C2 symmetric chair conformation of 3 is calculated to be 11.7 kJ KJ mol-1 more stable than the unsymmetrical twist-boat (3—TB) geometry. Interconversion of the chair conformation and 3-TB takes place via an unsymmetrical transition state, which is 37.6 kJ mol-1 less stable than the chair form. Conformatienal racemization of the chiral 3-TB takes place via the boat transition state.
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