Ab initio molecular orbital study of conformational properties of cyclohexyne, cycloheptyne, and cyclooctyne

Abstract 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/6‐31G* methods. The C 2 symmetric twist conformer of 1 is calculated to be more stable than the planar C 2 v geometry. Ring inversion of the envelope conformation of 2 takes place via C 2 symmetric twist transition state, which is 34.0 kJ mol −1 higher than the envelope form. The C 2 symmetric chair conformation of 3 is calculated to be 11.7 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. Conformational racemization of the chiral 3‐TB takes place via the boat transition state. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006