Transition state characteristics of planar singlet 2,4,6,8-cyclononatetraenylidenes and its halo derivatives, via ab initio

Calculations are carried out at HF/STO-3G, HF/6-31G*, MP2/6-31G*, B3LYP/6-31G* and B3LYP/6-311+G* levels of theory, for conversion of three series of cyclic conjugated non-planar allenes: 4-, 5-, and 6-X-1,2,4,6,8-cyclononapentaenes (1X, 2X, and 3X) to their mirror images (1′X, 2′X, and 3′X) through the corresponding singlet planar conjugated carbenic transition states: 3-, 4-, and 5-X-2,4,6,8-cyclononatetraenylidenes (1*X, 2*X, and 3*X), where X=H, F, Cl and Br. The employed halogens (F, Cl and Br) are chosen not to be directly attached to the allenic centers. For Series 1, the relative ease of racemization of 1X to 1′X through 1*X is: 1*H>1*Br>1*F≥1*Cl. For Series 2, the relative ease of racemization of 2X to 2′X through 2*X is: 2*H>2*F≥2*Cl≥2*Br. Finally, for Series 3, the relative ease of racemization of 3X to 3′X through 3*X is: 3*H>3*F≥3*Cl≥3*Br. Hence, the lowest racemization energy barrier encountered (26.8 kcal/mol) is for X=H, where 1H (=2H=3H) is converted to its mirror image 1′H (=2′H=3′H) through 1*H (=2*H=3*H) as a transition state.