CEPA Calculations for Rotation Barriers About CC Double Bonds: Ethylene, Allene, and Methylene-Cyclopropane

Abstract

The calculation of rotation harriers about CC double bonds has been a challenge to numerical quantum chemistry for a long time. While inversion barriers and rotation barriers about CC single bonds can be obtained by conventional quantum chemical methods to a rather high degree of accuracy (± 0.5 kcal/mol) all calculations for rotation barriers about CC double bonds that have been performed prior to 1977 yielded either completely wrong (errors of ± 30 kcal/mol) or only occasionally reasonable results. This holds for ab initio calculations as well as for semiempirical ones and is illustrated by two review articles 1,2 (which appeared in 1974 and 1977) on ab initio calculations of barrier heights. The situation improved largely in 1977 when three independent calculations3–5 on the rotation barrier of allene were successful in obtaining results close to each other and to the experimental value.