Transition structures, activation energies, energy differences and nucleus independent chemical shifts (NICS) values were calculated by ab initio quantum mechanical methods for the Cope rearrangements of a series of 2,8 (4,6)-disubstituted semibullvalenes (1 and 2) and 2,8:4,6-tetrasubstituted semibullvalenes (3 and 4). In considering π-electron donating or accepting substituents at positions 2,8 or 4,6 of semibullvalenes, it was found that the Cope rearrangements were all shifted toward 4,6-disubstituted semibullvalenes (2), which does not agree with Hoffmann’s early argument. However, their transition states are delocalized and bishomoaromatic. In 2,8:4,6-tetrasubstituted semibullvalenes, which we emphasized on, π-electron groups all shift the equilibrium toward 4, and their Cope barriers are reduced drastically and even eliminated. That can be ascribed to the BCO stabilization effects and the conjugative effects of π-electron groups. The halogen substituent parameters of the Cope rearrangement also have corresponding varieties with the change of the main quantum number.

Key words: transition state, geometry configuration, aromaticity, reaction barrier, energy difference