The strain energies and the pyramidalization parameters of adamantylideneadamantan (AD), trans-(1-methyl-2-adamantyliden)-1-methyladamantan (DMAD) and trans-(1-tert-butyl-2-adamantyliden)-1-tert-butyladamantan (DBAD) molecules were calculated by MM2 and AMBER molecular mechanic methods. The strain energy and the pyramidalization degree of the molecule are increased by increasing the volume of R groups at the allylic position of the double bond. The electron structure of the molecules were investigated by PM3 semiempirical method. Molecular mechanic and PM3 methods, the results of which agree with the X-ray results, showed that the double bond of AD molecule has a planar structure. The molecular complexes (1:1 pi-complexes) of the molecules formed with Br-2 were investigated by PM3 method and it was seen that the stable configuration of them had the axial structure. The electronic and the steric factors affecting the structure and the stability of the molecular complexes were studied. It was found that DMAD... Br-2 complex is more stable than AD... Br-2 complex which was in accordance with the experimental results. It was determined that DBAD... Br-2 complex containing the bulky tert-butyl group has the lowest stabilization energy among the investigated molecular complexes. (C) 2002 Elsevier Science B.V. All rights reserved.