The molecular and electronic structures of the dications of three homonuclear and three heteronuclear dichalcogenacyclooctanes (chalcogen β«Χ‘β¬ S, Se, or Te) were investigated by ab initio molecular orbital calculations. Four energy-minimum structures were located for each dication. Three of those (chairchair, boat-boat, and boat-chair) have the cis configuration with respect to the chalcogen lone pairs, and the remaining one has the trans configuration. The cis isomers were found to be much more stable than the trans isomer. Among the three cis structures, the stability is in the order of boat-chair ΟΎ boat-boat ΟΎ chairchair for all dications. This order can be explained by considering the nonbonding H β’ β’ β’ H interactions. The chair-chair structure (C 2v symmetry) of the 1,5-dithiacyclooctane dication has a very low vibrational frequency of a 2 symmetry, and its LUMO energy is lower than those of boat-boat and boat-chair. These can rationalize the fact that in the crystalline state the dication adopts a distorted C 2 chair-chair conformation. The transition states between the three conformers of the homonuclear dications were also located. The corresponding energy barriers are relatively low, which is consistent with their NMR spectra. The relative stabilities of the homonuclear and heteronuclear dications were elucidated on the basis of their energies and those of the corresponding neutral compounds. α§ 2000