AbstractThe characterisation of the nature and energy of the transannular interaction in saturated eight-membered cycles could be achieved using the formalism of Quantum Theory of Atoms In Molecules (Bader's QTAIM). Namely, the differential analysis of the field of the one-electron density reveal the pictures of bonding interactions for the "chair-boat" (CB) conformers of the cyclooctane and its several chalcogene heteroanalogs. The dominant pattern is the close non-valency interactions between two atoms in positions of "joining" the substructures of the "chair" and "boat" types.IntroductionThe conformational behaviour of the medium (i.e., of size 7 to 12) cycles are believed to include the transannular interactions between the atoms and/or groups that belong to the cycle, as the principal driving force.Principles of conformational analysis of medium-sized ringsTwo series of conformers are generally observed for compounds containing a saturated eight-memebered ring, corresponding to the "crown" (Cr), or "high symmetry", and "chair-boat" (CB), or "low symmetry", conformations of the cycle.