The structure of molecules or molecular ions that escapes description in terms of conventional rules of valency and stereochemistry. Nonclassical structures are characteristic of carbonium ions with hypercoordinated (see @HT07051@) carbon atoms, e.g., methanium ion 1, pyramidal dication C₆H₆²⁺ 2 (@I03291@ to benzene dication), and the molecular species whose structure cannot be adequately represented by the equilibrium (2-norbornyl @C00907@, 3) or @R05326@ of two or more classical structures. From the stereochemical point of view, those structures are assigned to the nonclassical type for which all tetracoordinate carbon bonds extend into a single hemisphere, i.e., the @V06588@ @A00346@ of a carbon atom is greater than 180°. A hypothetical example is pyramidane, 4, the structure of which corresponds to a local minimum on the C₅H₄ potential energy surface.