nonclassical structure

https://doi.org/10.1351/goldbook.NT07084
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 @[email protected]) carbon atoms, e.g., methanium ion 1, pyramidal dication C6H62+ 2 (@[email protected] to benzene dication), and the molecular species whose structure cannot be adequately represented by the equilibrium (2-norbornyl @[email protected], 3) or @[email protected] 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 @[email protected] @[email protected] 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.
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Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1955 [Terms] [Paper]