Preferred Label : nonclassical structure;
IUPAC definition : 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 hypercoordination) carbon atoms, e.g.,
methanium ion b 1 /b , pyramidal dication C sub 6 /sub H sub 6 /sub sup 2 /sup
b 2 /b (isomeric to benzene dication), and the molecular species whose structure
cannot be adequately represented by the equilibrium (2-norbornyl cation, b 3 /b )
or resonance 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 valence angle of a carbon
atom is greater than 180 . A hypothetical example is pyramidane, b 4 /b , the structure
of which corresponds to a local minimum on the C sub 5 /sub H sub 4 /sub potential
energy surface.;
Origin ID : NT07084;
See also
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 hypercoordination) carbon atoms, e.g.,
methanium ion b 1 /b , pyramidal dication C sub 6 /sub H sub 6 /sub sup 2 /sup
b 2 /b (isomeric to benzene dication), and the molecular species whose structure
cannot be adequately represented by the equilibrium (2-norbornyl cation, b 3 /b )
or resonance 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 valence angle of a carbon
atom is greater than 180 . A hypothetical example is pyramidane, b 4 /b , the structure
of which corresponds to a local minimum on the C sub 5 /sub H sub 4 /sub potential
energy surface.
