Nonclassical structure, definition

R. H. J. Amer. Chem. Soc. 73, 1542 (1951)), and Winstein who referred to the nonclassical structure of norbornyl, cholesteryl, and 3-phenyl-2-butyl cations. [Winstein, S, Trifan, D. J. Amer. Chem. Soc. 74, 1154 (1952).) Bartlett s definition is widely used for nonclassical ions stating "an ion is nonclassical if its ground state has delocalized bonding o-electrons. Also see Ref.2 ... 

In the context of the historical development of our knowledge of the structure of the 2-norbornyl cation since Winstein and Trifan s postulate in 1949, this comparison with the 1,2-dimethyl derivative is very important for aspects provided by the theory of scientific discoveries. As discussed in our book on aromatic diazo compounds (Zollinger, 1994, Chap. 9), verifications are never definitive, in science proofs are not possible (the term proof should only be used in mathematics) falsifications, however, can be definitive. It was shown that molecular orbital calculations and their application to experimental data on the IR and NMR spectra for the 2-norbornyl cation are consistent with a symmetrical, nonclassical cationic intermediate, but not with a rapid equilibrium between two classical intermediates. The... 

A real prospect for a correct definition of molecular structure on the basis of quantum mechanics emerged thanks to a topological approach to the analysis of the electron density p(q) distribution in a system characterized by this density. This approach developed by Bader [19], based on the topological properties of the charge distribution in molecular systems, provides an invariant pattern of the distribution of chemical bonds between the atoms in a molecule and determines unambiguously the type of a molecular structure for both the classical (Butlerov-type) and the nonclassical (see Ref. [20]) molecular systems. 

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