Hypercoordinated carbocations

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

For the CH with a Cs symmetrical structure 1 a 13C NMR chemical shift of 11.5 ppm is calculated. This is 7.1 ppm more shielded than that calculated for the hydrocarbon CH4 at the GIAO-MP2 level.19 The shielding effect in 1 is as expected for a hypercoordinated carbocation carbon.20 The calculated minimum energy structure for diprotonated methane (CHi+) 2 has C2v symmetry with two 3c-2e... 

For historical overviews on the 2-norbornyl cation, the bicyclobutonium ion and related hypercoordinated carbocations see (a) Nonclassical Ions, Reprints and Commentary, P. D. Bartlett, W.A. Benjamin, Inc, New York and Amsterdam 1965 (b) The Nonclassical Ion Problem, Brown, H.C. with comments by Schleyer, P.v.R. Plenum Press, New York, 1977... 

Siehl, H. -U Fuss, M. Silyl effects in hypercoordinated carbocations IN. Organosilicon Chemistry IV, From Molecules to Materials Editors Auner, N. Weis, J. 2000, Wiley-VCH Weinheim, p. 140 - 149. 

Figure 1.12. Different types of hypercoordinated carbocations formation from hydrocarbon precursors by protonation or alkylation and cleavage products.

The second class of carbocations contains one or more hypercoordinate carbon atoms. These hypercarbons are coordinated to five or more atoms within reasonable bonding distance. Hypercoordinate or nonclassical carbocations cannot be described by using 2c-2e single bonds alone, but necessitate the involvement of three- (or multi-) center, two-electron bonds, e.g., 3c-2e bonds. Each hypercarbon in the cation is always associated with eight electrons, although the ion, overall, is electron deficient. The methonium ion (Cd I5 ) may be considered the parent of the hypercoordinate carbocations (Fig. 5.1). 

Various spectroscopic techniques have been applied to the characterization of carbocations as stable ions in solution and in the solid state. The techniques that have found most application in the study of hypercoordinate carbocations were developed primarily to be capable of distinguishing trivalent cations undergoing rapid degenerate rearrangements from bona fide hypercoordinate ions. In recent years, theoretical studies have become also very useful in the study of hypercoordinate carbocations. Furthermore, many properties such as chemical shifts ( H, etc.) and vibrational spectra can be accurately computed. 

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