Trivalent cations equilibrating

The protracted and controversial discussions on the structure and dynamics of the 2-norbornyl cation have been recently summarised (Walling, 1983 Olah et al 1983 Brown, 1983 Grob, 1983). The question has been whether the 2-norbornyl cation has a symmetrically bridged hypercoordinated structure [99] with a pentacoordinated carbon atom or whether it is a rapidly equilibrating pair of trivalent cations [100], In the context of this review only the evidence from isotope effects on the nmr-spectra of the 2-norbomyl and related cations is discussed. 

The cross-polarization, magic-angle spinning method (CP MAS) has been applied by Myhre and Yannoni50 to cation 19 in the solid state at very low temperatures using 13C NMR spectroscopy. In the initial study, no convincing evidence for a frozen 2-butyl cation was obtained even at — 190°C. However, subsequently they managed to freeze out the equilibration of the 2-butyl cation (19) at —223°C.62 It behaves like a normal secondary trivalent carbocation. 

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