The structure of the 2-norbornyl cation: The pi-complex and beyond

Calculations at Becke3LYP, MP2, and QCISD levels of theory are presented fo r different molecular structures of the 2-norbornyl cation (1). With the ai d of the quantum theory of atoms in molecules (AIM) we show that 1 has a T- structure at the C-6-C-1-C-2 face (we view it as a Jr-complex with a tetrac oordinate carbon atom C-6) and that the ("nonclassical") bridged structure with a pentacoordinate Ch, which is not a minimum on the potential energy s urface, is only reached when the C-6-C-1 and C-6-C-2 internuclear distances are shortened to 165 pm. At the Becke3LYP/6-31G(d,p) level in the gas phas e this bridged species is 7.25 kcal mol(-1) higher in energy than the equil ibrium optimized geometry. NMR calculations show that C-13 chemical shifts of C-1, C-2, and C-6 Of the bridged 165 pm species differ significantly fro m the experimental values.