2- Norbomyl cation symmetrically bridged nonclassical

These results, which pertain to stable-ion conditions, provide strong evidence that foe most stable structure for foe norbomyl cation is foe symmetrically bridged nonclassical ion. How much stabilization does foe a bridging provide An estimate based on molecular mechanics calculations and a foermodynamic cycle suggests a stabilization of about 6 1 kcal/mol. An experimental value based on mass-spectrometric measurements is 11 kcal/mol. Gas-phase Itydride affinity and chloride affinity data also show foe norbomyl cation to be especially stable. ... 

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

While only an ancillary topic in this discussion, the nonclassical ion controversy, extensively reviewed elsewhere,has a foundation in the historical context of the Wagner-Meerwein rearrangement. After completing a solvolytic study of structurally similar exo- and e cto-2-norbomyl brosylates, Winstein and Trifan suggested that the reaction s cationic intermediate was instead a o-delocalized, symmetrically bridged norbomyl ion 9. This concept deviated from the accepted classical cation structure proposed by Meerwein as the equilibrium between 7 and 8, where the positive charge was considered to be localized on a single atom. 

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