In a comment appended to an earlier post, I mused about the magnitude of the force constant relating to the interconversion between a classical and a non-classical structure for the norbornyl cation. Most calculations indicate the force constant for an “isolated” symmetrical cation is +ve, which means it is a true minimum and not a transition state for a [1,2] shift.

It is not only the non-classical norbornyl cation that has proved controversial in the past. A colleague mentioned at lunch (thanks Paul!) that tri-coordinate group 14 cations such as R ₃ Si ⁺ have also had an interesting history.[cite]10.1021/ja990389u[/cite] Here I take a brief look at some of these systems. Their initial characterisations, as with the carbon analogues, was by ²⁹ Si NMR.