NMR Effects in Carbocations

We might anticipate, however, that the three center-two electron bonding of carbonium ions might lead to atypical values of Av. In particular, we might expect the chemical shifts of hypervalent carbons to be unusual, and indeed they are shifted far upheld from what is seen for carbenium centers. This leads to greatly reduced values of A.  

CHAPTER 14 ADVANCED CONCEPTS IN ELECTRONIC STRUCTURE THEORY 

The norbomenyl cation. Shown in schematic form are the limiting structures. Also shown is a calculated geometry along with the HOMO and LUMO. 

If an appropriately positioned olefin can interact with a cationic center to make a carbo-nium ion, we might expect that the edge of a cyclopropane ring could do the same. An example of this is the ion derived from pentacyclo[4.3.0.0 .0 .0 - 0]nonane, which is known as Coates ion (shown in the margin). Even the most ardent opponents of the non-classical ion concept accept that this is a bridged, symmetrical carbonium ion. The value of fully sup- 

The unusual chemical shifts associated with carbonium ions carry over to H NMR as well. Eq. 14.65 shows the novel bridged structure obtained when cyclodecanol is exposed to the highly acidic environment associated with stable ion media. The remarkable upheld shift seen for one proton confirms its bridging nature. This system is a beautiful example of the types of transannular effects discussed in Section 2.3.2. 

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