| Abstract
| - The structure of the camphenyl cation 1 has been studied in detail, using both experimental andcomputational approaches. Like others, we find only one structure on the camphenyl−isobornylcation PE surface, but this single structure shows some unusual features. These include a verysoft PE surface for movement along the C2−C6 axis (a nonbonding distance in a classical descriptionof the cation), and a result of this is that very high computational methods (optimization at MP4or QCI levels) are required in order to get structural minima that “fit” the experimental data. ThisPE surface has been probed computationally using fixed C2−C6 distances, and when one alsocalculates chemical shifts for these “fixed” structures, one sees calculated 13C NMR chemical shiftsfor the C2 carbon that are hugely dependent on this fixed distance value, giving near-linear slopesof ca. 25 ppm/0.1 Å distance change. Since this distance can vary over at least 0.6 Å with relativelysmall calculated energy changes, there is a total range of ca. 150 ppm involved here. In a secondpart of this work, and in response to a recent paper in which the historic Meerwein “carbocationintermediate” proposal was rejected, we have calculated solvation energies (SCI-PCM method) forfour carbocation systems, including 1. We find carbocation solvation energies (∈ = 10 “solvent”) of45−53 kcal/mol, and where comparison can be made, the data correlate well with the literature.On the basis of these results, we re-affirm the Meerwein “carbocation” mechanism, but in order toaccommodate only a single carbocation intermediate, we offer a description that amounts to a subtlevariation of both the nonclassical ion proposal and Meerwein's “two cation” mechanism, namelythat the camphenyl cation, 1, as a ground-state structure, can be described as only very weaklyinteracting in the C2−C6 bridging sense, but that the PE surface along this “bond” is so shallowthat an energy input of only 4−6 kcal/mol can produce a bridged “structure”. This mechanismexplains the preferred formation of exo products in both the camphenyl and isobornyl systems,isotopic exchange of chloride in camphenyl chloride, and it allows for partial racemization of thecamphenyl−isobornyl products in the reaction.
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