The historic camphenyl cation: A detailed structure evaluation including solvation energy calculations

The structure of the camphenyl cation 1 has been studied in detail, using b oth experimental and computational approaches. Like others, we find only on e structure on the camphenyl-isobornyl cation PE surface, but this single s tructure shows some unusual features. These include a very soft PE surface for movement along the C2-C6 axis (a nonbonding distance in a classical des cription of the cation), and a result of this is that very high computation al methods (optimization at MP4 or QCI levels) are required in order to get structural minima that "fit" the experimental data. This PE surface has be en probed computationally using fixed C2-C6 distances, and when one also ca lculates chemical shifts for these "fixed" structures, one sees calculated C-13 NMR chemical shifts for the C2 carbon that are hugely dependent on thi s fixed distance value, giving near-linear slopes of ca. 25 ppm/0.1 Angstro m distance change. Since this distance can vary over at least 0.6 Angstrom with relatively small calculated energy changes, there is a total range of ca. 150 ppm involved here, In a second part of this work, and in response t o a recent paper in which the historic Meerwein "carbocation intermediate" proposal was rejected, we have calculated solvation energies (SCI-PCM metho d) for four carbocation systems, including 1. We find carbocation solvation energies (is an element of = 10 "solvent") of 45-53 kcal/mol, and where co mparison can be made, the data correlate well with the literature. On the b asis of these results, we re-affirm the Meerwein "carbocation" mechanism, b ut in order to accommodate only a single carbocation intermediate, we offer a description that amounts to a subtle variation of both the nonclassical ion proposal and Meerwein's "two cation" mechanism, namely that the camphen yl cation, 1, as a ground-state structure, can be described as only very we akly interacting in the C2-C6 bridging sense, but that the PE surface along this "bond" is so shallow that an energy input of only 4-6 kcal/mol can pr oduce a bridged "structure". This mechanism explains the preferred formatio n of exo products in both the camphenyl and isobornyl systems, isotopic exc hange of chloride in camphenyl chloride, and it allows for partial racemiza tion of the camphenyl-isobornyl products in the reaction.