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A Nonclassical Conclusion
The concept of valence, which underlies the Periodic Table, originated in studies of reactivity rather than structure. Nonetheless, when studies in the mid-20th century suggested that the transient norbornyl cation (C7H11+) reacted as though a carbon center had adopted a formally pentacoordinate motif, this nonclassical structural hypothesis engendered tremendous controversy. Scholz et al. (p. 62) have now succeeded in characterizing the norbornyl cation by x-ray crystallography and confirm the symmetrical fivefold motif.
Abstract
After decades of vituperative debate over the classical or nonclassical structure of the 2-norbornyl cation, the long-sought x-ray crystallographic proof of the bridged, nonclassical geometry of this prototype carbonium ion in the solvated [C7H11]+[Al2Br7]– • CH2Br2 salt has finally been realized. This achievement required exceptional treatment. Crystals obtained by reacting norbornyl bromide with aluminum tribromide in CH2Br2 undergo a reversible order-disorder phase transition at 86 kelvin due to internal 6,1,2-hydride shifts of the 2-norbornyl cation moiety. Cooling with careful annealing gave a suitably ordered phase. Data collection at 40 kelvin and refinement revealed similar molecular structures of three independent 2-norbornyl cations in the unit cell. All three structures agree very well with quantum chemical calculations at the MP2(FC)/def2-QZVPP level of theory.
