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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.

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Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v. R.; Meyer, K.; Krossing, I. Science 2013, 341, 62Contributed by Steven Bachrach.Reposted from Computational Organic Chemistry with permissionA long sought-after data point critical to the non-classical...
A long sought-after data point critical to the non-classical cation story has finally been obtained. The elusive x-ray crystal structure of a norbornyl cation was finally solved.1 The [C7H11]+[Al2Br7]- salt was crystallized in CH2Br2 at low temperature (40...
Chemistry Riddle Solved - Chemists have explained the structure of a non-classical carbocation. They have captured the 2-norbornyl cation as a crystal and determining beyond doubt the structure of this unusual and instable carbon compound. The 2-norbornyl cation...