Armilenium is a
carbocation
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encoun ...
and was originally proposed as the first entirely organic
sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ) and heterocyclic de ...
. Named for its resemblance to an
armillary sphere
An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of objects in the sky (on the celestial sphere), consisting of a spherical framework of rings, centered on Earth or the Sun, that represent lines ...
, NMR evidence for the carbocation was first described by Melvin J. Goldstein and Stanley A. Klein at
Cornell University
Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to ...
in 1973. In subsequent
13C NMR experiments by Goldstein and Joseph P. Dinnocenzo in 1984, the carbocation was generated under stable ion conditions at lower temperature and at higher magnetic field than previously possible.
These experiments revealed the carbocation to be fluxional. Fitting of the dynamic NMR process ruled out the sandwich species even as an intermediate in the 20-fold degenerate rearrangement of the carbocation.
References
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Carbocations