German scientists solve nonclassical 2-norbornyl carbocation structure

Jul 05, 2013 by Bob Yirka report
Proposed 2-norbornyl cation structures. (A) Nonclassical Cs structure of the 2-norbornyl cation, depicted in 3c-2e and "pi" complex formulations. (B) Brown’s rapidly equilibrating C1 classical norbornyl cation enantiomers. Credit: (c) Science 5 July 2013: Vol. 341 no. 6141 pp. 62-64 DOI: 10.1126/science.1238849

(Phys.org) —A team of chemists working in Germany has finally, after decades of debate, solved the crystal structure of the nonclassical 2-norbornyl carbocation. In their paper published in the journal Science, the team describes the arduous process involved in the work they did that led to the eventual determination of the nonclassical crystal structure of the ion.

A debate has gone on for 64 years regarding the classical or nonclassical nature of the 2-norbornyl carbocation. It was in 1949 that Saul Winstein suggested their existence to explain the of substituted norbornane compounds. Other chemists such as Herbert Brown, reacted negatively to the suggestion because it meant accepting that carbon could be bonded to more than four other atoms. Brown suggested instead that a rapid equilibrium could occur to explain what chemists had been observing, which would allow them to remain categorized as classical.

Over the years, the debate has swung back and forth with various chemists arguing for one side or the other, with most eventually leaning towards the nonclassical 2-norbornyl carbocation structure—but now it appears, the chemists in this new effort have finally put the issue to rest. As it turned out, it appeared the vital condition that allowed for settlement of the argument came down to constructing an experiment that involved cooling the crystals with careful annealing at just the right temperature.

The breakthrough came at a lab on the campus of the University of Freiburg. The team used soft bromoaluminate to stabilize carbocations in a solid state. That allowed for the preparation of regular 2-norbornyl cation . Then, after much work revolving around how the crystals react to , the team arrived at a procedure that involved cooling a sample of the crystal to 40K, then allowing it to warm, then cooling it again—five or six times—doing so finally allowed the to reveal itself without cracking in the process.

With the true nonclassical structure of the crystals finally revealed, several have taken the opportunity to pronounce that the results of the work in Germany did little really but prove what most in the field already knew.

Explore further: Smartgels are thicker than water

More information: Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation, Science 5 July 2013: Vol. 341 no. 6141 pp. 62-64 DOI: 10.1126/science.1238849

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.

Related Stories

Breakthrough in chemical crystallography

Apr 05, 2013

A research team led by Professor Makoto Fujita of the University of Tokyo, Japan, and complemented by Academy Professor Kari Rissanen of the University of Jyväskylä, Finland, has made a fundamental breakthrough in single-crystal ...

Chemists grow crystals with a twist -- and untwist

Jul 16, 2010

(PhysOrg.com) -- Chemists from New York University and Russia's St. Petersburg State University have created crystals that can twist and untwist, pointing to a much more varied process of crystal growth than ...

Uranium crystals could reveal future of nuclear fuel

Jun 25, 2013

Mention the word "crystals" and few people think of nuclear fuel. Unless you are Eric Burgett. The Idaho State University professor is on a quest to create pure, single crystals of uranium and uranium oxide ...

Theory of crystal formation complete again

Feb 19, 2013

(Phys.org)—Exactly how a crystal forms from solution is a problem that has occupied scientists for decades. Researchers at Eindhoven University of Technology (TU/e), together with researchers from Germany ...

Recommended for you

Smartgels are thicker than water

Sep 19, 2014

Transforming substances from liquids into gels plays an important role across many industries, including cosmetics, medicine, and energy. But the transformation process, called gelation, where manufacturers ...

Separation of para and ortho water

Sep 18, 2014

(Phys.org) —Not all water is equal—at least not at the molecular level. There are two versions of the water molecule, para and ortho water, in which the spin states of the hydrogen nuclei are different. ...

User comments : 3

Adjust slider to filter visible comments by rank

Display comments: newest first

VENDItardE
2.5 / 5 (8) Jul 05, 2013
With the true nonclassical structure of the crystals finally revealed, several chemists have taken the opportunity to pronounce that the results of the work in Germany did little really but prove what most in the field already knew.......says all the "chemists" who were incapable of proving this themselves

komone
5 / 5 (1) Jul 05, 2013
Before this confirmation, surely "chemists knew" must have meant "chemists believed". Those two states are not the same. Beware the chemist (or teacher) who says they "know" when they actually mean they "believe"... without actual scientific proof.
NikFromNYC
1 / 5 (4) Jul 05, 2013
Human intuition overlaps well with organic reaction mechanisms. How molecules "think" is immediately relevant to the playful task of turning useless yellow oils into glistening medicines. At times, it overlaps perfectly.