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Physical Chemistry

Solving An Old Bonding Debate

Crystallography: 2-Norbornyl cation structure shows nonclassical bonding at last

by Jyllian Kemsley
July 8, 2013 | APPEARED IN VOLUME 91, ISSUE 27

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X-ray crystallography shows that the 2-norbornyl cation has a nonclassical two-electron, three-center structure, rather than classical single bonds.
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X-ray crystallography shows that the 2-norbornyl cation has a nonclassical two-electron, three-center structure, rather than classical single bonds.

Chemists have used a deep-freeze crystallization method to solve the structure for the 2-norbornyl cation, likely closing an acrimonious 50-year debate on the nature of a fundamental bonding concept. Instead of two electrons shared between two carbon atoms, as in conventional single-bond structures, this cation structure shows nonclassical bonding, with two electrons delocalized over three carbon atoms (Science 2013, DOI: 10.1126/science.1238849).

The structural analysis of the cation, a bridged cyclic hydrocarbon, was led by Ingo Krossing of the University of Freiburg and Karsten Meyer of the University of Erlangen-Nuremberg, both in Germany.

The debate centered on different interpretations of thermodynamic and spectroscopic data for the cation. Last year, University of Richmond chemist and historian Jeffrey I. Seeman characterized the interactions between the two sides as “crude and rude” (C&EN, Nov. 19, 2012, page 44). A crystal structure of the cation likely would have resolved the arguments, but that experimental evidence was elusive. Many other researchers have obtained 2-norbornyl cation crystals, but their molecular disorder stumped scientists, Krossing says.

Meyer, Krossing, and colleagues managed to obtain the structure by stabilizing the 2-norbornyl cation with a bromoaluminate counterion, crystallizing the product for several days at a chilly 245 K. They developed an annealing process of cooling the crystals even further to 40 K, then repeatedly warming and cooling the crystals to freeze out molecular disorder, allowing them to collect good X-ray data.

The structure reveals the two-electron, three-carbon system as having two C–C bonds of 1.80 Å, longer than a standard 1.54-Å C–C single bond. The third bond in the system is 1.39 Å long, similar to the delocalized C–C bonds of benzene.

The work is “a triumph of creative crystallography in taming an uncooperative cation,” says theoretical organic chemist Dean J. Tantillo of the University of California, Davis. Although the existing evidence had largely convinced chemists that the nonclassical structure existed in the gas phase and in solution, the new structure demonstrates that it exists in the solid state as well, Tantillo says.

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Comments
Bob Buntrock (July 11, 2013 10:59 AM)
Definitely read the article in Science. The 3rd named author is Paul Schleyer, currently at U. Georgia but formerly in the Erlangen group. Prior to that Paul was at Princeton when the NCC controversy came to a head. Paul did research in the area and was somewhere in the middle between the opposing camps (Bown, Winstein, et al.) although considering that the evidence seemed to favor NCCs. He also gave a course in early '63, which I took, on reactive intermediates, especially NCCs. The final was a take-home essay to describe the evidence, pro and con, for NCCs. It's only fitting that Paul is involved with the coup de grace.

Congratulations, Paul et al. Great job well done.
Charles J. Kelley (July 11, 2013 11:22 AM)
Well that's it. Organic Chemistry is now dead. The last mystery has been put out of its misery. My first, and lasting encounter with the "controversy" was in a departmental seminar at Indiana University in the fall of 1964. The seminar was advertized as "H. C. Brown is coming to town" (all the way from W. Lafayette to Bloomington). I was a new graduate student and was blissfully unaware of the controversy. It certainly got my attention, however, as the seminar ended in shouting between faculty members, my esteemed teachers, and the speaker (and later Nobel Laureate). One faculty member even stormed out of the room shouting over his shoulder. Research proceeded apace over the years. I later noted one paper funded by the American Cancer Society which was devoted completely to yet another inconclusive set of experiments to distinguish the "windshield-wiper" effect from the classical carbonium ions (as they were know at the time). Thank goodness I never devoted any research time to the pursuit.
R.I.P.
Bob Buntrock (July 12, 2013 4:45 PM)
Check out the cited C&EN article last Nov. A book, Nonclassical Ions (P. D. Bartlett, 1965) was even published which was a compendium of all of the papers published on the topic to that date. I didn't work for Paul Schleyer (the only research I did was for the course final), but we were well aware of the controversy and the principals all visited Princeton at some time or another. Seminars were pretty entertaining anyway but those on NCCs were almost rancorous. Bottom line: a great education in physical organic chemistry.

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