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Chemistry Riddle Solved

A Freiburg research team has explained the structure of a non-classical carbocation

Freiburg, Jul 05, 2013

Chemistry Riddle Solved

A model of the crystal structure of the 2-norbornyl cation: Slow cooling to -233 degrees Celsius enabled the scientists to determine the molecule's structure. © Ingo Krossing

With support from Prof. Paul von Rague Schleyer from the University of Georgia, USA, a team of researchers including Prof. Dr. Ingo Krossing, Dr. Daniel Himmel, and Franziska Scholz from the Institute of Inorganic and Analytical Chemistry of the University of Freiburg and Prof. Dr. Karsten Meyer from the University of Erlangen has succeeded in capturing the 2-norbornyl cation as a crystal and determining beyond doubt the structure of this unusual and instable carbon compound. The 2-norbornyl cation is a non-classical carbocation, a molecule with a positively charged carbon atom that enters into five instead of three bonds with other atoms. Their article in the journal Science has ended a 50-year-old controversy among chemists.

The structure of the 2-norbornyl cations is at odds with the common view of molecules, but indeed, it seems that certain chemical reactions can only be explained through the existence of this short-lived intermediate stage. Scientists succeeded long ago in synthesizing the molecule, but analyzing its structure is considerably more difficult. The reason is the rapid migration of the hydrogen atoms, even in isolated crystal. Up to now, this stumbling block has prevented scientists from furnishing clear evidence of precisely how the bonds are distributed in the molecule. In their article, the researchers now describe the method that ultimately enabled them to conduct an x-ray structure analysis on the 2-norbornyl cation. This method involves guiding x-rays through a crystal and then determining the structure of the molecule on the basis of the diffraction of the rays. In this special case, the scientists also had to take the additional difficult step of slowly lowering the temperature by means of cyclical temperature changes in order to freeze the migrating hydrogen atoms in a resting position at the extremely low temperature of -233 degrees Celsius.

As early as 1949, Chemistry Nobel laureate Prof. Herbert Charles Brown criticized the first description of non-classical carbocations by the chemist Prof. Saul Winstein and demanded clear experimental evidence. Classical carbocations with up to three bonds and a positive charge were also long regarded as a pipe dream, and accounts of them were even censored in journals. In the 1960s scientists finally succeeded in detecting them with the help of spectroscopic methods thanks to the work of Chemistry Nobel laureate Prof. George Andrew Olah. In the case of the non-classical carbocations, on the other hand, the extraordinary bonding situation of the carbon and the distribution of the positive charge over several carbon atoms led to a decades-long scientific dispute among leading chemists over the existence of these compounds – which has now finally been resolved by the findings of the Freiburg researchers.

Original publication:
F. Scholz, D. Himmel, F. W. Heinemann, P. v. R. Schleyer, K. Meyer and I. Krossing (2013): Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation. In: Science 341 (6141), S. 62-64.

Prof. Dr. Ingo Krossing
Institute of Inorganic and Analytical Chemistry
University of Freiburg
Phone: +49 (0)761 / 203-6122