Perturbation of Degeneracy of the Cope Rearrangement by the Crystal Lattice of the -Form of 1,5-Dimethylsemibullvalene-2,6-dicarbonitrile As Studied by Variable-Temperature Solid-State Carbon-13 Spectroscopy and X-ray Crystallography at Cryogenic Temperatures

J. Am. Chem. Soc., 122 (18), 4455 -4463, 2000. 10.1021/ja994256k S0002-7863(99)04256-0
Web Release Date: April 25, 2000

Alan Benesi, Rüdiger Bertermann, Hans Förster, Markus Heubes, Lloyd M. Jackman,* Tibor Koritsanszky, Peter Luger,* Andreas Mayer, Helmut Quast,* Maximilian Seefelder, and Dieter Zobel

Contribution from the Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Bruker Analytik GmbH, Silberstreifen, D-76287 Rheinstetten, Germany, Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, and Institut für Kristallographie der Freien Universität Berlin, Takusstrasse 6, D-14195 Berlin, Germany

Received December 2, 1999

Revised Manuscript Received February 14, 2000

Abstract:

X-ray crystallographic structures of two polymorphs (- and -forms) of 1,5-dimethylsemibullvalene-2,6-dicarbonitrile, in which the molecules undergo rapid, nondegenerate Cope rearrangement, have been obtained over the temperature range 20-295 K. CP-MAS carbon-13 NMR spectra of the -form have been observed over the temperature range 140-340 K. A generalized treatment for the perturbation of degeneracy has been developed and it is shown that the results of the X-ray and NMR methods are in excellent agreement. The thermodynamic parameters, H^P and S^P, for the perturbation of degeneracy in the -form are reported. A possible explanation for the difference in the magnitudes of the perturbation of degeneracy in the two crystalline forms is suggested. It is noted that for equilibrating systems, for which the barriers for interconversion are of the order of 1-10 kJ mol^-1, the transition between rapid and extremely slow equilibration occurs over a very narrow range at cryogenic temperatures, thus precluding direct observation of limiting structures.