Perturbation of degeneracy of the cope rearrangement by the crystal lattice of the beta-form of 1,5-dimethylsemibullvalene-2,6-dicarbonitrile studiedby variable-temperature solid-state carbon-13 spectroscopy and X-ray crystallography at cryogenic temperatures

X-ray crystallographic structures of two polymorphs (alpha- and beta-forms) of 1,5-dimethylsemibullvalene-2,6-dicarbonitrile, in which the molecules u ndergo rapid, nondegenerate Cope rearrangement; have been obtained over the temperature range 20-295 K. CP-MAS carbon-13 NMR spectra of the beta-form have been observed over the temperature range 140-340 K. A generalized trea tment 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, Delta H-P and Delta S-P, for the perturbation of degeneracy in the beta-form are reported. A possible explanation for th e 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 mo l(-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.